eval([ans '=' num2str(x0(i)) ';']); gives an error "The expression to the left of the equals sign is not a valid target for an assignment". what could be the problem?

eval([ans '=' num2str(x0(i)) ';']); gives an error "The expression to the left of the equals sign is not a valid target for an assignment". what could be the problem?

[Stephen23]

Avoid using eval for such trivial code:

https://www.mathworks.com/help/matlab/matlab_prog/string-evaluation.html

Answer 1

[Guillaume]

There are many thing wrong with

eval([ans '=' num2str(x0(i)) ';'])

• eval expect a string or char array. As per Mischa's answer the whole expression probably needs to be enclosed in quote.
• do not use ans as a variable name. It's a special variable used by matlab.
• and worst of all, is the use of eval. There's normally very little reason to use eval, in the above case, there's absolutely no reason, other than making the code harder to debug, more difficult to read and obviously harder to use since you can't get the syntax right

SomeVariableNameOtherThanANS = num2str(xi(0));

would do exactly the same.

Answer 2

[Mischa Kim]

Just wondering if the following would do the trick...

 eval('ans = num2str(x0(i));')

[Stephen23]

Why bother? There does not seem to be anything here that requires using the chainsaw eval. Why not just

X = num2str(x0(i));

???

https://www.mathworks.com/matlabcentral/answers/364851-assignin-and-eval-functions#comment_500697

[Mischa Kim]

I do not have enough information here to judge what is required and what is not. The question posed contains one single command.

[Balaji Jayaraman]

it is a code written for gradient descent function.

function [xsol, fsol] =  GradientDescent(f,varx,x0,epis,gamma)
for i=1:length(varx)
    nabf(i) = diff(f,varx(i));
end
nabf = vpa(nabf,5);
for i=1:length(varx)
    char(varx(i));
    eval([ans '=' num2str(x0(i)) ';']);
end
nabfloop = subs(nabf)';
while(double(norm(nabfloop))>=epis)
    x0 = x0 - gamma*double(nabfloop);
    x0
    for i=1:length(varx)
        char(varx(i));
        eval([ans '=' num2str(x0(i)) ';']);
    end
    nabfloop = subs(nabf)';
end
xsol = x0;
for i=1:length(varx)
    char(varx(i));
    eval([ans '=' num2str(x0(i)) ';']);
end
fsol = double(subs(f));
end

[Adam]

Most of that code appears to do nothing or create variables that nothing is done with!