Technical reasons behind formatting when incrementing by 1 in a 'for' loop?

All over the web, code samples have for loops which look like this:

for(int i = 0; i < 5; i++)

while I used the following format:

for(int i = 0; i != 5; ++i)

I do this because I believe it to be more efficient, but does this really matter in most cases?

Solution 1:

Everybody loves their micro-optimizations, but this would not make a difference as far as I can see. I compiled the two variations with g++ on for Intel processors without any fancy optimizations and the results are for

for(int i = 0; i < 5; i++)

    movl $0, -12(%ebp)
    jmp L2
L3:
    leal    -12(%ebp), %eax
    incl    (%eax)
L2:
    cmpl    $4, -12(%ebp)
    jle L3

for(int i = 0; i != 5; ++i)

    movl    $0, -12(%ebp)
    jmp L7
L8:
    leal    -12(%ebp), %eax
    incl    (%eax)
L7:
    cmpl    $5, -12(%ebp)
    jne L8

I think jle and jne should translate to equally fast instructions on most architectures. So for performance, you should not distinguish between the two. In general, I would agree that the first one is a little safer and I also think more common.

EDIT (2 years later): Since this thread recently got again a lot of attention, I would like to add that it will be difficult to answer this question generally. Which versions of code are more efficient is specifically not defined by the C-Standard [PDF] (and the same applies to C++ and probably also for C# ).

Section 5.1.2.3 Program execution

§1 The semantic descriptions in this International Standard describe the behavior of an abstract machine in which issues of optimization are irrelevant.

But it is reasonable to assume that a modern compiler will produce equally efficient code and I think that in only very rare cases will the loop-test and the counting expression be the bottleneck of a for-loop.

As for taste, I write

for(int i = 0; i < 5; ++i)

Solution 2:

If for some reason i jumps to 50 in the loop, your version would loop forever. The i < 5 is a sanity check.

Solution 3:

The form

for (int i = 0; i < 5; i++)

is idiomatic, so it's easier to read for experienced C programmers. Especially when used to iterate over an array. You should write idiomatic code whenever possible as it reads faster.

It is also a little safer in situations when you modify i inside the loop or use an increment different then 1. But it's a minor thing. It's best to carefully design your loop and add some asserts to catch broken assumptions early.

Solution 4:

If the increment rule changes slightly you immediately have an infinite loop. I much prefer the first end condition.

Solution 5:

It depends on the language.

C++ texts often suggest the second format as that will work with iterators which can be compared (!=) directly but not with a greater to or less than condition. Also pre increment can be faster than post increment as there is no need for a copy of the variable for comparison - however optimisers can deal with this.

For integers either form works. The common idiom for C is the first one whilst for C++ it is the second.

For C# and Java use I would foreach to loop over all things.

In C++ there is also the std::for_each function requiring a use of a functor which for simple cases is probably more complex than either example here and the Boost FOR_EACH which can look like the C# foreach but is complex inside.