Substituting Matrices in symbolic equations

Dear community,

I would like to ask for your help (if anyone has an idea) how to prevent the following miscalculation to happen:

Problem: Suppose I have two unknowns in a formula, where the dimensions of those are not known yet. Those two unkown variables get multiplied with each other, however when I substitute those two variables by matrices the " * " (multiplication symbol) will perform a elementwise multiplication. Is there anyway to force it to do a matrix multiplication. Attached you will find a little sample code.

The matrix multiplication should give: [2 1; 1 1] but instead I get back [1 1; 1 0]. The constraint of the problem is that in the beginning I have matrices inside a matrix.

Thanks in advance!

Adrian

clear all; close all; clc

syms Ku Kv
K = [Ku 0; 0 Kv];
Ku_mat= [1 1; 1 0];
Kv_mat = [1 1; 1 0];

z = det(K); % which gives back z = Ku*Kv

z=subs(z,{Ku,Kv},{Ku_mat, Kv_mat});

Answer 1

[madhan ravi]

At the moment it’s not possible using subs().

Resource: https://www.mathworks.com/help/symbolic/subs.html#btqh8bz-1

It would be better if you submit a feature enhancement request to the MathWorks.

A workaround:

z = det(K);
expre = regexprep(['@(',char(symvar(z)),')',char(z)], '\[|]','');
z1 = str2func(expre);
['Inputs must contain: ', char(symvar(z))]
Wanted = z1(Ku_mat,Kv_mat)

[Adrian Stein]

I couldn't run your version because my MATLAB version wouldn't know the matrix command. However I added two more line to your code would like to share it with the community. Thanks a lot for your help!

clear all; close all; clc
syms Ku Kv
K = [Ku 0; 0 Kv];
Ku_mat= [1 1; 1 0];
Kv_mat = [1 1; 1 0];

z = det(K);
expre = regexprep(['@(',char(symvar(z)),')',char(z)], '\[|]','');
expre = strrep(expre,'matrix(','');
expre = strrep(expre,'))',')');
z1 = str2func(expre);
['Inputs must contain: ', char(symvar(z))]
Wanted = z1(Ku_mat,Kv_mat)

[madhan ravi]

It worked for me in 2020 version.