How to eval an expression without an error?

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.

Answer 1

[Guillaume]

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

Answer 2

[Andy Doller]

Why not use the test for variables to exist

if exist('a') && exist('b')
    x = a + b;
else
    disp('did nothing')
end

did nothing