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How to eval an expression without an error?

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Mr M.
Mr M. on 16 Oct 2018
Answered: Andy Doller on 5 Dec 2023
I would like to eval an expression, for example eval('x=a+b;'); But in case of non-existing a or b, I dont want to get an error, just do nothing.

Answers (2)

Guillaume
Guillaume on 16 Oct 2018
Not withstanding that using eval is a really bad idea, if you do really want to do it and not stop on error, wrap it in a try statement.
try
eval('x=a+b');
end
I'd recommend you catch the error, to check that it is indeed due to a missing variable and not some syntax error in your eval:
try
eval('x=a+b');
catch mexc
if ~strcmp(mexc.identifier, 'MATLAB:UndefinedFunction') %the exception you get for undefined function or variable
rethrow(mexc); %not the error we expected, really error as something else happened
end
end

Andy Doller
Andy Doller on 5 Dec 2023
Why not use the test for variables to exist
if exist('a') && exist('b')
x = a + b;
else
disp('did nothing')
end
did nothing

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