Question

I was encountered with usage of function eval(expression) in somebody else’s code in matlab:
for example:

for n = 1 : 4
sn = int2str( n) ;
eval( [ 'saveas( fig'  sn  ', [ sName' sn ' ], ''fig'' ) ' ] );
end

MathWorks stuff in Matlab Help pointed out that:

Many common uses of the eval function are less efficient and are more difficult to read and debug than other MATLAB functions and language constructs.

After this, I find usage of this function in many other program languages, so as Python, JavaScript, PHP.

So I have a few questions:

1. Will usage of this function be enfluenced on the performance of my code?
2. If it will slow down execution, why does it occur?
3. If it slow down execution every time when called, what reason for use this function in principle?

Answer 1

1. Using eval here will certainly be slower than a non-eval version, but most likely it won’t be a bottleneck in you code. However, the performance is only one issue, maintenance (incl. debugging), as well as readability are other ones.
2. The slowdown occurs because Matlab uses a JIT compiler, and eval lines cannot be optimized.
3. Eval use is in most cases due to lack of knowledge about the Matlab functionality that would be appropriate instead. In this particular case, the issue is that the figure handles are stored in variable names called fig1 through fig4. If they had been stored in an array called fig, i.e. fig(1) etc, eval would have been unnecessary.

EDIT Here are two excellent articles by Loren Shure about why eval should be avoided in Matlab. Evading eval, and More on eval.