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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
Balaji Jayaraman on 8 Nov 2017
Commented: Adam on 8 Nov 2017
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?

Answers (2)

Guillaume
Guillaume on 8 Nov 2017
Edited: Guillaume on 8 Nov 2017
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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Mischa Kim
Mischa Kim on 8 Nov 2017
Just wondering if the following would do the trick...
eval('ans = num2str(x0(i));')

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Mischa Kim
Mischa Kim on 8 Nov 2017
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
Balaji Jayaraman on 8 Nov 2017
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
Adam on 8 Nov 2017
Most of that code appears to do nothing or create variables that nothing is done with!

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