Question

Im learning C and saw the first loop listed below in the book im reading. Im curious whats the difference between the two as I am used to using the second one and cant figure out the difference even though they return different results.

for(i = 0; i < 10; ++i){}

for(i = 0; i <= 10; i++){}

Answer 1

The first one iterates to 9, the second iterates to 10. That’s all.

The pre-/post- increment operation makes no difference.

Un-optimized code generated for both versions:

    for(int i = 0; i < 10; ++i)
00E517AE  mov         dword ptr [i],0  
00E517B5  jmp         wmain+30h (0E517C0h)  
00E517B7  mov         eax,dword ptr [i]  
00E517BA  add         eax,1  
00E517BD  mov         dword ptr [i],eax  
00E517C0  cmp         dword ptr [i],0Ah  
00E517C4  jge         wmain+53h (0E517E3h)  
    {
    }

    for(int i = 0; i <= 10; i++)
00E517E3  mov         dword ptr [i],0  
00E517EA  jmp         wmain+65h (0E517F5h)  
00E517EC  mov         eax,dword ptr [i]  
00E517EF  add         eax,1  
00E517F2  mov         dword ptr [i],eax  
00E517F5  cmp         dword ptr [i],0Ah  
00E517F9  jg          wmain+88h (0E51818h)  
    {
    }

So, even here, there is no performance penalty. The fact that i++ is slower than ++i is just not true (at least in this context, where it doesn’t make a difference). It would be slower for, say int y = i++, but in this case, the two would do different things, which is not the case here. The performance issue might have been valid on compilers from 20 years ago, but not anymore.