Symbolic Matrix members evaluation

Hello everyone,

I am using the symbolic toolbox to create symbolic matrices, then I need to evaluate the value of each members of the matrix.

So, here is what I have done so far.

clear;
clc;

A = sym('A', [2, 3], 'real')


for i = 1:2
    for j = 1:3      
        A(i, j) = i*j;   
        
        z  = str2sym ( strcat('A', num2str(i), '_', num2str(j) ) )

    end
end

Then, I obtain:

>> A
 
A =
 
[ 1, 2, 3]
[ 2, 4, 6]

However, when I want to get the member value, e.g: A1_3, I got the error "Unrecognized function or variable 'A1_3'."

That's why I created a string and conver to a symbolic value, and store in z. But I don't know how to get my Ai_j = A(i, j).

My goal is that each member Ai_j of the symbolic matrix gets the value of A(i, j). Can anyone help me with this ?

Thanks in advance.

Answer 1

[Ameer Hamza]

Why do you want to create variable names like A1_3, A2_3, etc., when you can directly index the matrix A. Creating a separate variable name for each element is not an efficient coding approach: https://www.mathworks.com/matlabcentral/answers/304528-tutorial-why-variables-should-not-be-named-dynamically-eval.

Also, MATLAB does not provide a good way of doing what you want to do.

[AMEHRI WALID]

Actually, I want to avoid directly indexing the matrix A because its size changes every time in my application, I only showed an example in my question, ot know if it is possible to perform something like this

[Ameer Hamza]

I believe there must be a way to use array in your case. But If you must want to do it in this way, then read about eval().

clear;
clc;
for i = 1:2
    for j = 1:3
        var_name = sprintf('A%d_%d', i, j);
        eval([var_name '=i*j']);
    end
end

It will create 6 symbolic variables.

[AMEHRI WALID]

Great, this is exactly what I was looking for.

Thank you very much!

[Ameer Hamza]

I am glad to be of help!

[Steven Lord]

Actually, I want to avoid directly indexing the matrix A because its size changes every time in my application

That's no problem. Use size or numel to avoid having to hard-code the size of the matrix.

function showAllElements(A)
fprintf("Input argument A is of size [" + num2str(size(A)) + "]." + newline);
for whichElement = 1:numel(A)
    fprintf("Element " + whichElement + " of A is %g.\n", A(whichElement));
end

Call this as:

showAllElements(magic(4))
showAllElements(rand(3, 5))