MATLAB: Need help with vector and eval

symbolicSymbolic Math Toolboxvectorization

Hi all, I am encountering a problem with vectorization and eval. So here is the code
syms a b c i
A = [a*i; b-i; c+i]
a = 10;
b = 2;
c = 3;
d = 4;
i = [1:3];
eval(A)
This would return a matrix as the following.
    10    20    30
     1     0    -1
     4     5     6
What I want is the calculation being performed as the followings
– For A(1), eval will be performed by using i = i(1)
– For A(2), eval will be performed by using i = i(2)
– For A(3), eval will be performed by using i = i(3)
So the return would be
10 0 6
I'd like to do it in a way that uses vectorization. Because in my real application, the array is much larger and the symbolic equation is a lot more complicated. Using for and if might sacrifice the efficiency.
Thanks for the help.
Steph

Best Answer

First of all, you should avoid using eval as much as possible: https://www.mathworks.com/matlabcentral/answers/304528-tutorial-why-variables-should-not-be-named-dynamically-eval.
Secondly, what you are trying to do can be done correctly using for loop but since, but here is a simple solution using arrayfun,
syms a b c i
A = [a*i; b-i; c+i];
aa = 10;
bb = 2;
cc = 3;
dd = 4;
ii = 1:3;
vector = cell2mat(arrayfun(@(s, i_) double(subs(s, {a b c i}, {aa bb cc i_})), A, ii', 'UniformOutput', 0));
I renamed the variables because your original variables names were overwriting previous declarations.
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