If statements inside or outside loops?

Question

Is it better if I do this:

foreach my $item ( @array ) {
   if ( $bool ) {
     .. code ..
   }
   else {
     .. code ..
   }
}

or

if ( $bool ) {
   foreach my $item ( @array ) {
   }
}
else {
   foreach my $item ( @array ) {
   }
}

Answer 1

I would leave premature optimization aside.

"Premature optimization is the root of all evil" - Donald Knuth

You should go for maintainability first and foremost. Group them in the way that makes more sense taking into account the logical structure of the code (such as grouping related statements together).

If you later determine that performance is an issue, try measuring with something like a profiler to see where the bottlenecks are. Chances are, it's not there. From Code Complete 2:

Barry Boehm reports that 20 percent of a program's routines consume 80 percent of its execution time. In his classic paper "An Empirical Study of Fortran Programs," Donald Knuth found that less than four percent of a program usually accounts for more than 50 percent of its run time.

We shouldn't try to guess where to optimize before it is necessary since most of us are really bad at guessing where that slow portion of our code is. Programmers who optimize as they go also spend about 96% of their time optimizing code that doesn't need to be optimized. Another thing to take into account is that code tuning (as in this example) considers a tradeoff between readability and maintainability for performance:

Focusing on optimization during initial development detracts from achieving other program objectives. Developers immerse themselves in algorithm analysis and arcane debates that in the end don't contribute much value to the user. Concerns such as correctness, information hiding, and readability become secondary goals, even though performance is easier to improve later than these other concerns are. Post hoc performance work typically affects less than five percent of a program's code. Would you rather go back and do performance work on five percent of the code or readability work on 100 percent?

I'm not saying don't optimize, but optimize code only in the end, when you have the luxury of the big picture and tools to point you in the right direction.

EXTRA: To answer the question of performance itself, though:

This ["unswitching" the code] is good for about a 20 percent time savings:

Language        Straight Time    Code-Tuned Time    Time Savings
C++             2.81             2.27               19%     
Java            3.97             3.12               21%
Visual Basic    2.78             2.77               <1%
Python          8.14             5.87               28%

A hazard distinct to this case is that the two loops have to be maintained in parallel. [...] you have to remember changing the code in both places, which is an annoyance for you and a maintenance headache for anyone else who has to work with the code.

This example also illustrates a key challenge in code tuning: the effect of any specific code tuning is not predictable. The code tuning produced significant improvements in three of the four languages but not in Visual Basic. To perform this specific optimization in this specific version of Visual Basic would produce less maintainable code without any offsetting gain in performance. The general lesson is that you must measure the effect of each specific optimization to be sure of its effect - no exceptions.

Check this other question here on SO. And this from the first edition of Code Complete.