How To Create A Matlab Function

Easily integrate Custom Functions in MATLAB with Python

MATLAB professional level functions in Python scripts

MATLAB implementation is usually quite reliable as it is developed by professionals. But the advantages of using Python are immense. In this post, I will show how you can integrate your custom MATLAB function into your Python script.

Defining a custom function in MATLAB

Let us make a custom function in MATLAB that we can use in Python. For demonstration, I will use an elementary function, but the same idea applies to any function.

Eigenvalues and eigenvectors in MATLAB

          function [V,D] = eigFunc(A)
%returns diagonal matrix D of eigenvalues and matrix V            
% whose columns are the corresponding right eigenvectors,            
% so that A*V = V*D.
[V, D] = eig(A);
end

I saved the above function as ei g Func. This function takes in a square matrix as input and outputs diagonal matrix D of eigenvalues and matrix V whose columns are the corresponding right eigenvectors [ see eig function in MATLAB].

Let's first use this function in MATLAB for test purpose.

          clear; close all; clc
A = gallery('lehmer',4);[V,D] = eigFunc(A)

This returns:

          V =              0.0693   -0.4422   -0.8105    0.3778
              -0.3618    0.7420   -0.1877    0.5322
              0.7694    0.0486    0.3010    0.5614
              -0.5219   -0.5014    0.4662    0.5088
            D =
                          0.2078         0         0         0
              0    0.4078         0         0
              0         0    0.8482         0
              0         0         0    2.5362

This function works excellent in MATLAB as expected (because the function is an exact copy of the eig function in MATLAB. But how can we use this function in Python?

Call MATLAB in Python

Using MATLAB Engine API for Python

The easiest way to use the matlab function in Python is by using the matlab.engine. You can install matlab library by following these two ways.

Install from inside MATLAB:

          cd (fullfile(matlabroot,'extern','engines','python'))
system('python setup.py install')

Install directly from the terminal:

Navigate to the MATLAB source location and compile the python

          python setup.py install

Please note that this MATLAB engine API will be installed for the specific version of python. If you are using anaconda, you can inspect the version of Python you are installing the MATLAB engine API for. It is essentially the same way you install other Python libraries. For details, visit here.

          import matlab
import matlab.engine
eng = matlab.engine.start_matlab()
A = [[1.0000,    0.5000,    0.3333,    0.2500],
            [0.5000,    1.0000,    0.6667,    0.5000],
            [0.3333,    0.6667,    1.0000,    0.7500],
            [0.2500,    0.5000,    0.7500,    1.0000]]
A = matlab.double(A)
V, D = eng.eigFunc(A, nargout=2)
print("V: ", V)
print("D: ", D)eng.quit()

Here is the output:

          V:  [[0.06939950784450351,-0.4421928183150595,-0.8104910184495989,0.37782737957175255],				[-0.3619020163563876,0.7419860358173743,-0.18770341448628555,0.5322133795757004],[0.7693553355549393,0.04873539080548356,0.30097912769034274,0.5613633351323756],[-0.5218266058004974,-0.5015447096377744,0.4661365700065611,0.5087893432606572]]D:  [[0.20775336892808516,0.0,0.0,0.0],[0.0,0.40783672775946245,0.0,0.0],[0.0,0.0,0.8482416513967358,0.0],[0.0,0.0,0.0,2.536168251915717]]

The results are the same as before. The nargout argument tells the matlab based function to output 2 results here.

Create Python Package: Using MATLAB Compiler & MATLAB Runtime

This is all well and good if you have MATLAB installed in your system. But what if you want to give your Python script to someone who does not have MATLAB installed on their system. In that case, you can build a Python library using the library compiler app in MATLAB. For details, visit Generate a Python Package and Build a Python Application.

(a) Select library compiler app in MATLAB (b) Fill in the information regarding the Python library (Image by author)

(a) Select MATLAB function to generate the Python library (b) Add example MATLAB script (Image by author)

(a) Click on the `package` to generate standalone component (b) Wait for the task to finish (Image by author)

MATLAB Runtime installation

Please note that the user needs to install MATLAB runtime to successfully using this library. MATLAB runtime helps in running compiled MATLAB applications or components without installing MATLAB. The runtime can be downloaded from here for Windows, Mac, and Linux OS, and it is free to download.

Import MATLAB based library for Python

          import eigFunc
eigFuncAnalyzer = eigFunc.initialize() #calls the matlab runtime
A = [[1.0000,    0.5000,    0.3333,    0.2500],
            [0.5000,    1.0000,    0.6667,    0.5000],
            [0.3333,    0.6667,    1.0000,    0.7500],
            [0.2500,    0.5000,    0.7500,    1.0000]]
A = array.array(A) %not tested
V, D = eigFuncAnalyzer.eigFunc(A, nargout=2)
print("V: ", V)
print("D: ", D)
eigFuncAnalyzer.terminate()

Please note that you can design your eigFunc.m in a way that you can simply load the mat data and you can use scipy.io.savemat function to save the python data to mat format. For details, see the scipy documentation.

Data Type Conversions

References

How to Call MATLAB from Python
Call MATLAB Functions from Python
Generate a Python Package and Build a Python Application