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?

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Balaji Jayaraman 2017-11-8

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评论： Adam 2017-11-8

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Stephen23 2017-11-8

Avoid using eval for such trivial code:

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

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Answer 1

Guillaume 2017-11-8

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编辑：Guillaume 2017-11-8

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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.

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Answer 2

Mischa Kim 2017-11-8

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

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

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Balaji Jayaraman 2017-11-8

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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