Need help with vector and eval

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Stephen on 14 May 2018
Commented: Walter Roberson on 15 May 2018
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];
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.

Accepted Answer

Ameer Hamza
Ameer Hamza on 14 May 2018
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.
Walter Roberson
Walter Roberson on 15 May 2018
The language produced by the Symbolic Toolbox is not quite the same as MATLAB. There are symbolic expressions that will produce the wrong answer or an error if you eval() then. subs() is the right thing to do for symbolic expressions.

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