language agnostic - Does the last element in a loop deserve a separate treatment?

Question

When reviewing, I sometimes encounter this kind of loop:

i = begin
while ( i != end ) {    
   // ... do stuff
   if ( i == end-1 (the one-but-last element) ) {
      ... do other stuff
   }
   increment i
}

Then I ask the question: would you write this?

i = begin
mid = ( end - begin ) / 2 // (the middle element)
while ( i != end ) {    
   // ... do stuff
   if ( i > mid ) {
      ... do other stuff
   }
   increment i
}

In my opinion, this beats the intention of writing a loop: you loop because there is something common to be done for each of the elements. Using this construct, for some of the elements you do something different. So, I conclude, you need a separate loop for those elements:

i = begin
mid = ( end - begin ) / 2 //(the middle element)
while ( i != mid ) {    
   // ... do stuff
   increment i
}

while ( i != end ) {
   // ... do stuff
   // ... do other stuff
   increment i
}

Now I even saw a question on SO on how to write the if-clause in a nice way... And I got sad: something isn't right here.

Am I wrong? If so, what's so good about cluttering the loop body with special cases, which you are aware of upfront, at coding time?

Answer 1

I don't think this question should be answered by a principle (e.g. "in a loop, treat every element equally"). Instead, you can look at two factors to evaluate if an implementation is good or bad:

1. Runtime effectivity - does the compiled code run fast, or would it be faster doing it differently?
2. Code maintainability - Is it easy (for another developer) to understand what is happening here?

If it is faster and the code is more readable by doing everything in one loop, do it that way. If it is slower and less readable, do it another way.

If it is faster and less readably, or slower but more readable, find out which of the factors matters more in your specific case, and then decide how to loop (or not to loop).