Welcome to CasADi’s documentation!¶
1. Introduction¶
CasADi is an opensource software tool for numerical optimization in general and optimal control (i.e. optimization involving differential equations) in particular. The project was started by Joel Andersson and Joris Gillis while PhD students at the Optimization in Engineering Center (OPTEC) of the KU Leuven under supervision of Moritz Diehl.
This document aims at giving a condensed introduction to CasADi. After reading it, you should be able to formulate and manipulate expressions in CasADi’s symbolic framework, generate derivative information efficiently using algorithmic differentiation, to set up, solve and perform forward and adjoint sensitivity analysis for systems of ordinary differential equations (ODE) or differentialalgebraic equations (DAE) as well as to formulate and solve nonlinear programs (NLP) problems and optimal control problems (OCP).
CasADi is available for C++, Python and MATLAB/Octave with little or no difference in performance. In general, the Python API is the best documented and is slightly more stable than the MATLAB API. The C++ API is stable, but is not ideal for getting started with CasADi since there is limited documentation and since it lacks the interactivity of interpreted languages like MATLAB and Python. The MATLAB module has been tested successfully for Octave (version 4.0.2 or later).
1.1. What CasADi is and what it is not¶
CasADi started out as a tool for algorithmic differentiation (AD) using a syntax borrowed from computer algebra systems (CAS), which explains its name. While AD still forms one of the core functionalities of the tool, the scope of the tool has since been considerably broadened, with the addition of support for ODE/DAE integration and sensitivity analysis, nonlinear programming and interfaces to other numerical tools. In its current form, it is a generalpurpose tool for gradientbased numerical optimization – with a strong focus on optimal control – and CasADi is just a name without any particular meaning.
It is important to point out that CasADi is not a conventional AD tool, that can be used to calculate derivative information from existing user code with little to no modification. If you have an existing model written in C++, Python or MATLAB/Octave, you need to be prepared to reimplement the model using CasADi syntax.
Secondly, CasADi is not a computer algebra system. While the symbolic core does include an increasing set of tools for manipulate symbolic expressions, these capabilities are very limited compared to a proper CAS tool.
Finally, CasADi is not an “optimal control problem solver”, that allows the user to enter an OCP and then gives the solution back. Instead, it tries to provide the user with a set of “building blocks” that can be used to implement generalpurpose or specificpurpose OCP solvers efficiently with a modest programming effort.
1.2. Help and support¶
If you find simple bugs or lack some feature that you think would be relatively easy for us to add, the simplest thing is simply to write to the forum, located at http://forum.casadi.org/. We check the forum regularly and try to respond as quickly as possible. The only thing we expect for this kind of support is that you cite us, cf. Section 1.3, whenever you use CasADi in scientific work.
If you want more help, we are always open for academic or industrial cooperation. An academic cooperation usually take the form of a coauthorship of a peer reviewed paper, and an industrial cooperation involves a negotiated consulting contract. Please contact us directly if you are interested in this.
1.3. Citing CasADi¶
If you use CasADi in published scientific work, please cite the following:
@Article{Andersson2018,
Author = {Joel A E Andersson and Joris Gillis and Greg Horn
and James B Rawlings and Moritz Diehl},
Title = {{CasADi}  {A} software framework for nonlinear optimization
and optimal control},
Journal = {Mathematical Programming Computation},
Year = {2018},
}
1.4. Reading this document¶
The goal of this document is to make the reader familiar with the syntax of CasADi and provide easily available building blocks to build numerical optimization and dynamic optimization software. Our explanation is mostly program code driven and provides little mathematical background knowledge. We assume that the reader already has a fair knowledge of theory of optimization theory, solution of initialvalue problems in differential equations and the programming language in question (C++, Python or MATLAB/Octave).
We will use Python and MATLAB/Octave syntax sidebyside in this guide, noting that the Python interface is more stable and better documented. Unless otherwise noted, the MATLAB/Octave syntax also applies to Octave. We try to point out the instances where has a diverging syntax. To facilitate switching between the programming languages, we also list the major differences in Chapter 10.
2. Obtaining and installing¶
CasADi is an opensource tool, available under LGPL license, which is a permissive license that allows the tool to be used royaltyfree also in commercial closedsource applications. The main restriction of LGPL is that if you decide to modify CasADi’s source code as opposed to just using the tool for your application, these changes (a “derivativework” of CasADi) must be released under LGPL as well.
The source code is hosted on GitHub and has a core written in selfcontained C++ code, relying on nothing but the C++ Standard Library. Its frontends to Python and MATLAB/Octave are fullfeatured and autogenerated using the tool SWIG. These frontends are unlikely to result in noticeable loss of efficiency. CasADi can be used on Linux, OS X and Windows.
For uptodate installation instructions, visit CasADi’s install section: http://install.casadi.org/
3. Symbolic framework¶
At the core of CasADi is a selfcontained symbolic framework that allows the user to construct symbolic expressions using a MATLAB inspired everythingisamatrix syntax, i.e. vectors are treated as nby1 matrices and scalars as 1by1 matrices. All matrices are sparse and use a general sparse format – compressed column storage (CCS) – to store matrices. In the following, we introduce the most fundamental classes of this framework.
3.1. The SX
symbolics¶
The SX
data type is used to represent matrices whose elements consist of symbolic expressions made up by a sequence of unary and binary operations. To see how it works in practice, start an interactive Python shell (e.g. by typing ipython
from a Linux terminal or inside a integrated development environment such as Spyder) or launch MATLAB’s or Octave’s graphical user interface. Assuming CasADi has been installed correctly, you can import the symbols into the workspace as follows:
from casadi import *
import casadi.*
Now create a variable x
using the syntax:
x = MX.sym("x")
x
x = MX.sym('x')
x =
x
This creates a 1by1 matrix, i.e. a scalar containing a symbolic primitive called x
. This is just the display name, not the identifier. Multiple variables can have the same name, but still be different. The identifier is the return value. You can also create vector or matrixvalued symbolic variables by supplying additional arguments to SX.sym
:
y = SX.sym('y',5)
Z = SX.sym('Z',4,2)
[y_0, y_1, y_2, y_3, y_4]
[[Z_0, Z_4],
[Z_1, Z_5],
[Z_2, Z_6],
[Z_3, Z_7]]
y = SX.sym('y',5)
Z = SX.sym('Z',4,2)
y =
[y_0, y_1, y_2, y_3, y_4]
Z =
[[Z_0, Z_4],
[Z_1, Z_5],
[Z_2, Z_6],
[Z_3, Z_7]]
which creates a 5by1 matrix, i.e. a vector, and a 4by2 matrix with symbolic primitives, respectively.
SX.sym
is a (static) function which returns an SX
instance. When variables have been declared, expressions can now be formed in an intuitive way:
f = x**2 + 10
f = sqrt(f)
print(f)
sqrt((10+sq(x)))
f = x^2 + 10;
f = sqrt(f);
display(f)
f =
sqrt((10+sq(x)))
You can also create constant SX
instances without any symbolic primitives:
B1 = SX.zeros(4,5)
: A dense 4by5 empty matrix with all zerosB2 = SX(4,5)
: A sparse 4by5 empty matrix with all zerosB4 = SX.eye(4)
: A sparse 4by4 matrix with ones on the diagonal
B1: @1=0,
[[@1, @1, @1, @1, @1],
[@1, @1, @1, @1, @1],
[@1, @1, @1, @1, @1],
[@1, @1, @1, @1, @1]]
B2:
[[00, 00, 00, 00, 00],
[00, 00, 00, 00, 00],
[00, 00, 00, 00, 00],
[00, 00, 00, 00, 00]]
B4: @1=1,
[[@1, 00, 00, 00],
[00, @1, 00, 00],
[00, 00, @1, 00],
[00, 00, 00, @1]]
Note the difference between a sparse matrix with structural zeros and a dense matrix with actual zeros. When printing an expression with structural zeros, these will be represented as 00
to distinguish them from actual zeros 0
.
The following list summarizes the most commonly used ways of constructing new SX
expressions:
SX.sym(name,n,m)
: Create an \(n\)by\(m\) symbolic primitiveSX.zeros(n,m)
: Create an \(n\)by\(m\) dense matrix with all zerosSX(n,m)
: Create an \(n\)by\(m\) sparse matrix with all structural zerosSX.ones(n,m)
: Create an \(n\)by\(m\) dense matrix with all onesSX.eye(n)
: Create an \(n\)by\(n\) diagonal matrix with ones on the diagonal and structural zeros elsewhere.SX(scalar_type)
: Create a scalar (1by1 matrix) with value given by the argument. This method can be used explicitly, e.g.SX(9)
, or implicitly, e.g.9 * SX.ones(2,2)
.SX(matrix_type)
: Create a matrix given a numerical matrix given as a NumPy or SciPy matrix (in Python) or as a dense or sparse matrix (in MATLAB/Octave). In MATLAB/Octave e.g.SX([1,2,3,4])
for a row vector,SX([1;2;3;4])
for a column vector andSX([1,2;3,4])
for a 2by2 matrix. This method can be used explicitly or implicitly.repmat(v,n,m)
: Repeat expression \(v\) \(n\) times vertically and \(m\) times horizontally.repmat(SX(3),2,1)
will create a 2by1 matrix with all elements 3.(Python only)
SX(list)
: Create a column vector (\(n\)by1 matrix) with the elements in the list, e.g.SX([1,2,3,4])
(note the difference between Python lists and MATLAB/Octave horizontal concatenation, which both uses square bracket syntax)(Python only)
SX(list of list)
: Create a dense matrix with the elements in the lists, e.g.SX([[1,2],[3,4]])
or a row vector (1by\(n\) matrix) usingSX([[1,2,3,4]])
.
3.1.1. Note on namespaces¶
In MATLAB, if the import casadi.*
command is omitted, you can still use CasADi by prefixing all the symbols with the package name, e.g. casadi.SX
instead of SX
, provided the casadi
package is in the path. We will not do this in the following for typographical reasons, but note that it is often preferable in user code. In Python, this usage corresponds to issuing import casadi
instead of from casadi import *
.
Unfortunately, Octave (version 4.0.3) does not implement MATLAB’s import
command. To work around this issue, we provide a simple
function import.m
that can be placed in Octave’s path enabling the compact syntax used in this guide.
3.1.2. Note for C++ users¶
In C++, all public symbols are defined in the casadi
namespace and require the inclusion of the casadi/casadi.hpp
header file.
The commands above would be equivalent to:
#include <casadi/casadi.hpp>
using namespace casadi;
int main() {
SX x = SX::sym("x");
SX y = SX::sym("y",5);
SX Z = SX::sym("Z",4,2)
SX f = pow(x,2) + 10;
f = sqrt(f);
std::cout << "f: " << f << std::endl;
return 0;
}
3.2. DM
¶
DM
is very similar to SX
, but with the difference that the nonzero elements are numerical values and not symbolic expressions. The syntax is also the same, except for functions such as SX.sym
, which have no equivalents.
DM
is mainly used for storing matrices in CasADi and as inputs and outputs of functions. It is not intended to be used for computationally intensive calculations. For this purpose, use the builtin dense or sparse data types in MATLAB, NumPy or SciPy matrices in Python or an expression template based library such as eigen
, ublas
or MTL
in C++. Conversion between the types is usually straightforward:
C = DM(2,3)
C_dense = C.full()
from numpy import array
C_dense = array(C) # equivalent
C_sparse = C.sparse()
from scipy.sparse import csc_matrix
C_sparse = csc_matrix(C) # equivalent
C = DM(2,3);
C_dense = full(C);
C_sparse = sparse(C);
More usage examples for SX
can be found in the example pack at http://install.casadi.org/. For documentation of particular functions of this class (and others), find the “C++ API” on http://docs.casadi.org/ and search for information about casadi::Matrix
.
3.3. The MX
symbolics¶
Let us perform a simple operation using the SX
above:
x = SX.sym('x',2,2)
y = SX.sym('y')
f = 3*x + y
print(f)
print(f.shape)
@1=3,
[[((@1*x_0)+y), ((@1*x_2)+y)],
[((@1*x_1)+y), ((@1*x_3)+y)]]
(2, 2)
x = SX.sym('x',2,2);
y = SX.sym('y');
f = 3*x + y;
disp(f)
disp(size(f))
@1=3,
[[((@1*x_0)+y), ((@1*x_2)+y)],
[((@1*x_1)+y), ((@1*x_3)+y)]]
2 2
As you can see, the output of this operation is a 2by2 matrix. Note how the multiplication and the addition were performed elementwise and new expressions (of type SX
) were created for each entry of the result matrix.
We shall now introduce a second, more general matrix expression type MX
. The MX
type allows, like SX
, to build up expressions consisting of a sequence of elementary operations. But unlike SX
, these elementary operations are not restricted to be scalar unary or binary operations (\(\mathbb{R} \rightarrow \mathbb{R}\) or \(\mathbb{R} \times \mathbb{R} \rightarrow \mathbb{R}\)). Instead, the elementary operations that are used to form MX
expressions are allowed to be general multiple sparsematrix valued input, multiple sparsematrix valued output functions: \(\mathbb{R}^{n_1 \times m_1} \times \ldots \times \mathbb{R}^{n_N \times m_N} \rightarrow \mathbb{R}^{p_1 \times q_1} \times \ldots \times \mathbb{R}^{p_M \times q_M}\).
The syntax of MX
mirrors that of SX
:
x = MX.sym('x',2,2)
y = MX.sym('y')
f = 3*x + y
print(f)
print(f.shape)
((3*x)+y)
(2, 2)
x = MX.sym('x',2,2);
y = MX.sym('y');
f = 3*x + y;
disp(f)
disp(size(f))
((3*x)+y)
2 2
Note how the result consists of only two operations (one multiplication and one addition) using MX
symbolics, whereas the SX
equivalent has eight (two for each element of the resulting matrix). As a consequence, MX
can be more economical when working with operations that are naturally vector or matrix valued with many elements. As we shall see in Chapter 4, it is also much more general since we allow calls to arbitrary functions that cannot be expanded in terms of elementary operations.
MX
supports getting and setting elements, using the same syntax as SX
, but the way it is implemented is very different. Test, for example, to print the element in the upperleft corner of a 2by2 symbolic variable:
x = MX.sym('x',2,2)
print(x[0,0])
x[0]
x = MX.sym('x',2,2);
x(1,1)
ans =
x[0]
The output should be understood as an expression that is equal to the first (i.e. index 0 in C++) structurally nonzero element of x
, unlike x_0
in the SX
case above, which is the name of a symbolic primitive in the first (index 0) location of the matrix.
Similar results can be expected when trying to set elements:
x = MX.sym('x',2)
A = MX(2,2)
A[0,0] = x[0]
A[1,1] = x[0]+x[1]
print('A:', A)
A: (project((zeros(2x2,1nz)[0] = x[0]))[1] = (x[0]+x[1]))
x = MX.sym('x',2);
A = MX(2,2);
A(1,1) = x(1);
A(2,2) = x(1)+x(2);
display(A)
A =
(project((zeros(2x2,1nz)[0] = x[0]))[1] = (x[0]+x[1]))
The interpretation of the (admittedly cryptic) output is that starting with an all zero sparse matrix, an element is assigned to x_0
. It is then projected to a matrix of different sparsity and an another element is assigned to x_0+x_1
.
Element access and assignment, of the type you have just seen, are examples of operations that can be used to construct expressions. Other examples of operations are matrix multiplications, transposes, concatenations, resizings, reshapings and function calls.
3.4. Mixing SX
and MX
¶
You can not multiply an SX
object with an MX
object, or perform any other operation to mix the two in the same expression graph. You can, however, in an MX
graph include calls to a Function
defined by SX
expressions. This will be demonstrated in Chapter 4. Mixing SX
and MX
is often a good idea since functions defined by SX
expressions have a much lower overhead per operation making it much faster for operations that are naturally written as a sequence of scalar operations. The SX
expressions are thus intended to be used for low level operations (for example the DAE right hand side in Section 4.4), whereas the MX
expressions act as a glue and enables the formulation of e.g. the constraint function of an NLP (which might contain calls to ODE/DAE integrators, or might simply be too large to expand as one big expression).
3.5. The Sparsity
class¶
As mentioned above, matrices in CasADi are stored using the compressed column storage (CCS) format. This is a standard format for sparse matrices that allows linear algebra operations such as elementwise operations, matrix multiplication and transposes to be performed efficiently. In the CCS format, the sparsity pattern is decoded using the dimensions – the number of rows and number of columns – and two vectors. The first vector contains the index of the first structurally nonzero element of each column and the second vector contains the row index for every nonzero element. For more details on the CCS format, see e.g. Templates for the Solution of Linear Systems on Netlib. Note that CasADi uses the CCS format for sparse as well as dense matrices.
Sparsity patterns in CasADi are stored as instances of the Sparsity
class, which is referencecounted, meaning that multiple matrices can share the same sparsity pattern, including MX
expression graphs and instances of SX
and DM
. The Sparsity
class is also cached, meaning that the creation of multiple instances of the same sparsity patterns is always avoided.
The following list summarizes the most commonly used ways of constructing new sparsity patterns:
Sparsity.dense(n,m)
: Create a dense \(n\)by\(m\) sparsity patternSparsity(n,m)
: Create a sparse \(n\)by\(m\) sparsity patternSparsity.diag(n)
: Create a diagonal \(n\)by\(n\) sparsity patternSparsity.upper(n)
: Create an upper triangular \(n\)by\(n\) sparsity patternSparsity.lower(n)
: Create a lower triangular \(n\)by\(n\) sparsity pattern
The Sparsity
class can be used to create nonstandard matrices, e.g.
print(SX.sym('x',Sparsity.lower(3)))
[[x_0, 00, 00],
[x_1, x_3, 00],
[x_2, x_4, x_5]]
disp(SX.sym('x',Sparsity.lower(3)))
[[x_0, 00, 00],
[x_1, x_3, 00],
[x_2, x_4, x_5]]
3.5.1. Getting and setting elements in matrices¶
To get or set an element or a set of elements in CasADi’s matrix types (SX
, MX
and DM
), we use square brackets in Python and round brackets in C++ and MATLAB. As is conventional in these languages, indexing starts from zero in C++ and Python but from one in MATLAB. In Python and C++, we allow negative indices to specify an index counted from the end. In MATLAB, use the end
keyword for indexing from the end.
Indexing can be done with one index or two indices. With two indices, you reference a particular row (or set or rows) and a particular column (or set of columns). With one index, you reference an element (or set of elements) starting from the upper left corner and columnwise to the lower right corner. All elements are counted regardless of whether they are structurally zero or not.
M = SX([[3,7],[4,5]])
print(M[0,:])
M[0,:] = 1
print(M)
[[3, 7]]
@1=1,
[[@1, @1],
[4, 5]]
M = SX([3,7;4,5]);
disp(M(1,:))
M(1,:) = 1;
disp(M)
[[3, 7]]
@1=1,
[[@1, @1],
[4, 5]]
Unlike Python’s NumPy, CasADi slices are not views into the data of the left hand side; rather, a slice access copies the data. As a result, the matrix \(m\) is not changed at all in the following example:
M = SX([[3,7],[4,5]])
M[0,:][0,0] = 1
print(M)
[[3, 7],
[4, 5]]
The getting and setting matrix elements is elaborated in the following. The discussion applies to all of CasADi’s matrix types.
Single element access is getting or setting by providing a rowcolumn pair or its flattened index (columnwise starting in the upper left corner of the matrix):
M = diag(SX([3,4,5,6]))
print(M)
[[3, 00, 00, 00],
[00, 4, 00, 00],
[00, 00, 5, 00],
[00, 00, 00, 6]]
M = diag(SX([3,4,5,6]));
disp(M)
[[3, 00, 00, 00],
[00, 4, 00, 00],
[00, 00, 5, 00],
[00, 00, 00, 6]]
print(M[0,0])
print(M[1,0])
print(M[1,1])
3
00
6
disp(M(1,1))
disp(M(2,1))
disp(M(end,end))
3
00
6
Slice access means setting multiple elements at once. This is significantly more efficient than setting the elements one at a time. You get or set a slice by providing a (start , stop , step) triple. In Python and MATLAB, CasADi uses standard syntax:
print(M[:,1])
print(M[1:,1:4:2])
[00, 4, 00, 00]
[[4, 00],
[00, 00],
[00, 6]]
disp(M(:,2))
disp(M(2:end,2:2:4))
[00, 4, 00, 00]
[[4, 00],
[00, 00],
[00, 6]]
In C++, CasADi’s Slice
helper class can be used. For the example above, this means M(Slice(),1)
and M(Slice(1,1),Slice(1,4,2))
, respectively.
List access is similar to (but potentially less efficient than) slice access:
M = SX([[3,7,8,9],[4,5,6,1]])
print(M)
print(M[0,[0,3]], M[[5,6]])
[[3, 7, 8, 9],
[4, 5, 6, 1]]
[[3, 9]] [6, 7]
M = SX([3 7 8 9; 4 5 6 1]);
disp(M)
disp(M(1,[1,4]))
disp(M([6,numel(M)5]))
[[3, 7, 8, 9],
[4, 5, 6, 1]]
[[3, 9]]
[[6, 7]]
3.6. Arithmetic operations¶
CasADi supports most standard arithmetic operations such as addition, multiplications, powers, trigonometric functions etc:
x = SX.sym('x')
y = SX.sym('y',2,2)
print(sin(y)x)
[[(sin(y_0)x), (sin(y_2)x)],
[(sin(y_1)x), (sin(y_3)x)]]
x = SX.sym('x');
y = SX.sym('y',2,2);
disp(sin(y)x)
[[(sin(y_0)x), (sin(y_2)x)],
[(sin(y_1)x), (sin(y_3)x)]]
In C++ and Python (but not in MATLAB), the standard multiplication operation (using *
) is reserved for elementwise multiplication (in MATLAB .*
). For matrix multiplication, use A @ B
or (mtimes(A,B)
in Python 3.4+):
print(y*y, y@y)
[[sq(y_0), sq(y_2)],
[sq(y_1), sq(y_3)]]
[[(sq(y_0)+(y_2*y_1)), ((y_0*y_2)+(y_2*y_3))],
[((y_1*y_0)+(y_3*y_1)), ((y_1*y_2)+sq(y_3))]]
disp(y.*y)
disp(y*y)
[[sq(y_0), sq(y_2)],
[sq(y_1), sq(y_3)]]
[[(sq(y_0)+(y_2*y_1)), ((y_0*y_2)+(y_2*y_3))],
[((y_1*y_0)+(y_3*y_1)), ((y_1*y_2)+sq(y_3))]]
As is customary in MATLAB, multiplication using *
and .*
are equivalent when either of the arguments is a scalar.
Transposes are formed using the syntax A.T
in Python, A.T()
in C++ and with
A
or A.'
in MATLAB:
print(y)
print(y.T)
[[y_0, y_2],
[y_1, y_3]]
[[y_0, y_1],
[y_2, y_3]]
disp(y)
disp(y')
[[y_0, y_2],
[y_1, y_3]]
[[y_0, y_1],
[y_2, y_3]]
Reshaping means changing the number of rows and columns but retaining the number of elements and the relative location of the nonzeros. This is a computationally very cheap operation which is performed using the syntax:
x = SX.eye(4)
print(reshape(x,2,8))
@1=1,
[[@1, 00, 00, 00, 00, @1, 00, 00],
[00, 00, @1, 00, 00, 00, 00, @1]]
x = SX.eye(4);
reshape(x,2,8)
ans =
@1=1,
[[@1, 00, 00, 00, 00, @1, 00, 00],
[00, 00, @1, 00, 00, 00, 00, @1]]
Concatenation means stacking matrices horizontally or vertically. Due to the columnmajor way of storing elements in CasADi, it is most efficient to stack matrices horizontally. Matrices that are in fact column vectors (i.e. consisting of a single column), can also be stacked efficiently vertically. Vertical and horizontal concatenation is performed using the functions vertcat
and horzcat
(that take a variable amount of input arguments) in Python and C++ and with square brackets in MATLAB:
x = SX.sym('x',5)
y = SX.sym('y',5)
print(vertcat(x,y))
[x_0, x_1, x_2, x_3, x_4, y_0, y_1, y_2, y_3, y_4]
x = SX.sym('x',5);
y = SX.sym('y',5);
disp([x;y])
[x_0, x_1, x_2, x_3, x_4, y_0, y_1, y_2, y_3, y_4]
print(horzcat(x,y))
[[x_0, y_0],
[x_1, y_1],
[x_2, y_2],
[x_3, y_3],
[x_4, y_4]]
disp([x,y])
[[x_0, y_0],
[x_1, y_1],
[x_2, y_2],
[x_3, y_3],
[x_4, y_4]]
There are also variants of these functions that take a list (in Python) or a cell array (in Matlab) as inputs:
L = [x,y]
print(hcat(L))
[[x_0, y_0],
[x_1, y_1],
[x_2, y_2],
[x_3, y_3],
[x_4, y_4]]
L = {x,y};
disp([L{:}])
[[x_0, y_0],
[x_1, y_1],
[x_2, y_2],
[x_3, y_3],
[x_4, y_4]]
Horizontal and vertical split are the inverse operations of the above introduced horizontal and vertical concatenation. To split up an expression horizontally into \(n\) smaller expressions, you need to provide, in addition to the expression being split, a vector offset of length \(n+1\). The first element of the offset vector must be 0 and the last element must be the number of columns. Remaining elements must follow in a nondecreasing order. The output \(i\) of the split operation then contains the columns \(c\) with \(\textit{offset}[i] \le c < \textit{offset}[i+1]\). The following demonstrates the syntax:
x = SX.sym('x',5,2)
w = horzsplit(x,[0,1,2])
print(w[0], w[1])
[x_0, x_1, x_2, x_3, x_4] [x_5, x_6, x_7, x_8, x_9]
x = SX.sym('x',5,2);
w = horzsplit(x,[0,1,2]);
disp(w(1)), disp(w(2))
{
[1,1] =
casadi.SX object with properties:
}
{
[1,1] =
casadi.SX object with properties:
}
The vertsplit operation works analogously, but with the offset vector referring to rows:
w = vertsplit(x,[0,3,5])
print(w[0], w[1])
[[x_0, x_5],
[x_1, x_6],
[x_2, x_7]]
[[x_3, x_8],
[x_4, x_9]]
w = vertsplit(x,[0,3,5]);
disp(w{1}), disp(w{2})
[[x_0, x_5],
[x_1, x_6],
[x_2, x_7]]
[[x_3, x_8],
[x_4, x_9]]
Note that it is always possible to use slice element access instead of horizontal and vertical split, for the above vertical split:
w = [x[0:3,:], x[3:5,:]]
print(w[0], w[1])
[[x_0, x_5],
[x_1, x_6],
[x_2, x_7]]
[[x_3, x_8],
[x_4, x_9]]
w = {x(1:3,:), x(4:5,:)};
disp(w{1}), disp(w{2})
[[x_0, x_5],
[x_1, x_6],
[x_2, x_7]]
[[x_3, x_8],
[x_4, x_9]]
For SX
graphs, this alternative way is completely equivalent, but for MX
graphs using horzsplit
/vertsplit
is significantly more efficient when all the split expressions are needed.
Inner product, defined as \(<A,B> := \text{tr}(A \, B) = \sum_{i,j} \, A_{i,j} \, B_{i,j}\) are created as follows:
x = SX.sym('x',2,2)
print(dot(x,x))
(((sq(x_0)+sq(x_1))+sq(x_2))+sq(x_3))
x = SX.sym('x',2,2);
disp(dot(x,x))
(((sq(x_0)+sq(x_1))+sq(x_2))+sq(x_3))
Many of the above operations are also defined for the Sparsity
class (Section 3.5), e.g. vertcat
, horzsplit
, transposing, addition (which returns the union of two sparsity patterns) and multiplication (which returns the intersection of two sparsity patterns).
3.7. Querying properties¶
You can check if a matrix or sparsity pattern has a certain property by calling an appropriate member function. e.g.
y = SX.sym('y',10,1)
print(y.shape)
(10, 1)
y = SX.sym('y',10,1);
size(y)
ans =
10 1
Note that in MATLAB, obj.myfcn(arg)
and myfcn(obj, arg)
are both valid ways of calling a member function myfcn
. The latter variant is probably preferable from a style viewpoint.
Some commonly used properties for a matrix A
are:
A.size1()
The number of rows
A.size2()
The number of columns
A.shape
(in MATLAB “size”) The shape, i.e. the pair (nrow,*ncol*)
A.numel()
The number of elements, i.e \(\textit{nrow} * \textit{ncol}\)
A.nnz()
The number of structurally nonzero elements, equal toA.numel()
if dense.
A.sparsity()
Retrieve a reference to the sparsity pattern
A.is_dense()
Is a matrix dense, i.e. having no structural zeros
A.is_scalar()
Is the matrix a scalar, i.e. having dimensions 1by1?
A.is_column()
Is the matrix a vector, i.e. having dimensions \(n\)by1?
A.is_square()
Is the matrix square?
A.is_triu()
Is the matrix upper triangular?
A.is_constant()
Are the matrix entries all constant?
A.is_integer()
Are the matrix entries all integervalued?
The last queries are examples of queries for which false negative returns are allowed. A matrix for which A.is_constant()
is true is guaranteed to be constant, but is not guaranteed to be nonconstant if A.is_constant()
is false. We recommend you to check the API documentation for a particular function before using it for the first time.
3.8. Linear algebra¶
CasADi supports a limited number of linear algebra operations, e.g. for solution of linear systems of equations:
A = MX.sym('A',3,3)
b = MX.sym('b',3)
print(solve(A,b))
(A\b)
A = MX.sym('A',3,3);
b = MX.sym('b',3);
disp(A\b)
(A\b)
3.9. Calculus – algorithmic differentiation¶
The single most central functionality of CasADi is algorithmic (or automatic) differentiation (AD). For a function \(f: \mathbb{R}^N \rightarrow \mathbb{R}^M\):
forward mode directional derivatives can be used to calculate Jacobiantimesvector products:
Similarly, reverse mode directional derivatives can be used to calculate Jacobiantransposedtimesvector products:
Both forward and reverse mode directional derivatives are calculated at a cost proportional to evaluating \(f(x)\), regardless of the dimension of \(x\).
CasADi is also able to generate complete, sparse Jacobians efficiently. The algorithm for this is very complex, but essentially consists of the following steps:
Automatically detect the sparsity pattern of the Jacobian
Use graph coloring techniques to find a few forward and/or directional derivatives needed to construct the complete Jacobian
Calculate the directional derivatives numerically or symbolically
Assemble the complete Jacobian
Hessians are calculated by first calculating the gradient and then performing the same steps as above to calculate the Jacobian of the gradient in the same way as above, while exploiting symmetry.
3.9.1. Syntax¶
An expression for a Jacobian is obtained using the syntax:
A = SX.sym('A',3,2)
x = SX.sym('x',2)
print(jacobian(A@x,x))
[[A_0, A_3],
[A_1, A_4],
[A_2, A_5]]
A = SX.sym('A',3,2);
x = SX.sym('x',2);
jacobian(A*x,x)
ans =
[[A_0, A_3],
[A_1, A_4],
[A_2, A_5]]
When the differentiated expression is a scalar, you can also calculate the gradient in the matrix sense:
print(gradient(dot(A,A),A))
[[(A_0+A_0), (A_3+A_3)],
[(A_1+A_1), (A_4+A_4)],
[(A_2+A_2), (A_5+A_5)]]
gradient(dot(A,A),A)
ans =
[[(A_0+A_0), (A_3+A_3)],
[(A_1+A_1), (A_4+A_4)],
[(A_2+A_2), (A_5+A_5)]]
Note that, unlike jacobian
, gradient
always returns a dense vector.
Hessians, and as a byproduct gradients, are obtained as follows:
[H,g] = hessian(dot(x,x),x)
print('H:', H)
H: @1=2,
[[@1, 00],
[00, @1]]
[H,g] = hessian(dot(x,x),x);
display(H)
H =
@1=2,
[[@1, 00],
[00, @1]]
For calculating a Jacobiantimesvector product, the jtimes
function – performing forward mode AD – is often more efficient than creating the full Jacobian and performing a matrixvector multiplication:
A = DM([[1,3],[4,7],[2,8]])
x = SX.sym('x',2)
v = SX.sym('v',2)
f = mtimes(A,x)
print(jtimes(f,x,v))
[(v_0+(3*v_1)), ((4*v_0)+(7*v_1)), ((2*v_0)+(8*v_1))]
A = [1 3;4 7;2 8];
x = SX.sym('x',2);
v = SX.sym('v',2);
f = A*x;
jtimes(f,x,v)
ans =
[(v_0+(3*v_1)), ((4*v_0)+(7*v_1)), ((2*v_0)+(8*v_1))]
The jtimes
function optionally calculates the transposedJacobiantimesvector product, i.e. reverse mode AD:
w = SX.sym('w',3)
f = mtimes(A,x)
print(jtimes(f,x,w,True))
[(((2*w_2)+(4*w_1))+w_0), (((8*w_2)+(7*w_1))+(3*w_0))]
w = SX.sym('w',3);
f = A*x;
jtimes(f,x,w,true)
ans =
[(((2*w_2)+(4*w_1))+w_0), (((8*w_2)+(7*w_1))+(3*w_0))]
4. Function objects¶
CasADi allows the user to create function objects, in C++ terminology often referred to as functors. This includes functions that are defined by a symbolic expression, ODE/DAE integrators, QP solvers, NLP solvers etc.
Function objects are typically created with the syntax:
f = functionname(name, arguments, ..., [options])
The name is mainly a display name that will show up in e.g. error messages or as comments in generated C code. This is followed by a set of arguments, which is class dependent. Finally, the user can pass an options structure for customizing the behavior of the class. The options structure is a dictionary type in Python, a struct in MATLAB or CasADi’s Dict
type in C++.
A Function
can be constructed by passing a list of input expressions and a list of output expressions:
x = SX.sym('x',2)
y = SX.sym('y')
f = Function('f',[x,y],\
[x,sin(y)*x])
print(f)
f:(i0[2],i1)>(o0[2],o1[2]) SXFunction
x = SX.sym('x',2);
y = SX.sym('y');
f = Function('f',{x,y},...
{x,sin(y)*x});
disp(f)
f:(i0[2],i1)>(o0[2],o1[2]) SXFunction
which defines a function \(f : \mathbb{R}^{2} \times \mathbb{R} \rightarrow \mathbb{R}^{2} \times \mathbb{R}^{2}, \quad (x,y) \mapsto (x,\sin(y) x)\). Note that all function objects in CasADi, including the above, are multiple matrixvalued input, multiple, matrixvalued output.
MX
expression graphs work the same way:
x = MX.sym('x',2)
y = MX.sym('y')
f = Function('f',[x,y],\
[x,sin(y)*x])
print(f)
f:(i0[2],i1)>(o0[2],o1[2]) MXFunction
x = MX.sym('x',2);
y = MX.sym('y');
f = Function('f',{x,y},...
{x,sin(y)*x});
disp(f)
f:(i0[2],i1)>(o0[2],o1[2]) MXFunction
When creating a Function
from expressions like that, it is always advisory to name the inputs and outputs as follows:
f = Function('f',[x,y],\
[x,sin(y)*x],\
['x','y'],['r','q'])
print(f)
f:(x[2],y)>(r[2],q[2]) MXFunction
f = Function('f',{x,y},...
{x,sin(y)*x},...
{'x','y'},{'r','q'});
disp(f)
f:(x[2],y)>(r[2],q[2]) MXFunction
Naming inputs and outputs is preferred for a number of reasons:
No need to remember the number or order of arguments
Inputs or outputs that are absent can be left unset
More readable and less error prone syntax.
For Function
instances – to be encountered later – that are not created directly from expressions,
the inputs and outputs are named automatically.
4.1. Calling function objects¶
MX
expressions may contain calls to Function
derived functions. Calling a function object is both done for the numerical evaluation and, by passing symbolic arguments, for embedding a call to the function object into an expression graph (cf. also Section 4.4).
To call a function object, you either pass the argument in the correct order:
r0, q0 = f(1.1,3.3)
print('r0:',r0)
print('q0:',q0)
r0: [1.1, 1.1]
q0: [0.17352, 0.17352]
[r0, q0] = f(1.1,3.3);
disp(r0)
disp(q0)
[1.1, 1.1]
[0.17352, 0.17352]
or the arguments and their names as follows, which will result in a dictionary (dict
in Python, struct
in MATLAB and std::map<std::string, MatrixType>
in C++):
res = f(x=1.1, y=3.3)
print('res:', res)
res: {'q': DM([0.17352, 0.17352]), 'r': DM([1.1, 1.1])}
res = f('x',1.1,'y',3.3);
disp(res)
scalar structure containing the fields:
q =
casadi.DM object with properties:
r =
casadi.DM object with properties:
When calling a function object, the dimensions (but not necessarily the sparsity patterns) of the evaluation arguments have to match those of the function inputs, with two exceptions:
A row vector can be passed instead of a column vector and vice versa.
A scalar argument can always be passed, regardless of the input dimension. This has the meaning of setting all elements of the input matrix to that value.
When the number of inputs to a function object is large or changing, an alternative syntax to the above is to use the call function which takes a Python list / MATLAB cell array or, alternatively, a Python dict / MATLAB struct. The return value will have the same type:
arg = [1.1,3.3]
res = f.call(arg)
print('res:', res)
arg = {'x':1.1,'y':3.3}
res = f.call(arg)
print('res:', res)
res: [DM([1.1, 1.1]), DM([0.17352, 0.17352])]
res: {'q': DM([0.17352, 0.17352]), 'r': DM([1.1, 1.1])}
arg = {1.1,3.3};
res = f.call(arg);
disp(res)
arg = struct('x',1.1,'y',3.3);
res = f.call(arg);
disp(res)
{
[1,1] =
casadi.DM object with properties:
[1,2] =
casadi.DM object with properties:
}
scalar structure containing the fields:
q =
casadi.DM object with properties:
r =
casadi.DM object with properties:
4.2. Converting MX
to SX
¶
A function object defined by an MX
graph that only contains builtin operations (e.g. elementwise operations such as addition, square root, matrix multiplications and calls to SX
functions), can be converted into a function defined purely by an SX
graph using the syntax:
sx_function = mx_function.expand()
This might speed up the calculations significantly, but might also cause extra memory overhead.
4.3. Nonlinear rootfinding problems¶
Consider the following system of equations:
where the first equation uniquely defines \(z\) as a function of \(x_1\), ldots, \(x_n\) by the implicit function theorem and the remaining equations define the auxiliary outputs \(y_1\), ldots, \(y_m\).
Given a function \(g\) for evaluating \(g_0\), ldots, \(g_m\), we can use CasADi to automatically formulate a function \(G: \{z_{\text{guess}}, x_1, x_2, \ldots, x_n\} \rightarrow \{z, y_1, y_2, \ldots, y_m\}\). This function includes a guess for \(z\) to handle the case when the solution is nonunique. The syntax for this, assuming \(n=m=1\) for simplicity, is:
z = SX.sym('x',nz)
x = SX.sym('x',nx)
g0 = sin(x+z)
g1 = cos(xz)
g = Function('g',[z,x],[g0,g1])
G = rootfinder('G','newton',g)
print(G)
G:(i0,i1)>(o0,o1) Newton
z = SX.sym('x',nz);
x = SX.sym('x',nx);
g0 = sin(x+z);
g1 = cos(xz);
g = Function('g',{z,x},{g0,g1});
G = rootfinder('G','newton',g);
disp(G)
G:(i0,i1)>(o0,o1) Newton
where the rootfinder
function expects a display name, the name of a solver plugin
(here a simple fullstep Newton method) and the residual function.
Rootfinding objects in CasADi are differential objects and derivatives can be calculated exactly to arbitrary order.
See also
4.4. Initialvalue problems and sensitivity analysis¶
CasADi can be used to solve initialvalue problems in ODE or DAE. The problem formulation used is a DAE of semiexplicit form with quadratures:
For solvers of ordinary differential equations, the second equation and the algebraic variables \(z\) must be absent.
An integrator in CasADi is a function that takes the state at the initial time x0
, a set of parameters p
and controls u
, and a guess for the algebraic variables (only for DAEs) z0
and returns the state vector xf
, algebraic variables zf
and the quadrature state qf
at a number of output times. The control vector u
is assumed to be piecewise constant and has the same grid discretization as the output grid.
The freely available SUNDIALS suite (distributed along with CasADi) contains the two popular integrators CVodes and IDAS for ODEs and DAEs respectively. These integrators have support for forward and adjoint sensitivity analysis and when used via CasADi’s Sundials interface, CasADi will automatically formulate the Jacobian information, which is needed by the backward differentiation formula (BDF) that CVodes and IDAS use. Also automatically formulated will be the forward and adjoint sensitivity equations.
4.4.1. Creating integrators¶
Integrators are created using CasADi’s integrator
function. Different integrators schemes and interfaces are implemented as plugins, essentially shared libraries that are loaded at runtime.
Consider for example the DAE:
An integrator, using the “idas” plugin, can be created using the syntax:
x = SX.sym('x'); z = SX.sym('z'); p = SX.sym('p')
dae = {'x':x, 'z':z, 'p':p, 'ode':z+p, 'alg':z*cos(z)x}
F = integrator('F', 'idas', dae)
print(F)
F:(x0,z0,p,u[0],adj_xf[],adj_zf[],adj_qf[])>(xf,zf,qf[0],adj_x0[],adj_z0[],adj_p[],adj_u[]) IdasInterface
x = SX.sym('x'); z = SX.sym('z'); p = SX.sym('p');
dae = struct('x',x,'z',z,'p',p,'ode',z+p,'alg',z*cos(z)x);
F = integrator('F', 'idas', dae);
disp(F)
F:(x0,z0,p,u[0],adj_xf[],adj_zf[],adj_qf[])>(xf,zf,qf[0],adj_x0[],adj_z0[],adj_p[],adj_u[]) IdasInterface
This will result in an integration from \(t_0=0\) until \(t_f=1\), i.e. a single output time. We can evaluate the function object using the initial condition \(x(0)=0\), parameter \(p=0.1\) and the guess for the algebraic variable at the initial time \(z(0)=0\) as follows:
r = F(x0=0, z0=0, p=0.1)
print(r['xf'])
0.1724
r = F('x0',0,'z0',0,'p',0.1);
disp(r.xf)
0.1724
Note that the time horizon is always fixed. It can be changed to its default values \(t_0=0\) and \(t_f=1\) by adding two additional argument to the constructor, after the DAE. \(t_f\) can either be a single value or a vector of values. It may include the initial time.
4.4.2. Sensitivity analysis¶
From a usage point of view, an integrator behaves just like the function objects created from expressions earlier in the chapter. You can use member functions in the Function class to generate new function objects corresponding to directional derivatives (forward or reverse mode) or complete Jacobians. Then evaluate these function objects numerically to obtain sensitivity information. The documented example “sensitivity_analysis” (available in CasADi’s example collection for Python, MATLAB and C++) demonstrate how CasADi can be used to calculate first and second order derivative information (forwardoverforward, forwardoveradjoint, adjointoveradjoint) for a simple DAE.
4.5. Nonlinear programming¶
Note
This section assumes familiarity with much of the material that comes above. There is also a higherlevel interface in Chapter 9. That interface can be learned standalone.
The NLP solvers distributed with or interfaced to CasADi solves parametric NLPs of the following form:
where \(x \in \mathbb{R}^{nx}\) is the decision variable and \(p \in \mathbb{R}^{np}\) is a known parameter vector.
An NLP solver in CasADi is a function that takes the parameter value (p
), the bounds (lbx
, ubx
, lbg
, ubg
) and a guess for the primaldual solution (x0
, lam_x0
, lam_g0
) and returns the optimal solution. Unlike integrator objects, NLP solver functions are currently not differentiable functions in CasADi.
There are several NLP solvers interfaced with CasADi. The most popular one is IPOPT, an opensource primaldual interior point method which is included in CasADi installations. Others, that require the installation of thirdparty software, include SNOPT, WORHP and KNITRO. Whatever the NLP solver used, the interface will automatically generate the information that it needs to solve the NLP, which may be solver and option dependent. Typically an NLP solver will need a function that gives the Jacobian of the constraint function and a Hessian of the Lagrangian function (\(L(x,\lambda) = f(x) + \lambda^{\text{T}} \, g(x))\) with respect to \(x\).
4.5.1. Creating NLP solvers¶
NLP solvers are created using CasADi’s nlpsol
function. Different solvers and interfaces are implemented as plugins.
Consider the following form of the socalled Rosenbrock problem:
A solver for this problem, using the “ipopt” plugin, can be created using the syntax:
x = SX.sym('x'); y = SX.sym('y'); z = SX.sym('z')
nlp = {'x':vertcat(x,y,z), 'f':x**2+100*z**2, 'g':z+(1x)**2y}
S = nlpsol('S', 'ipopt', nlp)
print(S)
S:(x0[3],p[],lbx[3],ubx[3],lbg,ubg,lam_x0[3],lam_g0)>(x[3],f,g,lam_x[3],lam_g,lam_p[]) IpoptInterface
x = SX.sym('x'); y = SX.sym('y'); z = SX.sym('z');
nlp = struct('x',[x;y;z], 'f',x^2+100*z^2, 'g',z+(1x)^2y);
S = nlpsol('S', 'ipopt', nlp);
disp(S)
S:(x0[3],p[],lbx[3],ubx[3],lbg,ubg,lam_x0[3],lam_g0)>(x[3],f,g,lam_x[3],lam_g,lam_p[]) IpoptInterface
Once the solver has been created, we can solve the NLP, using [2.5,3.0,0.75]
as an initial guess, by evaluating the
function S
:
r = S(x0=[2.5,3.0,0.75],\
lbg=0, ubg=0)
x_opt = r['x']
print('x_opt: ', x_opt)
This is Ipopt version 3.14.11, running with linear solver MUMPS 5.4.1.
Number of nonzeros in equality constraint Jacobian...: 3
Number of nonzeros in inequality constraint Jacobian.: 0
Number of nonzeros in Lagrangian Hessian.............: 2
Total number of variables............................: 3
variables with only lower bounds: 0
variables with lower and upper bounds: 0
variables with only upper bounds: 0
Total number of equality constraints.................: 1
Total number of inequality constraints...............: 0
inequality constraints with only lower bounds: 0
inequality constraints with lower and upper bounds: 0
inequality constraints with only upper bounds: 0
iter objective inf_pr inf_du lg(mu) d lg(rg) alpha_du alpha_pr ls
0 6.2500000e+01 0.00e+00 9.00e+01 1.0 0.00e+00  0.00e+00 0.00e+00 0
1 1.8457621e+01 1.48e02 4.10e+01 1.0 4.10e01 2.0 1.00e+00 1.00e+00f 1
2 7.8031530e+00 3.84e03 8.76e+00 1.0 2.63e01 1.5 1.00e+00 1.00e+00f 1
3 7.1678278e+00 9.42e05 1.04e+00 1.0 9.32e02 1.0 1.00e+00 1.00e+00f 1
4 6.7419924e+00 6.18e03 9.95e01 1.0 2.69e01 0.6 1.00e+00 1.00e+00f 1
5 5.4363330e+00 7.03e02 1.04e+00 1.7 8.40e01 0.1 1.00e+00 1.00e+00f 1
6 1.2144815e+00 1.52e+00 1.32e+00 1.7 3.21e+00 0.4 1.00e+00 1.00e+00f 1
7 1.0214718e+00 2.51e01 1.17e+01 1.7 1.33e+00 0.9 1.00e+00 1.00e+00h 1
8 3.1864085e01 1.04e03 7.53e01 1.7 3.58e01  1.00e+00 1.00e+00f 1
9 3.7092062e66 3.19e01 2.57e32 1.7 5.64e01  1.00e+00 1.00e+00f 1
iter objective inf_pr inf_du lg(mu) d lg(rg) alpha_du alpha_pr ls
10 0.0000000e+00 0.00e+00 0.00e+00 1.7 3.19e01  1.00e+00 1.00e+00h 1
Number of Iterations....: 10
(scaled) (unscaled)
Objective...............: 0.0000000000000000e+00 0.0000000000000000e+00
Dual infeasibility......: 0.0000000000000000e+00 0.0000000000000000e+00
Constraint violation....: 0.0000000000000000e+00 0.0000000000000000e+00
Variable bound violation: 0.0000000000000000e+00 0.0000000000000000e+00
Complementarity.........: 0.0000000000000000e+00 0.0000000000000000e+00
Overall NLP error.......: 0.0000000000000000e+00 0.0000000000000000e+00
Number of objective function evaluations = 11
Number of objective gradient evaluations = 11
Number of equality constraint evaluations = 11
Number of inequality constraint evaluations = 0
Number of equality constraint Jacobian evaluations = 11
Number of inequality constraint Jacobian evaluations = 0
Number of Lagrangian Hessian evaluations = 10
Total seconds in IPOPT = 0.006
EXIT: Optimal Solution Found.
S : t_proc (avg) t_wall (avg) n_eval
nlp_f  27.00us ( 2.45us) 14.30us ( 1.30us) 11
nlp_g  76.00us ( 6.91us) 30.80us ( 2.80us) 11
nlp_grad_f  45.00us ( 3.75us) 20.50us ( 1.71us) 12
nlp_hess_l  33.00us ( 3.30us) 15.50us ( 1.55us) 10
nlp_jac_g  34.00us ( 2.83us) 16.60us ( 1.38us) 12
total  13.21ms ( 13.21ms) 6.61ms ( 6.61ms) 1
x_opt: [0, 1, 0]
r = S('x0',[2.5,3.0,0.75],...
'lbg',0,'ubg',0);
x_opt = r.x;
disp(x_opt)
This is Ipopt version 3.14.11, running with linear solver MUMPS 5.4.1.
Number of nonzeros in equality constraint Jacobian...: 3
Number of nonzeros in inequality constraint Jacobian.: 0
Number of nonzeros in Lagrangian Hessian.............: 2
Total number of variables............................: 3
variables with only lower bounds: 0
variables with lower and upper bounds: 0
variables with only upper bounds: 0
Total number of equality constraints.................: 1
Total number of inequality constraints...............: 0
inequality constraints with only lower bounds: 0
inequality constraints with lower and upper bounds: 0
inequality constraints with only upper bounds: 0
iter objective inf_pr inf_du lg(mu) d lg(rg) alpha_du alpha_pr ls
0 6.2500000e+01 0.00e+00 9.00e+01 1.0 0.00e+00  0.00e+00 0.00e+00 0
1 1.8457621e+01 1.48e02 4.10e+01 1.0 4.10e01 2.0 1.00e+00 1.00e+00f 1
2 7.8031530e+00 3.84e03 8.76e+00 1.0 2.63e01 1.5 1.00e+00 1.00e+00f 1
3 7.1678278e+00 9.42e05 1.04e+00 1.0 9.32e02 1.0 1.00e+00 1.00e+00f 1
4 6.7419924e+00 6.18e03 9.95e01 1.0 2.69e01 0.6 1.00e+00 1.00e+00f 1
5 5.4363330e+00 7.03e02 1.04e+00 1.7 8.40e01 0.1 1.00e+00 1.00e+00f 1
6 1.2144815e+00 1.52e+00 1.32e+00 1.7 3.21e+00 0.4 1.00e+00 1.00e+00f 1
7 1.0214718e+00 2.51e01 1.17e+01 1.7 1.33e+00 0.9 1.00e+00 1.00e+00h 1
8 3.1864085e01 1.04e03 7.53e01 1.7 3.58e01  1.00e+00 1.00e+00f 1
9 3.7092062e66 3.19e01 2.57e32 1.7 5.64e01  1.00e+00 1.00e+00f 1
iter objective inf_pr inf_du lg(mu) d lg(rg) alpha_du alpha_pr ls
10 0.0000000e+00 0.00e+00 0.00e+00 1.7 3.19e01  1.00e+00 1.00e+00h 1
Number of Iterations....: 10
(scaled) (unscaled)
Objective...............: 0.0000000000000000e+00 0.0000000000000000e+00
Dual infeasibility......: 0.0000000000000000e+00 0.0000000000000000e+00
Constraint violation....: 0.0000000000000000e+00 0.0000000000000000e+00
Variable bound violation: 0.0000000000000000e+00 0.0000000000000000e+00
Complementarity.........: 0.0000000000000000e+00 0.0000000000000000e+00
Overall NLP error.......: 0.0000000000000000e+00 0.0000000000000000e+00
Number of objective function evaluations = 11
Number of objective gradient evaluations = 11
Number of equality constraint evaluations = 11
Number of inequality constraint evaluations = 0
Number of equality constraint Jacobian evaluations = 11
Number of inequality constraint Jacobian evaluations = 0
Number of Lagrangian Hessian evaluations = 10
Total seconds in IPOPT = 0.007
EXIT: Optimal Solution Found.
S : t_proc (avg) t_wall (avg) n_eval
nlp_f  28.00us ( 2.55us) 14.80us ( 1.35us) 11
nlp_g  84.00us ( 7.64us) 34.60us ( 3.15us) 11
nlp_grad_f  48.00us ( 4.00us) 23.00us ( 1.92us) 12
nlp_hess_l  38.00us ( 3.80us) 17.70us ( 1.77us) 10
nlp_jac_g  36.00us ( 3.00us) 18.10us ( 1.51us) 12
total  15.20ms ( 15.20ms) 7.73ms ( 7.73ms) 1
[0, 1, 0]
4.6. Quadratic programming¶
CasADi provides interfaces to solve quadratic programs (QPs). Supported solvers are the opensource solvers qpOASES (distributed with CasADi), OOQP, OSQP and PROXQP as well as the commercial solvers CPLEX and GUROBI.
There are two different ways to solve QPs in CasADi, using a highlevel interface and a lowlevel interface. They are described in the following.
4.6.1. Highlevel interface¶
The highlevel interface for quadratic programming mirrors that of nonlinear programming, i.e. expects a problem of the form (4.5.1), with the restriction that objective function \(f(x,p)\) must be a convex quadratic function in \(x\) and the constraint function \(g(x,p)\) must be linear in \(x\). If the functions are not quadratic and linear, respectively, the solution is done at the current linearization point, given by the “initial guess” for \(x\).
If the objective function is not convex, the solver may or may not fail to find a solution or the solution may not be unique.
To illustrate the syntax, we consider the following convex QP:
To solve this problem with the highlevel interface, we simply replace nlpsol
with qpsol
and use a QP solver plugin such as the with CasADi distributed qpOASES:
x = SX.sym('x'); y = SX.sym('y')
qp = {'x':vertcat(x,y), 'f':x**2+y**2, 'g':x+y10}
S = qpsol('S', 'qpoases', qp)
print(S)
S:(x0[2],p[],lbx[2],ubx[2],lbg,ubg,lam_x0[2],lam_g0)>(x[2],f,g,lam_x[2],lam_g,lam_p[]) MXFunction
x = SX.sym('x'); y = SX.sym('y');
qp = struct('x',[x;y], 'f',x^2+y^2, 'g',x+y10);
S = qpsol('S', 'qpoases', qp);
disp(S)
S:(x0[2],p[],lbx[2],ubx[2],lbg,ubg,lam_x0[2],lam_g0)>(x[2],f,g,lam_x[2],lam_g,lam_p[]) MXFunction
The created solver object S
will have the same input and output signature as the solver objects
created with nlpsol
. Since the solution is unique, it is less important to provide an initial guess:
r = S(lbg=0)
x_opt = r['x']
print('x_opt: ', x_opt)
#################### qpOASES  QP NO. 1 #####################
Iter  StepLength  Info  nFX  nAC
++++
0  1.866661e07  ADD CON 0  1  1
1  8.333622e10  REM BND 1  0  1
2  1.000000e+00  QP SOLVED  0  1
x_opt: [5, 5]
r = S('lbg',0);
x_opt = r.x;
disp(x_opt)
#################### qpOASES  QP NO. 1 #####################
Iter  StepLength  Info  nFX  nAC
++++
0  1.866661e07  ADD CON 0  1  1
1  8.333622e10  REM BND 1  0  1
2  1.000000e+00  QP SOLVED  0  1
[5, 5]
4.6.2. Lowlevel interface¶
The lowlevel interface, on the other hand, solves QPs of the following form:
Encoding problem (4.6.1) in this form, omitting bounds that are infinite, is straightforward:
H = 2*DM.eye(2)
A = DM.ones(1,2)
g = DM.zeros(2)
lba = 10.
H = 2*DM.eye(2);
A = DM.ones(1,2);
g = DM.zeros(2);
lba = 10;
To create a solver instance, instead of passing symbolic expressions for the QP, we now pass the sparsity patterns of the matrices \(H\) and \(A\).
Since we used CasADi’s DM
type above, we can simply query the sparsity patterns:
qp = {}
qp['h'] = H.sparsity()
qp['a'] = A.sparsity()
S = conic('S','qpoases',qp)
print(S)
S:(h[2x2,2nz],g[2],a[1x2],lba,uba,lbx[2],ubx[2],x0[2],lam_x0[2],lam_a0,q[],p[])>(x[2],cost,lam_a,lam_x[2]) QpoasesInterface
qp = struct;
qp.h = H.sparsity();
qp.a = A.sparsity();
S = conic('S','qpoases',qp);
disp(S)
S:(h[2x2,2nz],g[2],a[1x2],lba,uba,lbx[2],ubx[2],x0[2],lam_x0[2],lam_a0,q[],p[])>(x[2],cost,lam_a,lam_x[2]) QpoasesInterface
The returned Function
instance will have a different input/output signature compared to the highlevel interface, one that includes the matrices \(H\) and \(A\):
r = S(h=H, g=g, \
a=A, lba=lba)
x_opt = r['x']
print('x_opt: ', x_opt)
#################### qpOASES  QP NO. 1 #####################
Iter  StepLength  Info  nFX  nAC
++++
0  1.866661e07  ADD CON 0  1  1
1  8.333622e10  REM BND 1  0  1
2  1.000000e+00  QP SOLVED  0  1
x_opt: [5, 5]
r = S('h', H, 'g', g,...
'a', A, 'lba', lba);
x_opt = r.x;
disp(x_opt)
#################### qpOASES  QP NO. 1 #####################
Iter  StepLength  Info  nFX  nAC
++++
0  1.866661e07  ADD CON 0  1  1
1  8.333622e10  REM BND 1  0  1
2  1.000000e+00  QP SOLVED  0  1
[5, 5]
4.7. Forloop equivalents¶
When modeling using expression graphs in CasADi, it is a common pattern to use of forloop constructs of the host language (C++/Python/Matlab).
The graph size will grow linearly with the loop size \(n\), and so will the construction time of the expression graph and the initialization time of functions using that expression.
We offer some special constructs that improve on this complexity.
4.7.1. Map¶
Suppose you are interested in computing a function \(f : \mathbb{R}^{n} \rightarrow \mathbb{R}^{m}\) repeatedly on all columns of a matrix \(X \in \mathbb{R}^{n \times N}\), and aggregating all results in a result matrix \(Y \in \mathbb{R}^{m \times N}\):
N = 4
X = MX.sym("X",1,N)
print(f)
ys = []
for i in range(N):
ys.append(f(X[:,i]))
Y = hcat(ys)
F = Function('F',[X],[Y])
print(F)
f:(i0)>(o0) SXFunction
F:(i0[1x4])>(o0[1x4]) MXFunction
N = 4;
X = MX.sym('X',1,N);
disp(f)
ys = {};
for i=1:N
ys{end+1} = f(X(:,i));
end
Y = [ys{:}];
F = Function('F',{X},{Y});
disp(F)
f:(i0)>(o0) SXFunction
F:(i0[1x4])>(o0[1x4]) MXFunction
The aggregate function \(F : \mathbb{R}^{n \times N} \rightarrow \mathbb{R}^{m \times N}\) can be obtained with the map
construct:
F = f.map(N)
print(F)
map4_f:(i0[1x4])>(o0[1x4]) Map
F = f.map(N);
disp(F)
map4_f:(i0[1x4])>(o0[1x4]) Map
CasADi can be instructed to parallelize when \(F\) gets evaluated. In the following example, we dedicate 2 threads for the map
task.
F = f.map(N,"thread",2)
print(F)
threadmap2_map2_f:(i0[1x4])>(o0[1x4]) ThreadMap
F = f.map(N,'thread',2);
disp(F)
threadmap2_map2_f:(i0[1x4])>(o0[1x4]) ThreadMap
The map
operation supports primitive functions \(f\) with multiple inputs/outputs which can also be matrices. Aggregation will always happen horizontally.
The map
operation exhibits constant graph size and initialization time.
4.7.2. Fold¶
In case each forloop iteration depends on the result from the previous iteration, the fold
construct applies. In the following, the x
variable acts as an accumulater that is initialized by \(x_0 \in \mathbb{R}^{n}\):
x = x0
for i in range(N):
x = f(x)
F = Function('F',[x0],[x])
print(F)
F:(i0)>(o0) MXFunction
x = x0;
for i=1:N
x = f(x);
end
F = Function('F',{x0},{x});
disp(F)
F:(i0)>(o0) MXFunction
For a given function \(f : \mathbb{R}^{n} \rightarrow \mathbb{R}^{n}\), the result function \(F : \mathbb{R}^{n} \rightarrow \mathbb{R}^{n}\) can be obtained with the fold
construct:
F = f.fold(N)
print(F)
fold_f:(i0)>(o0) MXFunction
F = f.fold(N);
disp(F)
fold_f:(i0)>(o0) MXFunction
In case intermediate accumulator values are desired as output (\(\mathbb{R}^{n} \rightarrow \mathbb{R}^{n \times N}\)), use mapaccum
instead of fold
.
The fold
/mapaccum
operation supports primitive functions \(f\) with multiple inputs/outputs which can also be matrices.
The first input and output are used for accumulating, while the remainder inputs are read columnwise over the iterations.
The map
/mapaccum
operation exhibits a graph size and initialization time that scales logarithmically with \(n\).
4.7.3. Conditional evaluation¶
It is possible to include conditional evaluation of expressions in CasADi expression graphs by constucting conditional
function instances. This function takes a number of existing Function
instances, \(f_1\), \(f_2\), \(f_n\) as well as a “default” function \(f_{def}\). All these functions must have the same input and output signatures, i.e. the same number of inputs and outputs with the same dimensions:
x = SX.sym("x")
f0 = Function("f0",[x],[sin(x)])
f1 = Function("f1",[x],[cos(x)])
f2 = Function("f2",[x],[tan(x)])
f_cond = Function.conditional('f_cond', [f0, f1], f2)
print(f_cond)
f_cond:(i0,i1)>(o0) Switch
x = SX.sym('x');
f0 = Function('f0',{x},{sin(x)});
f1 = Function('f1',{x},{cos(x)});
f2 = Function('f2',{x},{tan(x)});
f_cond = Function.conditional('f_cond', {f0, f1}, f2);
disp(f_cond);
f_cond:(i0,i1)>(o0) Switch
The result is a new function instance with the same input/output signature, but with one additional input corresponding to an index. Its evaluation corresponds to:
A function above can be missing (i.e. be a null pointer Function()
), in which case all outputs will be evaluated to NaN. Note that the evaluation is “shortcircuiting”, i.e. only the relevant function is evaluated. This also applies to any derivative calculation.
A common special case is when there is only a single case in addition to the default case. This is equivalent to an ifthenelse statement, which can be written with the shorthand:
x = SX.sym("x")
f_true = Function("f_true",[x],[cos(x)])
f_false = Function("f_false",[x],[sin(x)])
f_cond = Function.if_else('f_cond', f_true, f_false)
print(f_cond)
f_cond:(i0,i1)>(o0) Switch
x = SX.sym('x');
f_true = Function('f_true',{x},{cos(x)});
f_false = Function('f_false',{x},{sin(x)});
f_cond = Function.if_else('f_cond', f_true, f_false);
disp(f_cond);
f_cond:(i0,i1)>(o0) Switch
Note that conditional expressions such as these can result in nonsmooth expressions that may not converge if used if used in a gradientbased optimization algorithm.
Footnotes
5. Generating Ccode¶
The numerical evaluation of function objects in CasADi normally takes place in virtual machines, implemented as part of CasADi’s symbolic framework. But CasADi also supports the generation of selfcontained Ccode for a large subset of function objects.
Ccode generation is interesting for a number of reasons:
Speeding up the evaluation time. As a rule of thumb, the numerical evaluation of autogenerated code, compiled with code optimization flags, can be between 4 and 10 times faster than the same code executed in CasADi’s virtual machines.
Allowing code to be compiled on a system where CasADi is not installed, such as an embedded system. All that is needed to compile the generated code is a C compiler.
Debugging and profiling functions. The generated code is essentially a mirror of the evaluation that takes place in the virtual machines and if a particular operation is slow, this is likely to show up when analyzing the generated code with a profiling tool such as
gprof
. By looking at the code, it is also possible to detect what is potentially done in a suboptimal way. If the code is very long and takes a long time to compile, it is an indication that some functions need to be broken up in smaller, but nested functions.
5.1. Syntax for generating code¶
Generated C code can be as simple as calling the generate
member function for a Function
instance.
x = MX.sym('x',2)
y = MX.sym('y')
f = Function('f',[x,y],\
[x,sin(y)*x],\
['x','y'],['r','q'])
f.generate('gen.c')
print(open('gen.c','r').read())
/* This file was automatically generated by CasADi 3.6.3.
* It consists of:
* 1) content generated by CasADi runtime: not copyrighted
* 2) template code copied from CasADi source: permissively licensed (MIT0)
* 3) user code: owned by the user
*
*/
#ifdef __cplusplus
extern "C" {
#endif
/* How to prefix internal symbols */
#ifdef CASADI_CODEGEN_PREFIX
#define CASADI_NAMESPACE_CONCAT(NS, ID) _CASADI_NAMESPACE_CONCAT(NS, ID)
#define _CASADI_NAMESPACE_CONCAT(NS, ID) NS ## ID
#define CASADI_PREFIX(ID) CASADI_NAMESPACE_CONCAT(CODEGEN_PREFIX, ID)
#else
#define CASADI_PREFIX(ID) gen_ ## ID
#endif
#include <math.h>
#ifndef casadi_real
#define casadi_real double
#endif
#ifndef casadi_int
#define casadi_int long long int
#endif
/* Add prefix to internal symbols */
#define casadi_copy CASADI_PREFIX(copy)
#define casadi_f0 CASADI_PREFIX(f0)
#define casadi_s0 CASADI_PREFIX(s0)
#define casadi_s1 CASADI_PREFIX(s1)
/* Symbol visibility in DLLs */
#ifndef CASADI_SYMBOL_EXPORT
#if defined(_WIN32)  defined(__WIN32__)  defined(__CYGWIN__)
#if defined(STATIC_LINKED)
#define CASADI_SYMBOL_EXPORT
#else
#define CASADI_SYMBOL_EXPORT __declspec(dllexport)
#endif
#elif defined(__GNUC__) && defined(GCC_HASCLASSVISIBILITY)
#define CASADI_SYMBOL_EXPORT __attribute__ ((visibility ("default")))
#else
#define CASADI_SYMBOL_EXPORT
#endif
#endif
void casadi_copy(const casadi_real* x, casadi_int n, casadi_real* y) {
casadi_int i;
if (y) {
if (x) {
for (i=0; i<n; ++i) *y++ = *x++;
} else {
for (i=0; i<n; ++i) *y++ = 0.;
}
}
}
static const casadi_int casadi_s0[6] = {2, 1, 0, 2, 0, 1};
static const casadi_int casadi_s1[5] = {1, 1, 0, 1, 0};
/* f:(x[2],y)>(r[2],q[2]) */
static int casadi_f0(const casadi_real** arg, casadi_real** res, casadi_int* iw, casadi_real* w, int mem) {
casadi_int i;
casadi_real *rr;
const casadi_real *cs;
casadi_real *w0=w+0, w1;
/* #0: @0 = input[0][0] */
casadi_copy(arg[0], 2, w0);
/* #1: output[0][0] = @0 */
casadi_copy(w0, 2, res[0]);
/* #2: @1 = input[1][0] */
w1 = arg[1] ? arg[1][0] : 0;
/* #3: @1 = sin(@1) */
w1 = sin( w1 );
/* #4: @0 = (@1*@0) */
for (i=0, rr=w0, cs=w0; i<2; ++i) (*rr++) = (w1*(*cs++));
/* #5: output[1][0] = @0 */
casadi_copy(w0, 2, res[1]);
return 0;
}
CASADI_SYMBOL_EXPORT int f(const casadi_real** arg, casadi_real** res, casadi_int* iw, casadi_real* w, int mem){
return casadi_f0(arg, res, iw, w, mem);
}
CASADI_SYMBOL_EXPORT int f_alloc_mem(void) {
return 0;
}
CASADI_SYMBOL_EXPORT int f_init_mem(int mem) {
return 0;
}
CASADI_SYMBOL_EXPORT void f_free_mem(int mem) {
}
CASADI_SYMBOL_EXPORT int f_checkout(void) {
return 0;
}
CASADI_SYMBOL_EXPORT void f_release(int mem) {
}
CASADI_SYMBOL_EXPORT void f_incref(void) {
}
CASADI_SYMBOL_EXPORT void f_decref(void) {
}
CASADI_SYMBOL_EXPORT casadi_int f_n_in(void) { return 2;}
CASADI_SYMBOL_EXPORT casadi_int f_n_out(void) { return 2;}
CASADI_SYMBOL_EXPORT casadi_real f_default_in(casadi_int i) {
switch (i) {
default: return 0;
}
}
CASADI_SYMBOL_EXPORT const char* f_name_in(casadi_int i) {
switch (i) {
case 0: return "x";
case 1: return "y";
default: return 0;
}
}
CASADI_SYMBOL_EXPORT const char* f_name_out(casadi_int i) {
switch (i) {
case 0: return "r";
case 1: return "q";
default: return 0;
}
}
CASADI_SYMBOL_EXPORT const casadi_int* f_sparsity_in(casadi_int i) {
switch (i) {
case 0: return casadi_s0;
case 1: return casadi_s1;
default: return 0;
}
}
CASADI_SYMBOL_EXPORT const casadi_int* f_sparsity_out(casadi_int i) {
switch (i) {
case 0: return casadi_s0;
case 1: return casadi_s0;
default: return 0;
}
}
CASADI_SYMBOL_EXPORT int f_work(casadi_int *sz_arg, casadi_int* sz_res, casadi_int *sz_iw, casadi_int *sz_w) {
if (sz_arg) *sz_arg = 4;
if (sz_res) *sz_res = 3;
if (sz_iw) *sz_iw = 0;
if (sz_w) *sz_w = 3;
return 0;
}
#ifdef __cplusplus
} /* extern "C" */
#endif
x = MX.sym('x',2);
y = MX.sym('y');
f = Function('f',{x,y},...
{x,sin(y)*x},...
{'x','y'},{'r','q'});
f.generate('gen.c');
type('gen.c')
/* This file was automatically generated by CasADi 3.6.3.
* It consists of:
* 1) content generated by CasADi runtime: not copyrighted
* 2) template code copied from CasADi source: permissively licensed (MIT0)
* 3) user code: owned by the user
*
*/
#ifdef __cplusplus
extern "C" {
#endif
/* How to prefix internal symbols */
#ifdef CASADI_CODEGEN_PREFIX
#define CASADI_NAMESPACE_CONCAT(NS, ID) _CASADI_NAMESPACE_CONCAT(NS, ID)
#define _CASADI_NAMESPACE_CONCAT(NS, ID) NS ## ID
#define CASADI_PREFIX(ID) CASADI_NAMESPACE_CONCAT(CODEGEN_PREFIX, ID)
#else
#define CASADI_PREFIX(ID) gen_ ## ID
#endif
#include <math.h>
#ifndef casadi_real
#define casadi_real double
#endif
#ifndef casadi_int
#define casadi_int long long int
#endif
/* Add prefix to internal symbols */
#define casadi_copy CASADI_PREFIX(copy)
#define casadi_f0 CASADI_PREFIX(f0)
#define casadi_s0 CASADI_PREFIX(s0)
#define casadi_s1 CASADI_PREFIX(s1)
/* Symbol visibility in DLLs */
#ifndef CASADI_SYMBOL_EXPORT
#if defined(_WIN32)  defined(__WIN32__)  defined(__CYGWIN__)
#if defined(STATIC_LINKED)
#define CASADI_SYMBOL_EXPORT
#else
#define CASADI_SYMBOL_EXPORT __declspec(dllexport)
#endif
#elif defined(__GNUC__) && defined(GCC_HASCLASSVISIBILITY)
#define CASADI_SYMBOL_EXPORT __attribute__ ((visibility ("default")))
#else
#define CASADI_SYMBOL_EXPORT
#endif
#endif
void casadi_copy(const casadi_real* x, casadi_int n, casadi_real* y) {
casadi_int i;
if (y) {
if (x) {
for (i=0; i<n; ++i) *y++ = *x++;
} else {
for (i=0; i<n; ++i) *y++ = 0.;
}
}
}
static const casadi_int casadi_s0[6] = {2, 1, 0, 2, 0, 1};
static const casadi_int casadi_s1[5] = {1, 1, 0, 1, 0};
/* f:(x[2],y)>(r[2],q[2]) */
static int casadi_f0(const casadi_real** arg, casadi_real** res, casadi_int* iw, casadi_real* w, int mem) {
casadi_int i;
casadi_real *rr;
const casadi_real *cs;
casadi_real *w0=w+0, w1;
/* #0: @0 = input[0][0] */
casadi_copy(arg[0], 2, w0);
/* #1: output[0][0] = @0 */
casadi_copy(w0, 2, res[0]);
/* #2: @1 = input[1][0] */
w1 = arg[1] ? arg[1][0] : 0;
/* #3: @1 = sin(@1) */
w1 = sin( w1 );
/* #4: @0 = (@1*@0) */
for (i=0, rr=w0, cs=w0; i<2; ++i) (*rr++) = (w1*(*cs++));
/* #5: output[1][0] = @0 */
casadi_copy(w0, 2, res[1]);
return 0;
}
CASADI_SYMBOL_EXPORT int f(const casadi_real** arg, casadi_real** res, casadi_int* iw, casadi_real* w, int mem){
return casadi_f0(arg, res, iw, w, mem);
}
CASADI_SYMBOL_EXPORT int f_alloc_mem(void) {
return 0;
}
CASADI_SYMBOL_EXPORT int f_init_mem(int mem) {
return 0;
}
CASADI_SYMBOL_EXPORT void f_free_mem(int mem) {
}
CASADI_SYMBOL_EXPORT int f_checkout(void) {
return 0;
}
CASADI_SYMBOL_EXPORT void f_release(int mem) {
}
CASADI_SYMBOL_EXPORT void f_incref(void) {
}
CASADI_SYMBOL_EXPORT void f_decref(void) {
}
CASADI_SYMBOL_EXPORT casadi_int f_n_in(void) { return 2;}
CASADI_SYMBOL_EXPORT casadi_int f_n_out(void) { return 2;}
CASADI_SYMBOL_EXPORT casadi_real f_default_in(casadi_int i) {
switch (i) {
default: return 0;
}
}
CASADI_SYMBOL_EXPORT const char* f_name_in(casadi_int i) {
switch (i) {
case 0: return "x";
case 1: return "y";
default: return 0;
}
}
CASADI_SYMBOL_EXPORT const char* f_name_out(casadi_int i) {
switch (i) {
case 0: return "r";
case 1: return "q";
default: return 0;
}
}
CASADI_SYMBOL_EXPORT const casadi_int* f_sparsity_in(casadi_int i) {
switch (i) {
case 0: return casadi_s0;
case 1: return casadi_s1;
default: return 0;
}
}
CASADI_SYMBOL_EXPORT const casadi_int* f_sparsity_out(casadi_int i) {
switch (i) {
case 0: return casadi_s0;
case 1: return casadi_s0;
default: return 0;
}
}
CASADI_SYMBOL_EXPORT int f_work(casadi_int *sz_arg, casadi_int* sz_res, casadi_int *sz_iw, casadi_int *sz_w) {
if (sz_arg) *sz_arg = 4;
if (sz_res) *sz_res = 3;
if (sz_iw) *sz_iw = 0;
if (sz_w) *sz_w = 3;
return 0;
}
#ifdef __cplusplus
} /* extern "C" */
#endif
This will create a C file gen.c
containing the function f
and all its dependencies and required helper functions.
We will return to how this file can be used in Section 5.1 and the structure of the generated code is
described in Section 5.3 below.
You can generate a C file containing multiple CasADi functions by working with CasADi’s CodeGenerator
class:
f = Function('f',[x],[sin(x)])
g = Function('g',[x],[cos(x)])
C = CodeGenerator('gen.c')
C.add(f)
C.add(g)
C.generate()
f = Function('f',{x},{sin(x)});
g = Function('g',{x},{cos(x)});
C = CodeGenerator('gen.c');
C.add(f);
C.add(g);
C.generate();
Both the generate
function and the CodeGenerator
constructor take an optional
options dictionary as an argument, allowing customization of the code generation. Two useful
options are main
, which generates a main entry point, and mex
,
which generates a mexFunction entry point:
f = Function('f',[x],[sin(x)])
opts = dict(main=True, \
mex=True)
f.generate('gen.c',opts)
f = Function('f',{x},{sin(x)});
opts = struct('main', true,...
'mex', true);
f.generate('gen.c',opts);
This enables executing the function from the command line and MATLAB, respectively, as described in Section 5.2 below.
If you plan to link directly against the generated code in some C/C++ application,
a useful option is with_header
, which controls the creation of a header file
containing declarations of the functions with external linkage, i.e. the API of
the generated code, described in Section 5.3 below.
Here is a list of available options for the CodeGenerator
class:
Option 
Default value 
Description 


true 
Include comments in generated code 

false 
Generate an MATLAB/Octave MEX entry point 

false 
Generated code is C++ instead of C 

false 
Generate a command line interface 


Floating point type 


Integer type 

false 
Generate a header file 

false 
Generate a simplified C API 

2 
Number of spaces per indentation level 
5.2. Using the generated code¶
The generated C code can be used in a number of different ways:
The code can be compiled into a dynamically linked library (DLL), from which a
Function
instance can be created using CasADi’sexternal
function. Optionally, the user can rely on CasADi to carry out the compilation justintime.The generated code can be compiled into MEX function and executed from MATLAB.
The generated code can be executed from the command line.
The user can link, statically or dynamically, the generated code to his or her C/C++ application, accessing the C API of the generated code.
The code can be compiled into a dynamically linked library and the user can then manually access the C API using
dlopen
on Linux/OS X orLoadLibrary
on Windows.
This is elaborated in the following.
5.2.1. CasADi’s external
function¶
The external
command allows the user to create a Function
instance
from a dynamically linked library with the entry points described by the
C API described in Section 5.3. Since the autogenerated files are
selfcontained [1], the compilation – on Linux/OSX – can be as easy as issuing:
gcc fPIC shared gen.c o gen.so
from the command line. Or, equivalently using MATLAB’s system
command
or Python’s os.system
command. Assuming gen.c
was created as
described in the previous section, we can then create a Function
f
as follows:
f = external('f', './gen.so')
print(f(3.14))
0.00159265
f = external('f', './gen.so');
disp(f(3.14))
0.00159265
We can also rely on CasADi performing the compilation justintime
using CasADi’s Importer
class. This is a plugin class, which at the
time of writing had two supported plugins, namely 'clang'
, which invokes
the LLVM/Clang compiler framework (distributed with CasADi), and 'shell'
,
which invokes the system compiler via the command line:
C = Importer('gen.c','shell')
f = external('f',C)
print(f(3.14))
0.00159265
C = Importer('gen.c','shell');
f = external('f',C);
disp(f(3.14))
0.00159265
CasADi  20230517 20:14:25 WARNING("Failed to remove ./tmp_casadi_compiler_shellkRq1w4.so") [.../casadi/solvers/shell_compiler.cpp:66]
CasADi  20230517 20:14:25 WARNING("Failed to remove ./tmp_casadi_compiler_shellkRq1w4.o") [.../casadi/solvers/shell_compiler.cpp:67]
We will return to the external
function in Section 6.3.
5.2.2. Calling generated code from MATLAB¶
An alternative way of executing generated code is to compile the code into a
MATLAB MEX function and call from MATLAB. This assumes that the mex
option
was set to “true” during the code generation, cf. Section 5.1.
The generated MEX function takes the function name as its first argument,
followed by the function inputs:
%mex gen.c largeArrayDims % Matlab
mex gen.c DMATLAB_MEX_FILE % Octave
disp(gen('f', 3.14))
1.5927e03
Note that the result of the execution is always a MATLAB sparse matrix by default. Compiler flags DCASASI_MEX_ALWAYS_DENSE
and DCASASI_MEX_ALLOW_DENSE
may be set to influence this behaviour.
5.2.3. Calling generated code from the command line¶
Another option is to execute the generated code from the Linux/OSX command line.
This is possible if the main
option was set to “true” during the code
generation, cf. Section 5.1. This is useful if you e.g. want
to profile the generated with a tool such as gprof
.
When executing the generated code, the function name is passed as a command line argument. The nonzero entries of all the inputs need to be passed via standard input and the function will return the output nonzeros for all the outputs via standard output:
# Command line
echo 3.14 3.14 > gen_in.txt
gcc gen.c o gen
./gen f < gen_in.txt > gen_out.txt
cat gen_out.txt # returns 0.00159265 0.00159265
5.2.4. Linking against generated code from a C/C++ application¶
The generated code is written so that it can be linked with directly from a C/C++
application. If the with_header
option was set to “true” during the
code generation, a header file with declarations of all the exposed entry points
of the file. Using this header file requires an understanding of CasADi’s
codegen API, as described in Section 5.3 below. Symbols that are
not exposed are prefixed with a filespecific prefix, allowing an
application to link against multiple generated functions without risking
symbol conflicts.
5.2.5. Dynamically loading generated code from a C/C++ application¶
A variant of above is to compile the generated code into a shared library,
but directly accessing the exposed symbols rather than relying on CasADi’s
external
function. This also requires an understanding of the structure
of the generated code.
In CasADi’s example collection, codegen_usage.cpp
demonstrates how this
can be done.
5.3. API of the generated code¶
The API of the generated code consists of a number of functions with external
linkage. In addition to the actual execution, there are functions for memory
management as well as meta information about the inputs and outputs.
These functions are described in the following. Below, assume that the name of
function we want to access is fname
. To see what these functions actually
look like in code and when they are called, we refer to the
codegen_usage.cpp
example.
5.3.1. Reference counting¶
void fname_incref(void);
void fname_decref(void);
A generated function may need to e.g. read in some data or initialize some data structures before first call. This is typically not needed for functions generated from CasADi expressions, but may be required e.g. when the generated code contains calls to external functions. Similarly, memory might need to be deallocated after usage.
To keep track of the ownership, the generated code contains two functions for
increasing and decreasing a reference counter.
They are named fname_incref
and fname_decref
, respectively. These
functions have no input argument and return void.
Typically, some initialization may take place upon the first call to
fname_incref
and subsequent calls will only increase some internal counter.
The fname_decref
, on the other hand, decreases the internal counter and
when the counter hits zero, a deallocation – if any – takes place.
5.3.2. Number of inputs and outputs¶
casadi_int fname_n_in(void);
casadi_int fname_n_out(void);
The number of function inputs and outputs can be obtained by calling the
fname_n_in
and fname_n_out
functions, respectively. These functions
take no inputs and return the number of input or outputs (casadi_int
is an alias for long long int
).
5.3.3. Names of inputs and outputs¶
const char* fname_name_in(casadi_int ind);
const char* fname_name_out(casadi_int ind);
The functions fname_name_in
and fname_name_out
return the name
of a particular input or output. They take the index of the input or output,
starting with index 0, and return a const char*
with the name as a
nullterminated C string. Upon failure, these functions will return a null
pointer.
5.3.4. Sparsity patterns of inputs and outputs¶
const casadi_int* fname_sparsity_in(casadi_int ind);
const casadi_int* fname_sparsity_out(casadi_int ind);
The sparsity pattern for a given input or output is obtained by calling
fname_sparsity_in
and fname_sparsity_out
, respectively.
These functions take the input or output index and return a pointer to a field
of constant integers (const casadi_int*
). This is a compact representation
of the compressed column storage (CCS) format that CasADi uses,
cf. Section 3.5.
The integer field pointed to is structured as follows:
The first two entries are the number of rows and columns, respectively. In the following referred to as
nrow
andncol
.If the third entry is
1
, the pattern is dense and any remaining entries are discarded.If the third entry is
0
, that entry plus subsequentncol
entries form the nonzero offsets for each column,colind
in the following. E.g. column \(i\) will consist of the nonzero indices ranging fromcolind[i]
tocolind[i+1]
. The last entry,colind[ncol]
, will be equal to the number of nonzeros,nnz
.Finally, if the sparsity pattern is not dense, i.e. if
nnz
\(\ne\)nrow
*ncol
, then the lastnnz
entries will contain the row indices.
Upon failure, these functions will return a null pointer.
5.3.5. Memory objects¶
A function may contain some mutable memory, e.g. for caching the latest factorization or keeping track of evaluation statistics. When multiple functions need to call the same function without conflicting, they each need to work with a different memory object. This is especially important for evaluation in parallel on a shared memory architecture, in which case each thread should access a different memory object.
void* fname_alloc_mem(void);
Allocates a memory object which will be passed to the numerical evaluation.
int fname_init_mem(void* mem);
(Re)initializes a memory object. Returns 0 upon successful return;
int fname_free_mem(void* mem);
Frees a memory object. Returns 0 upon successful return;
5.3.6. Work vectors¶
int fname_work(casadi_int* sz_arg, casadi_int* sz_res, casadi_int* sz_iw, casadi_int* sz_w);
To allow the evaluation to be performed efficiently with a small memory
footprint, the user is expected to pass four work arrays. The function
fname_work
returns the length of these arrays, which have entries
of type const double*
, double*
, casadi_int
and double
,
respectively.
The return value of the function is nonzero upon failure.
5.3.7. Numerical evaluation¶
int fname(const double** arg, double** res,
casadi_int* iw, double* w, void* mem);
Finally, the function fname
, performs the actual evaluation. It takes
as input arguments the four work vectors and a memory object created using fname_alloc_mem
(or NULL if absent).
The length of the work vectors must be at least the lengths provided
by the fname_work
command and the index of the memory object must be strictly
smaller than the value returned by fname_n_mem
.
The nonzeros of the function inputs are pointed to by the
first entries of the arg
work vector and are unchanged by the evaluation.
Similarly, the output nonzeros are pointed to by the first entries of the
res
work vector and are also unchanged (i.e. the pointers are unchanged,
not the actual values).
The return value of the function is nonzero upon failure.
Footnotes
An exception is when code is generated for a function that in turn contains calls to external functions.
6. Userdefined function objects¶
There are situations when rewriting userfunctions using CasADi symbolics is not possible or practical. To tackle this, CasADi provides a number of ways to embed a call to a “black box” function defined in the language CasADi is being used from (C++, MATLAB or Python) or in C. That being said, the recommendation is always to try to avoid this when possible, even if it means investing a lot of time reimplementing existing code. Functions defined using CasADi symbolics are almost always more efficient, especially when derivative calculation is involved, since a lot more structure can typically be exploited.
Depending on the circumstances, the user can implement custom Function
objects in a number of different ways, which will be elaborated on in the following sections:
Subclassing
FunctionInternal
: Section 6.1Subclassing
Callback
: Section 6.2Importing a function with
external
: Section 6.3Justintime compile a C language string: Section 6.4
Replace the function call with a lookup table: Section 6.5
6.1. Subclassing FunctionInternal
¶
All function objects presented in Chapter 4 are implemented
in CasADi as C++ classes inheriting from the FunctionInternal
abstract
base class. In principle, a user with familiarity with C++ programming, can
implement a class inheriting from FunctionInternal
,
overloading the virtual methods of this class. The best reference for doing so
is the C++ API documentation, choosing “switch to internal” to expose the internal
API.
Since FunctionInternal
is not considered part of the stable, public API,
we advice against this in general, unless the plan is to contribution to CasADi’s
source.
6.2. Subclassing Callback
¶
The Callback
class provides a public API to FunctionInternal
and inheriting from this class has the same effect as inheriting directly from
FunctionInternal
. Thanks to crosslanguage polymorphism, it
is possible to implement the exposed methods of Callback
from either
Python, MATLAB/Octave or C++.
The derived class consists of the following parts:
A constructor or a static function replacing the constructor
A number of virtual functions, all optional, that can be overloaded in order to get the desired behavior. This includes the number of inputs and outputs using
get_n_in
andget_n_out
, their names usingget_name_in
andget_name_out
and their sparsity patternsget_sparsity_in
andget_sparsity_out
.An optional
init
function called duing the object construction.A function for numerical evaluation.
Optional functions for derivatives. You can choose to work with finite differences (
enable_fd
), supply a full Jacobian (has_jacobian
,get_jacobian
), or choose to supply forward/reverse sensitivities (has_forward
,get_forward
,has_reverse
,get_reverse
).
For a complete list of functions, see the C++ API documentation for Callback
.
Also see the callback.py
example.
The usage from the different languages are described in the following.
6.2.1. Python¶
class MyCallback(Callback):
def __init__(self, name, d, opts={}):
Callback.__init__(self)
self.d = d
self.construct(name, opts)
# Number of inputs and outputs
def get_n_in(self): return 1
def get_n_out(self): return 1
# Initialize the object
def init(self):
print('initializing object')
# Evaluate numerically
def eval(self, arg):
x = arg[0]
f = sin(self.d*x)
return [f]
The implementation should include a constructor, which should begin with a call to
the base class constructor using
Callback.__init__(self)
and end with a call to
initialize object construction using self.construct(name, opts)
.
This function can be used as any builtin CasADi function with the important caveat that when embedded in graphs, the ownership of the class will not be shared between all references. So it is important that the user does not allow the Python class to go out of scope while it is still needed in calculations.
# Use the function
f = MyCallback('f', 0.5)
print(f(2))
x = MX.sym("x")
print(f(x))
initializing object
0.841471
f(x){0}
6.2.2. MATLAB¶
In MATLAB, a custom function class can be defined as follows, in a file
MyCallback.m
:
classdef MyCallback < casadi.Callback
properties
d
end
methods
function self = MyCallback(name, d)
self@casadi.Callback();
self.d = d;
construct(self, name);
end
% Number of inputs and outputs
function v=get_n_in(self)
v=1;
end
function v=get_n_out(self)
v=1;
end
% Initialize the object
function init(self)
disp('initializing object')
end
% Evaluate numerically
function arg = eval(self, arg)
x = arg{1};
f = sin(self.d * x);
arg = {f};
end
end
end
This function can be used as any builtin CasADi function, but as for Python,
the ownership of the class will not be shared between all references.
So the user must not allow a class instance to get deleted while it is still
in use, e.g. by making it persistent
.
% Use the function
f = MyCallback('f', 0.5);
res = f(2);
disp(res)
x = MX.sym('x');
disp(f(x))
6.2.3. C++¶
In C++, the syntax is as follows:
#include "casadi/casadi.hpp"
using namespace casadi;
class MyCallback : public Callback {
// Data members
double d;
public:
// Constructor
MyCallback(const std::string& name, double d,
const Dict& opts=Dict()) : d(d) {
construct(name, opts);
}
// Destructor
~MyCallback() override {}
// Number of inputs and outputs
casadi_int get_n_in() override { return 1;}
casadi_int get_n_out() override { return 1;}
// Initialize the object
void init() override() {
std::cout << "initializing object" << std::endl;
}
// Evaluate numerically
std::vector<DM> eval(const std::vector<DM>& arg) const override {
DM x = arg.at(0);
DM f = sin(d*x);
return {f};
}
};
A class created this way can be used as any other Function
instance,
but with the important difference that the user is responsible to managing
the memory of this class.
int main() {
MyCallback f("f", 0.5);
std::vector<DM> arg = {2};
std::vector<DM> res = f(arg);
std::cout << res << std::endl;
return 0;
}
6.3. Importing a function with external
¶
The basic usage of CasADi’s external
function was demonstrated in
Section 5.2 in the context of using autogenerated code. The
same function can also be used for importing a userdefined function, as long as
it also uses the C API described in Section 5.3.
The following sections expands on this.
6.3.1. Default functions¶
It is usually not necessary to define all the functions defined in
Section 5.3. If fname_incref
and fname_decref
are absent, it is assumed that no memory management is needed. If no
names of inputs and outputs are provided, they will be given default names.
Sparsity patterns are in general assumed to be scalar by default, unless the
function corresponds to a derivative of another function (see below), in which
case they are assumed to be dense and of the correct dimension.
Furthermore, work vectors are assumed not to be needed if fname_work
has
not been implemented.
6.3.2. Meta information as comments¶
If you rely on CasADi’s justintime compiler, you can provide meta information as a comment in the C code instead of implementing the actual callback function.
The structure of such meta information should be as follows:
/*CASADIMETA
:fname_N_IN 1
:fname_N_OUT 2
:fname_NAME_IN[0] x
:fname_NAME_OUT[0] r
:fname_NAME_OUT[1] s
:fname_SPARSITY_IN[0] 2 1 0 2
*/
6.3.3. Derivatives¶
The external function can be made differentiable by providing functions for
calculating derivatives. During derivative calculations, CasADi will look for
symbols in the same file/shared library that follows a certain
naming convention. For example, you can specify a Jacobian for all the
outputs with respect to all inputs for a function named fname
by
implementing a function named jac_fname
. Similary, you can specify
a function for calculating one forward directional derivative by providing a
function named fwd1_fname
, where 1 can be replaced by 2, 4, 8, 16,
32 or 64 for calculating multiple forward directional derivatives at once.
For reverse mode directional derivatives, replace fwd
with adj
.
This is an experimental feature.
6.4. Justintime compile a C language string¶
In the previous section we showed how to specify a C file with functions for numerical evaluation and meta information. As was shown, this file can be justintime compiled by CasADi’s interface to Clang. There exists a shorthand for this approach, where the user simply specifies the source code as a C language string.
body =\
'r[0] = x[0];'+\
'while (r[0]<s[0]) {'+\
' r[0] *= r[0];'+\
'}'
f = Function.jit('f',body,\
['x','s'],['r'],\
{"compiler":"shell"})
print(f)
f:(x,s)>(r) JitFunction
body =[...
'r[0] = x[0];',...
'while (r[0]<s[0]) {',...
' r[0] *= r[0];',...
'}'];
f = Function.jit('f',body,...
{'x','s'},{'r'},...
struct('compiler','shell'));
disp(f)
f:(x,s)>(r) JitFunction
CasADi  20230517 20:14:29 WARNING("Failed to remove jit_tmpryFspM.c") [.../casadi/core/function_internal.cpp:127]
CasADi  20230517 20:14:29 WARNING("Failed to remove ./tmp_casadi_compiler_shellOCpqVk.so") [.../casadi/solvers/shell_compiler.cpp:66]
CasADi  20230517 20:14:29 WARNING("Failed to remove ./tmp_casadi_compiler_shellOCpqVk.o") [.../casadi/solvers/shell_compiler.cpp:67]
These four arguments of Function.jit
are mandatory:
The function name, the C source as a string and the names of inputs and outputs.
In the C source, the input/output names correspond to arrays of type casadi_real_t
containing the nonzero elements of the function inputs and outputs. By default,
all inputs and outputs are scalars (i.e. 1by1 and dense). To specify a different sparsity pattern, provide two additional function arguments containing vectors/lists
of the sparsity patterns:
sp = Sparsity.scalar()
f = Function.jit('f',body,\
['x','s'], ['r'],\
[sp,sp], [sp],\
{"compiler":"shell"})
print(f)
f:(x,s)>(r) JitFunction
sp = Sparsity.scalar();
f = Function.jit('f',body,...
{'x','s'}, {'r'},...
{sp,sp}, {sp},...
struct('compiler','shell'));
disp(f)
f:(x,s)>(r) JitFunction
CasADi  20230517 20:14:36 WARNING("Failed to remove jit_tmp9vC5ah.c") [.../casadi/core/function_internal.cpp:127]
CasADi  20230517 20:14:36 WARNING("Failed to remove ./tmp_casadi_compiler_shellPvpRVJ.so") [.../casadi/solvers/shell_compiler.cpp:66]
CasADi  20230517 20:14:36 WARNING("Failed to remove ./tmp_casadi_compiler_shellPvpRVJ.o") [.../casadi/solvers/shell_compiler.cpp:67]
Both variants accept an optional 5th (or 7th) argument in the form of an options dictionary.
6.5. Using lookuptables¶
Lookuptables can be created using CasADi’s interpolant
function. Different interpolating schemes are implemented as plugins, similar to nlpsol
or integrator
objects. In addition to the identifier name and plugin, the interpolant
function expects a set of grid points with the corresponding numerical values.
The result of an interpolant
call is a CasADi Function object that is differentiable, and can be embedded into CasADi computational graphs by calling with MX arguments. Furthermore, C code generation is fully supported for such graphs.
Currently, two plugins exist for interpolant
: 'linear'
and 'bspline'
. They are intended to behave simiarly to MATLAB/Octave’s interpn
with the method set to 'linear'
or 'spline'
– corresponding to a multilinear interpolation and a (by default cubic) spline interpolation with
notaknot boundary conditions.
In the case of bspline
, coefficients will be sought at construction time that fit the provided data. Alternatively, you may also use the more lowlevel Function.bspline
to supply the coefficients yourself. The default degree of the bspline is 3 in each dimension. You may deviate from this default by passing a degree
option.
We will walk through the syntax of interpolant
for the 1D and 2D versions, but the syntax in fact generalizes to an arbitrary number of dimensions.
6.5.1. 1D lookup tables¶
A 1D spline fit can be done in CasADi/Python as follows, compared with the corresponding method in SciPy:
import casadi as ca
import numpy as np
xgrid = np.linspace(1,6,6)
V = [1,1,2,3,0,2]
lut = ca.interpolant('LUT','bspline',[xgrid],V)
print(lut(2.5))
# Using SciPy
import scipy.interpolate as ip
interp = ip.InterpolatedUnivariateSpline(xgrid, V)
print(interp(2.5))
1.35
1.3500000000000005
In MATLAB/Octave, the corresponding code reads:
xgrid = 1:6;
V = [1 1 2 3 0 2];
lut = casadi.interpolant('LUT','bspline',{xgrid},V);
lut(2.5)
% Using MATLAB/Octave builtin
interp1(xgrid,V,2.5,'spline')
ans =
1.35
ans = 1.3500
Note in particular that the grid
and values
arguments to interpolant
must be numerical in nature.
6.5.2. 2D lookup tables¶
In two dimensions, we get the following in Python, also compared to SciPy for reference:
xgrid = np.linspace(5,5,11)
ygrid = np.linspace(4,4,9)
X,Y = np.meshgrid(xgrid,ygrid,indexing='ij')
R = np.sqrt(5*X**2 + Y**2)+ 1
data = np.sin(R)/R
data_flat = data.ravel(order='F')
lut = ca.interpolant('name','bspline',[xgrid,ygrid],data_flat)
print(lut([0.5,1]))
# Using Scipy
interp = ip.RectBivariateSpline(xgrid, ygrid, data)
print(interp.ev(0.5,1))
0.245507
0.24550661674668917
or, in MATLAB/Octave compared to the builtin functions:
xgrid = 5:1:5;
ygrid = 4:1:4;
[X,Y] = ndgrid(xgrid, ygrid);
R = sqrt(5*X.^2 + Y.^2)+ 1;
V = sin(R)./R;
lut = interpolant('LUT','bspline',{xgrid, ygrid},V(:));
lut([0.5 1])
% Using Matlab builtin
interpn(X,Y,V,0.5,1,'spline')
ans =
0.245507
ans = 0.2455
In particular note how the values
argument had to be flatten to a onedimensional array.
6.6. Derivative calculation using finite differences¶
CasADi 3.3 introduced support for finite difference calculation for all function objects, in particular including external functions defined as outlined in Section 6.2, Section 6.3 or Section 6.4 (for lookup tables, Section 6.5, analytical derivatives are available).
Finite difference derivative are disabled by default, with the exception of Function.jit
, and to enable it, you must set the option 'enable_fd'
to True
/true
:
f = external('f', './gen.so',\
dict(enable_fd=True))
e = jacobian(f(x),x)
D = Function('d',[x],[e])
print(D(0))
1
f = external('f', './gen.so',...
struct('enable_fd',true));
e = jacobian(f(x),x);
D = Function('d',{x},{e});
disp(D(0))
1
cf. Section 5.1.
The 'enable_fd'
options enables CasADi to use finite differences, if
analytical derivatives are not available. To force CasADi to use finite differences,
you can set 'enable_forward'
, 'enable_reverse'
and 'enable_jacobian'
to False
/false
, corresponding to the three types of analytical derivative information that CasADi works with.
The default method is central differences with a step size determined by estimates of roundoff errors and truncation errors of the function. You can change the method by setting the option 'fd_method'
to 'forward'
(corresponding to first order forward differences), 'backward'
(corresponding to first order backward differences) and 'smoothing'
for a secondorder accurate discontinuity avoiding scheme, suitable when derivatives need to be calculated at the edges of a domain. Additional algorithmic options for the finite differences are available by setting 'fd_options'
option.
7. The DaeBuilder
class¶
The DaeBuilder
class in CasADi is an auxiliary class intended to
facilitate the modeling complex dynamical systems for later use with optimal
control algorithms. This class has lower level of abstraction than
a physical modeling language such as Modelica (cf. Section 7.2),
while still being higher level than working directly with CasADi symbolic
expressions. In particular, it can be used to interface physical models
available in the Functional Mockup Interface (FMI) format or be used
as a building block for constructing a domain specific modeling environments.
 There are in principle three different ways to use
DaeBuilder
: One can use the class to stepbystep construct a structured system of differentialalgebraic equations (DAE), which can then be used to interface to simulation or optimization in CasADi. There is also experimental support for exporting the models in the FMI format for use in other tools.
One can use the class to import existing models available in the FMI format. As of CasADi 3.6, we support import of standard FMUs, where the model equations are available only by calls to DLLs. The FMU import support has been tested for challenging FMUs, but is still under development as of this writing.
One can use the class to import existing models in a symbolic XML format. This format was supported in the JModelica.org tool and has experimental support in OpenModelica. This format is not in active development due to the limited availability of tools to generate models in the format.
7.1. Constructing a DaeBuilder
instance¶
Consider the following simple DAE corresponding to a controlled rocket subject to quadratic air friction term and gravity, which loses mass as it uses up fuel:
where the three states correspond to height, velocity and mass, respectively. \(u\) is the thrust of the rocket and \((a,b)\) are parameters.
To construct a DAE formulation for this problem, start with an empty
DaeBuilder
instance and add the input and output expressions stepbystep
as follows.
dae = DaeBuilder('rocket')
# Add input expressions
a = dae.add_p('a')
b = dae.add_p('b')
u = dae.add_u('u')
h = dae.add_x('h')
v = dae.add_x('v')
m = dae.add_x('m')
# Add output expressions
hdot = v
vdot = (ua*v**2)/mg
mdot = b*u**2
dae.set_ode('h', hdot)
dae.set_ode('v', vdot)
dae.set_ode('m', mdot)
# Specify initial conditions
dae.set_start('h', 0)
dae.set_start('v', 0)
dae.set_start('m', 1)
# Add meta information
dae.set_unit('h','m')
dae.set_unit('v','m/s')
dae.set_unit('m','kg')
dae = DaeBuilder('rocket')
% Add input expressions
a = dae.add_p('a');
b = dae.add_p('b');
u = dae.add_u('u');
h = dae.add_x('h');
v = dae.add_x('v');
m = dae.add_x('m');
% Add output expressions
hdot = v;
vdot = (ua*v^2)/mg;
mdot = b*u^2;
dae.set_ode('h', hdot);
dae.set_ode('v', vdot);
dae.set_ode('m', mdot);
% Specify initial conditions
dae.set_start('h', 0);
dae.set_start('v', 0);
dae.set_start('m', 1);
% Add meta information
dae.set_unit('h','m');
dae.set_unit('v','m/s');
dae.set_unit('m','kg');
dae =
nx = 0, nz = 0, nq = 0, ny = 0, np = 0, nc = 0, nd = 0, nw = 0, nu = 0
Other input and output expressions can be added in an analogous way. For a full
list of functions, see the C++ API documentation for DaeBuilder
.
7.2. Symbolic import of OCPs from Modelica¶
Note: JModelica.org is no longer offered by Modelon. Its closedsource successor code, OCT, does retain CasADi interoperability however. Details of how to use OCT to generate CasADi expressions is best described in OCT’s user guide. The text in the following refers to the legacy Modelica interoperability support.
7.2.1. Legacy symbolic import from XML files¶
An alternative to model directly in CasADi, as above, is to use an advanced
physical modeling language such as Modelica to specify the model. For this,
CasADi offers interoperability with the opensource JModelica.org compiler, which
is written specifically with optimal control in mind. Model import from
JModelica.org is possible in two different ways; using the JModelica.org’s
CasadiInterface
or via DaeBuilder
’s
parse_fmi
command.
We recommend the former approach, since it is being actively maintained and refer to JModelica.org’s user guide for details on how to extract CasADi expressions.
In the following, we will outline the legacy approach, using
parse_fmi
.
7.2.2. Legacy import of a modelDescription.xml
file¶
To see how to use the Modelica import, look at thermodynamics_example.py in CasADi’s example collection.
Assuming that the Modelica/Optimica model ModelicaClass.ModelicaModel
is defined in the files file1.mo
and file2.mop
, the Python compile command is:
from pymodelica import compile_jmu
jmu_name=compile_jmu('ModelicaClass.ModelicaModel', \
['file1.mo','file2.mop'],'auto','ipopt',\
{'generate_xml_equations':True, 'generate_fmi_me_xml':False})
This will generate a jmu
file, which is essentially a zip file containing, among other things, the file modelDescription.xml
. This XMLfile contains a symbolic representation of the optimal control problem and can be inspected in a standard XML editor.
from zipfile import ZipFile
sfile = ZipFile(jmu_name','r')
mfile = sfile.extract('modelDescription.xml','.')
Once a modelDescription.xml
file is available, it can be imported
using the parse_fmi
command:
ocp = DaeBuilder()
ocp.parse_fmi('modelDescription.xml')
7.3. Function factory¶
Once a DaeBuilder
has been formulated and possibly reformulated to
a satisfactory form, we can generate CasADi functions corresponding to the
input and output expressions outlined in secdaebuilder_io
.
For example, to create a function for the ODE righthandside for the rocket
model in Section 7.1, simply provide a display
name of the function being created, a list of input expressions
and a list of output expressions:
f = dae.create('f',\
['x','u','p'],['ode'])
f = dae.create('f',...
{'x','u','p'},{'ode'});
Using a naming convention, we can also create Jacobians, e.g. for the ‘ode’ output with respect to ‘x’:
f = dae.create('f',\
['x','u','p'],\
['jac_ode_x'])
f = dae.create('f',...
{'x','u','p'},
{'jac_ode_x'});
Functions with second order information can be extracted by first creating
a named linear combination of the output expressions using add_lc
and then requesting its Hessian:
dae.add_lc('gamma',['ode'])
hes = dae.create('hes',\
['x','u','p','lam_ode'],\
['hes_gamma_x_x'])
dae.add_lc('gamma',{'ode'});
hes = dae.create('hes',...
{'x','u','p','lam_ode'},...
{'hes_gamma_x_x'});
It is also possible to simply extract the symbolic expressions from the
DaeBuilder
instance and manually create CasADi functions.
For example, dae.x
contains all the expressions corresponding to ‘x’,
dae.ode
contains the expressions corresponding to ‘ode’, etc.
Footnotes
FMI development group. Functional Mockup Interface for Model Exchange and CoSimulation. https://www.fmistandard.org/, July 2014. Specification, FMI 2.0. Section 3.1, pp. 71–72
8. Optimal control with CasADi¶
CasADi can be used to solve optimal control problems (OCP) using a variety of methods, including direct (a.k.a. discretizethenoptimize) and indirect (a.k.a. optimizethendiscretize) methods, allatonce (e.g. collocation) methods and shootingmethods requiring embedded solvers of initial value problems in ODE or DAE. As a user, you are in general expected to write your own OCP solver and CasADi aims as making this as easy as possible by providing powerful highlevel building blocks. Since you are writing the solver yourself (rather than calling an existing “blackbox” solver), a basic understanding of how to solve OCPs is indispensable. Good, selfcontained introductions to numerical optimal control can be found in the recent textbooks by Biegler [1] or Betts [2] or Moritz Diehl’s lecture notes on numerical optimal control.
8.1. A simple test problem¶
To illustrate some of the methods, we will consider the following test problem, namely driving a Van der Pol oscillator to the origin, while trying to minimize a quadratic cost:
with \(T=10\).
In CasADi’s examples collection [3], you find codes for solving optimal control problems using a variety of different methods.
In the following, we will discuss three of the most important methods, namely direct single shooting, direct multiple shooting and direct collocation.
8.1.1. Direct singleshooting¶
In the direct single shooting method, the control trajectory is parameterized using some piecewise smooth approximation, typically piecewise constant.
Using an explicit expression for the controls, we can then eliminate the whole state trajectory from the optimization problem, ending up with an NLP in only the discretized controls.
In CasADi’s examples collection, you will find the codes
direct_single_shooting.py
and direct_single_shooting.m
for Python and MATLAB/Octave, respectively. These codes implement the direct single
shooting method and solves it with IPOPT, relying on CasADi to calculate derivatives.
To obtain the discrete time dynamics from the continuous time dynamics, a
simple fixedstep RungeKutta 4 (RK4) integrator is implemented using CasADi symbolics.
Simple integrator codes like these are often useful in the context of optimal control,
but care must be taken so that they accurately solve the initialvalue
problem.
The code also shows how the RK4 scheme can be replaced by a more advanced integrator, namely the CVODES integrator from the SUNDIALS suite, which implements a variable stepsize, variable order backward differentiation formula (BDF) scheme. An advanced integrator like this is useful for larger systems, systems with stiff dynamics, for DAEs and for checking a simpler scheme for consistency.
8.1.2. Direct multipleshooting¶
The direct_multiple_shooting.py
and direct_multiple_shooting.m
codes, also in CasADi’s examples collection, implement the direct multiple
shooting method. This is very similar to the direct single shooting method,
but includes the state at certain shooting nodes as decision variables in
the NLP and includes equality constraints to ensure continuity of the trajectory.
The direct multiple shooting method is often superior to the direct single shooting method, since “lifting” the problem to a higher dimension is known to often improve convergence. The user is also able to initialize with a known guess for the state trajectory.
The drawback is that the NLP solved gets much larger, although this is often compensated by the fact that it is also much sparser.
8.1.3. Direct collocation¶
Finally, the direct_collocation.py
and direct_collocation.m
codes implement the direct collocation method. In this case, a parameterization
of the entire state trajectory, as piecewise loworder polynomials, are included
as decision variables in the NLP. This removes the need for the formulation
of the discrete time dynamics completely.
The NLP in direct collocation is even larger than that in direct multiple shooting, but is also even sparser.
Footnotes
Lorenz T. Biegler, Nonlinear Programming: Concepts, Algorithms, and Applications to Chemical Processes , SIAM 2010
John T. Betts, Practical Methods for Optimal Control Using Nonlinear Programming , SIAM 2001
You can obtain this collection as an archive named examples_pack.zip
in CasADi’s download area
9. Opti stack¶
The Opti stack is a collection of CasADi helper classes that provides a close correspondence between mathematical NLP notation, e.g.
and computer code:
opti = casadi.Opti()
x = opti.variable()
y = opti.variable()
opti.minimize( (yx**2)**2 )
opti.subject_to( x**2+y**2==1 )
opti.subject_to( x+y>=1 )
opti.solver('ipopt')
sol = opti.solve()
print(sol.value(x))
print(sol.value(y))
0.7861513776531158
0.6180339888825889
opti = casadi.Opti();
x = opti.variable();
y = opti.variable();
opti.minimize( (yx^2)^2 );
opti.subject_to( x^2+y^2==1 );
opti.subject_to( x+y>=1 );
opti.solver('ipopt');
sol = opti.solve();
sol.value(x)
sol.value(y)
ans = 0.7862
ans = 0.6180
The main characteristics of the Opti stack are:
Allows natural syntax for constraints.
Indexing/bookkeeping of decision variables is hidden.
Closer mapping of numerical datatype to the host language: no encounter with
DM
.
By default, Opti will assume a nonlinear program. To solve quadratic programs, supply the string ‘conic’ as argument to the Opti constructor.
9.1. Problem specification¶
Variables: Declare any amount of decision variables:
x = opti.variable()
: scalarx = opti.variable(5)
: column vectorx = opti.variable(5,3)
: matrixx = opti.variable(5,5,'symmetric')
: symmetric matrix
The order in which you declare the variables is respected by the solver. Note that the variables are in fact plain MX symbols. You may perform any CasADi MX operations on them, e.g. embedding integrator calls.
Parameters: Declare any amount of parameters. You must fix them to a specific numerical value before solving, and you may overwrite this value at any time.
p = opti.parameter()
opti.set_value(p, 3)
p = opti.parameter();
opti.set_value(p, 3)
Objective: Declare an objective using an expression that may involve all variables or parameters. Calling the command again discards the old objective.
opti.minimize( sin(x*(yp)) )
opti.minimize( sin(x*(yp)) )
Constraints: Declare any amount of equality/inequality constraints:
opti.subject_to( sqrt(x+y) >= 1)
: inequalityopti.subject_to( sqrt(x+y) > 1)
: same as aboveopti.subject_to( 1<= sqrt(x+y) )
: same as aboveopti.subject_to( 5*x+y==1 )
: equality
You may also throw in several constraints at once:
opti.subject_to([x*y>=1,x==3])
opti.subject_to({x*y>=1,x==3});
You may declare double inequalities:
opti.subject_to( opti.bounded(0,x,1) )
opti.subject_to( 0<=x<=1 );
When the bounds of the double inequalities are free of variables, the constraint will be passed on efficiently to solvers that support them (notably IPOPT).
You may make elementwise (in)equalities with vectors:
x = opti.variable(5,1)
opti.subject_to( x*p<=3 )
x = opti.variable(5,1);
opti.subject_to( x*p<=3 )
Elementwise (in)equalities for matrices are not supported with a natural syntax, since there is an ambiguity with semidefiniteness constraints. The workaround is to vectorize first:
A = opti.variable(5,5)
opti.subject_to( vec(A)<=3 )
A = opti.variable(5,5);
opti.subject_to( A(:)<=3 );
Each subject_to
command adds to the set of constraints in the problem specification.
Use subject_to()
to empty this set and start over.
Solver:
You must always declare the solver
(numerical backend).
An optional dictionary of CasADi plugin options can be given as second argument.
An optional dictionary of solver
options can be given as third argument.
p_opts = {"expand":True}
s_opts = {"max_iter": 100}
opti.solver("ipopt",p_opts,
s_opts)
p_opts = struct('expand',true);
s_opts = struct('max_iter',100);
opti.solver('ipopt',p_opts,
s_opts);
Initial guess: You may provide initial guesses for decision variables (or simple mappings of decision variables). When no initial guess is provided, numerical zero is assumed.
opti.set_initial(x, 2)
opti.set_initial(10*x[0], 2)
opti.set_initial(x, 2);
opti.set_initial(10*x(1), 2)
9.2. Problem solving and retrieving¶
9.2.1. Solving¶
After setting up the problem, you may call the solve method, which constructs a CasADi nlpsol
and calls it.
sol = opti.solve()
This is Ipopt version 3.14.11, running with linear solver MUMPS 5.4.1.
Number of nonzeros in equality constraint Jacobian...: 2
Number of nonzeros in inequality constraint Jacobian.: 2
Number of nonzeros in Lagrangian Hessian.............: 3
Total number of variables............................: 2
variables with only lower bounds: 0
variables with lower and upper bounds: 0
variables with only upper bounds: 0
Total number of equality constraints.................: 1
Total number of inequality constraints...............: 1
inequality constraints with only lower bounds: 1
inequality constraints with lower and upper bounds: 0
inequality constraints with only upper bounds: 0
iter objective inf_pr inf_du lg(mu) d lg(rg) alpha_du alpha_pr ls
0 0.0000000e+00 1.00e+00 1.00e+00 1.0 0.00e+00  0.00e+00 0.00e+00 0
1 1.5180703e+00 2.32e01 3.95e+08 1.0 1.11e+00  9.90e01 1.00e+00h 1
2 9.1720733e01 1.38e02 8.65e+08 1.0 1.05e01 10.0 1.00e+00 1.00e+00f 1
3 8.9187743e01 4.67e05 9.29e+06 1.0 7.19e03  1.00e+00 1.00e+00h 1
4 8.9179090e01 5.46e10 2.17e+02 1.0 2.42e05  1.00e+00 1.00e+00h 1
5 8.5961118e01 2.43e04 3.39e+00 1.0 1.56e02  1.00e+00 1.00e+00f 1
6 5.0768568e01 3.58e02 2.18e+00 1.0 1.89e01  3.86e01 1.00e+00f 1
7 4.4751644e01 4.12e04 6.30e+07 1.0 1.96e02 9.5 1.00e+00 1.00e+00h 1
8 4.4707538e01 4.25e08 2.27e+04 1.0 2.53e04  1.00e+00 1.00e+00h 1
9 4.4688522e01 9.34e09 2.20e+00 1.0 9.32e05  1.00e+00 1.00e+00h 1
iter objective inf_pr inf_du lg(mu) d lg(rg) alpha_du alpha_pr ls
10 2.9995484e02 3.46e01 1.19e+00 1.0 5.68e01  5.45e01 1.00e+00f 1
11 3.9964883e04 3.56e02 5.81e02 1.0 2.09e01  1.00e+00 1.00e+00h 1
12 3.7581923e06 5.63e04 5.70e03 2.5 2.69e02  1.00e+00 1.00e+00h 1
13 7.6573980e10 1.48e06 3.13e05 3.8 1.11e03  1.00e+00 1.00e+00h 1
14 4.8778322e14 2.53e10 7.52e09 5.7 1.29e05  1.00e+00 1.00e+00h 1
15 8.8029044e20 1.60e14 6.47e13 8.6 9.88e08  1.00e+00 1.00e+00h 1
Number of Iterations....: 15
(scaled) (unscaled)
Objective...............: 8.8029044107981477e20 8.8029044107981477e20
Dual infeasibility......: 6.4749359014643689e13 6.4749359014643689e13
Constraint violation....: 1.5987211554602254e14 1.5987211554602254e14
Variable bound violation: 0.0000000000000000e+00 0.0000000000000000e+00
Complementarity.........: 2.5060006300485104e09 2.5060006300485104e09
Overall NLP error.......: 2.5060006300485104e09 2.5060006300485104e09
Number of objective function evaluations = 16
Number of objective gradient evaluations = 16
Number of equality constraint evaluations = 16
Number of inequality constraint evaluations = 16
Number of equality constraint Jacobian evaluations = 16
Number of inequality constraint Jacobian evaluations = 16
Number of Lagrangian Hessian evaluations = 15
Total seconds in IPOPT = 0.008
EXIT: Optimal Solution Found.
solver : t_proc (avg) t_wall (avg) n_eval
nlp_f  61.00us ( 3.81us) 29.50us ( 1.84us) 16
nlp_g  163.00us ( 10.19us) 59.20us ( 3.70us) 16
nlp_grad_f  82.00us ( 4.82us) 40.80us ( 2.40us) 17
nlp_hess_l  131.00us ( 8.73us) 64.20us ( 4.28us) 15
nlp_jac_g  117.00us ( 6.88us) 57.50us ( 3.38us) 17
total  17.01ms ( 17.01ms) 8.51ms ( 8.51ms) 1
sol = opti.solve();
This is Ipopt version 3.14.11, running with linear solver MUMPS 5.4.1.
Number of nonzeros in equality constraint Jacobian...: 2
Number of nonzeros in inequality constraint Jacobian.: 2
Number of nonzeros in Lagrangian Hessian.............: 3
Total number of variables............................: 2
variables with only lower bounds: 0
variables with lower and upper bounds: 0
variables with only upper bounds: 0
Total number of equality constraints.................: 1
Total number of inequality constraints...............: 1
inequality constraints with only lower bounds: 1
inequality constraints with lower and upper bounds: 0
inequality constraints with only upper bounds: 0
iter objective inf_pr inf_du lg(mu) d lg(rg) alpha_du alpha_pr ls
0 0.0000000e+00 1.00e+00 1.00e+00 1.0 0.00e+00  0.00e+00 0.00e+00 0
1 1.5180703e+00 2.32e01 3.95e+08 1.0 1.11e+00  9.90e01 1.00e+00h 1
2 9.1720733e01 1.38e02 8.65e+08 1.0 1.05e01 10.0 1.00e+00 1.00e+00f 1
3 8.9187743e01 4.67e05 9.29e+06 1.0 7.19e03  1.00e+00 1.00e+00h 1
4 8.9179090e01 5.46e10 2.17e+02 1.0 2.42e05  1.00e+00 1.00e+00h 1
5 8.5961118e01 2.43e04 3.39e+00 1.0 1.56e02  1.00e+00 1.00e+00f 1
6 5.0768568e01 3.58e02 2.18e+00 1.0 1.89e01  3.86e01 1.00e+00f 1
7 4.4751644e01 4.12e04 6.30e+07 1.0 1.96e02 9.5 1.00e+00 1.00e+00h 1
8 4.4707538e01 4.25e08 2.27e+04 1.0 2.53e04  1.00e+00 1.00e+00h 1
9 4.4688522e01 9.34e09 2.20e+00 1.0 9.32e05  1.00e+00 1.00e+00h 1
iter objective inf_pr inf_du lg(mu) d lg(rg) alpha_du alpha_pr ls
10 2.9995484e02 3.46e01 1.19e+00 1.0 5.68e01  5.45e01 1.00e+00f 1
11 3.9964883e04 3.56e02 5.81e02 1.0 2.09e01  1.00e+00 1.00e+00h 1
12 3.7581923e06 5.63e04 5.70e03 2.5 2.69e02  1.00e+00 1.00e+00h 1
13 7.6573980e10 1.48e06 3.13e05 3.8 1.11e03  1.00e+00 1.00e+00h 1
14 4.8778322e14 2.53e10 7.52e09 5.7 1.29e05  1.00e+00 1.00e+00h 1
15 8.8029044e20 1.60e14 6.47e13 8.6 9.88e08  1.00e+00 1.00e+00h 1
Number of Iterations....: 15
(scaled) (unscaled)
Objective...............: 8.8029044107981477e20 8.8029044107981477e20
Dual infeasibility......: 6.4749359014643689e13 6.4749359014643689e13
Constraint violation....: 1.5987211554602254e14 1.5987211554602254e14
Variable bound violation: 0.0000000000000000e+00 0.0000000000000000e+00
Complementarity.........: 2.5060006300485104e09 2.5060006300485104e09
Overall NLP error.......: 2.5060006300485104e09 2.5060006300485104e09
Number of objective function evaluations = 16
Number of objective gradient evaluations = 16
Number of equality constraint evaluations = 16
Number of inequality constraint evaluations = 16
Number of equality constraint Jacobian evaluations = 16
Number of inequality constraint Jacobian evaluations = 16
Number of Lagrangian Hessian evaluations = 15
Total seconds in IPOPT = 0.009
EXIT: Optimal Solution Found.
solver : t_proc (avg) t_wall (avg) n_eval
nlp_f  65.00us ( 4.06us) 30.50us ( 1.91us) 16
nlp_g  144.00us ( 9.00us) 63.80us ( 3.99us) 16
nlp_grad_f  95.00us ( 5.59us) 47.70us ( 2.81us) 17
nlp_hess_l  158.00us ( 10.53us) 77.90us ( 5.19us) 15
nlp_jac_g  135.00us ( 7.94us) 67.90us ( 3.99us) 17
total  18.22ms ( 18.22ms) 9.21ms ( 9.21ms) 1
The call will fail with an error if the solver fails to convergence. You may still inspect the nonconverged solution (see Section 9.3).
You may call solve
any number of times. You will always get an immutable copy of the problem specification and its solution.
Consecutively calling solve
will not help the convergence of the problem.
To warm start a solver, you need to explicitly transfer the solution of one problem to the initial value of the next.
sol1 = opti.solve()
print(sol1.stats()["iter_count"])
# Solving again makes no difference
sol1 = opti.solve()
print(sol1.stats()["iter_count"])
# Passing initial makes a difference
opti.set_initial(sol1.value_variables())
sol2 = opti.solve()
print(sol2.stats()["iter_count"])
15
15
4
sol1 = opti.solve();
sol1.stats.iter_count
% Solving again makes no difference
sol1 = opti.solve();
sol1.stats.iter_count
% Passing initial makes a difference
opti.set_initial(sol1.value_variables());
sol2 = opti.solve();
sol2.stats.iter_count
ans = 15
ans = 15
ans = 4
In order to initialize the dual variables, e.g. when solving a set of similar optimization problems, you can use the following syntax:
sol = opti.solve()
lam_g0 = sol.value(opti.lam_g)
opti.set_initial(opti.lam_g, lam_g0)
sol = opti.solve();
lam_g0 = sol.value(opti.lam_g);
opti.set_initial(opti.lam_g, lam_g0);
9.2.2. Numerical value at the solution¶
Afterwards, you may retrieve the numerical values of variables (or expressions of those variables) at the solution:
sol.value(x)
: value of a decision variablesol.value(p)
: value of a parametersol.value(sin(x+p))
: value of an expressionsol.value(jacobian(opti.g,opti.x))
: value of constraint jacobian
Note that the return type of value
is sparse when applicable.
9.2.3. Numerical value at other points¶
You may pass a list of overruling assignment expressions to value
.
In the following code, we are asking for the value of the objective,
using all optimal values at the solution, except for y
, which we set equal to 2.
Note that such statement does not modify the actual optimal value of y
in a permanent way.
obj = (yx**2)**2
opti.minimize(obj)
print(sol.value(obj,[y==2]))
1.909830056703819
obj = (yx^2)^2;
opti.minimize(obj);
sol.value(obj,{y==2})
ans = 1.9098
A related usage pattern is to evaluate an expression at the initial guess:
print(sol.value(x**2+y,opti.initial()))
0.0
sol.value(x**2+y,opti.initial())
ans = 0
9.2.4. Dual variables¶
In order to obtain dual variables (Lagrange multipliers) of constraints, make sure you save the constraint expression first:
con = sin(x+y)>=1
opti.subject_to(con)
sol = opti.solve()
print(sol.value(opti.dual(con)))
0.258679501736367
con = sin(x+y)>=1;
opti.subject_to(con);
sol = opti.solve();
sol.value(opti.dual(con))
ans = 0.2587
9.3. Extras¶
It may well happen that the solver does not find an optimal solution. In such cases, you may still access the nonconverged solution through debug mode:
opti.debug.value(x)
Related, you may inspect the value of an expression, at the initial guess that you supplied to the solver:
opti.debug.value(x,opti.initial())
In case the solver stops due to problem infeasibility, you may identify the problematic constraints with:
opti.debug.show_infeasibilities()
In case the solver reports NaN/Inf at a certain location, you may find out which constraint or variable is to blame by looking at its description:
opti.debug.x_describe(index)
opti.debug.g_describe(index)
You may specify a callback function; it will be called at each iteration of the solver, with the current iteration number as argument. To plot the progress of the solver, you may access the nonconverged solution through debug mode:
opti.callback(lambda i: plot(opti.debug.value(x)))
opti.callback(@(i) plot(opti.debug.value(x)))
The callback may be cleared from the Opti stack by calling the Callback
function without arguments.
You may construct a regular CasADi Function out of an Opti object, by calling the to_function
function.
The supplied list of input arguments may contain parameters, decision variables and dual variables.
Inputs corresponding to variables are taken as initial guesses.
If you desire this Function to throw a runtime error when the solver fails, you may pass the error_on_fail
as solver option.
f = opti.to_function("f",[x,y],[x**2+y])
print(f(0, 0))
1.23607
f = opti.to_function('f',{x,y},{x^2+y});
disp(f(0, 0))
1.23607
10. Difference in usage from different languages¶
10.1. General usage¶
Example:
Task 
Python 
C++ 
MATLAB/Octave 

Starting CasADi 
from casadi import *

#include <casadi/casadi.hpp>
using namespace casadi;

import casadi.*

Printing object 
print(A)

std::cout << A;

disp(A)

Printing with type information 

std::cout << repr(A);


Get (extended) representation, 



Calling a class function 
SX.zeros(3,4)

SX::zeros(3,4)

SX.zeros(3,4)

Creating a dictionary (e.g. for options) 
d = dict(opt1=opt1)
#or
d = {'opt1':opt1}
#or
d = {}; a['opt1'] = opt1

Dict a;
a["opt1"] = opt1;

a = struct;
a.opt1 = opt1;
#or
a = struct('opt1',opt1);

Creating a symbol 
MX.sym("x",2,2)

MX::sym("x",2,2)

MX.sym('x',2,2)

Creating a function 
Function("f",[x,y],[x+y])

Function("f",{x,y},{x+y})

Function('f',{x,y},{x+y})

Calling a function (form 1) 
z=f(x,y)

z = f({x,y})

z=f(x,y)

Calling a function (form 2) 
res = f(x=x,y=y)
res["z"]

auto res =\
f({{"x",x},{"y",y}});
res["z"]

res=f('x',x,'y',y)
res.z

Create an NLP solver 
nlp = {"x":x,"f":f}
nlpsol("S","ipopt",nlp)

MXDict nlp =\
{{"x",x},{"f",f}};
nlpsol("S","ipopt",nlp);

nlp=struct('x',x,'f',f);
nlpsol('S','ipopt',nlp);

10.2. List of operations¶
The following is a list of the most important operations. Operations that differ between the different languages are marked with a star (*). This list is neither complete, nor does it show all the variants of each operation. Further information is available in the API documentation.
Task 
Python 
C++ 
MATLAB/Octave 

Addition, subtraction 
x+y, xy, x

x+y, xy, x

x+y, xy, x

Elementwise multiplication, division 
x*y, x/y

x*y, x/y

x.*y, x./y

Matrix multiplication 
x @ y
# or before Py3.4:
mtimes(x,y)

mtimes(x,y)

x*y

Linear system solve 
solve(A,b)

solve(A,b)

A\b

Natural exponential function and logarithm 
exp(x), log(x)

exp(x), log(x)

exp(x), log(x)

Exponentiation 
x**y

pow(x,y)

x^y, x.^y

Square root 
sqrt(x)

sqrt(x)

sqrt(x)

Trigonometric functions 
sin(x), cos(x), tan(x)

sin(x), cos(x), tan(x)

sin(x), cos(x), tan(x)

Inverse trigonometric 
asin(x), acos(x)

asin(x), acos(x)

asin(x), acos(x)

Two argument arctangent 
atan2(y, x)

atan2(y, x)

atan2(y, x)

Hyperbolic functions 
sinh(x), cosh(x), tanh(x)

sinh(x), cosh(x), tanh(x)

sinh(x), cosh(x), tanh(x)

Inverse hyperbolic 
asinh(x), acosh(x)

asinh(x), acosh(x)

asinh(x), acosh(x)

Inequalities 
a<b, a<=b, a>b, a>=b

a<b, a<=b, a>b, a>=b

a<b, a<=b, a>b, a>=b

(Not) equal to 
a==b, a!=b

a==b, a!=b

a==b, a~=b

Logical and 
logic_and(a, b)

a && b

a & b

Logical or 
logic_or(a, b)

a  b

a  b

Logical not 
logic_not(a)

!a

~a

Round to integer 
floor(x), ceil(x)

floor(x), ceil(x)

floor(x), ceil(x)

Modulus after division 
fmod(x, y), remainder(x,y)

fmod(x, y), remainder(x,y)

rem(x, y), remainder(x,y)

Absolute value 
fabs(x)

fabs(x)

abs(x)

Norm 
norm_1(x)

norm_1(x)

norm(x,1)

Sign function 
sign(x)

sign(x)

sign(x)

(Inverse) error function 
erf(x), erfinv(x)

erf(x), erfinv(x)

erf(x), erfinv(x)

Elementwise min and max 
fmin(x, y), fmax(x, y)

fmin(x, y), fmax(x, y)

min(x, y), max(x, y)

Global min and max 
mmin(x), mmax(x)

mmin(x), mmax(x)

mmin(x), mmax(x)

Index of first nonzero 
find(x)

find(x)

find(x)

Ifthenelse 
if_else(c, x, y)

if_else(c, x, y)

if_else(c, x, y)

Transpose 
A.T

A.T()

A',A.'

Inner product 
dot(x, y)

dot(x, y)

dot(x, y)

Horizontal/vertical concatenation 
horzcat(x, y)
vertcat(x, y)
hcat([x, y])
vcat([x, y])

horzcat(x, y)
vertcat(x, y)
MX::horzcat({x,y})
MX::vertcat({x,y})

[x,y]
[x; y]
c = {x,y};
[c{:}]
vertcat(c{:})

Horizontal/vertical split (inverse of concatenation) 
vertsplit(x)
horzsplit(x)

vertsplit(x)
horzsplit(x)

vertsplit(x)
horzsplit(x)

Element access 
# 0based
A[i,j]
A[i]

// 0based
A(i,j)
A(i)

% 1based
A(i,j)
A(i)

Element assignment 
# 0based
A[i,j] = b
A[i] = b

// 0based
A(i,j) = b
A(i) = b

% 1based
A(i,j) = b
A(i) = b

Nonzero access 
# 0based
A.nz[k]

// 0based
A.nz(k)

(currently unsupported)

Nonzero assignment 
# 0based
A.nz[k] = b

// 0based
A.nz(k) = b

(currently unsupported)

Project to a different sparsity 
project(x, s)

project(x, s)

project(x, s)
