Will strlen be calculated multiple times if used i

Question

I'm not sure if the following code can cause redundant calculations, or is it compiler-specific?

for (int i = 0; i < strlen(ss); ++i)
{
    // blabla
}

Will strlen() be calculated every time when i increases?

Answer 1

Yes, strlen(ss) will be calculated every time the code runs.

Answer 2

Yes. strlen will be calculated everytime when i increases.

If you didn't change ss with in the loop means it won't affect logic otherwise it will affect.

It is safer to use following code.

int length = strlen(ss);

for ( int i = 0; i < length ; ++ i )
{
 // blabla
}

Answer 3

well, I noticed that someone is saying that it is optimized by default by any "clever" modern compiler. By the way look at results without optimization. I tried:
Minimal C code:

#include <stdio.h>
#include <string.h>

int main()
{
 char *s="aaaa";

 for (int i=0; i<strlen(s);i++)
  printf ("a");
 return 0;
}

My compiler: g++ (Ubuntu/Linaro 4.6.3-1ubuntu5) 4.6.3
Command for generation of assembly code: g++ -S -masm=intel test.cpp

Gotten assembly code at the output:
    ...
    L3:
mov DWORD PTR [esp], 97
call    putchar
add DWORD PTR [esp+40], 1
    .L2:
     THIS LOOP IS HERE
    **<b>mov    ebx, DWORD PTR [esp+40]
mov eax, DWORD PTR [esp+44]
mov DWORD PTR [esp+28], -1
mov edx, eax
mov eax, 0
mov ecx, DWORD PTR [esp+28]
mov edi, edx
repnz scasb</b>**
     AS YOU CAN SEE it's done every time
mov eax, ecx
not eax
sub eax, 1
cmp ebx, eax
setb    al
test    al, al
jne .L3
mov eax, 0
     .....

Answer 4

Not common nowadays but 20 years ago on 16 bit platforms, I'd recommend this:

for ( char* p = str; *p; p++ ) { /* ... */ }

Even if your compiler isn't very smart in optimization, the above code can result in good assembly code yet.

Answer 5

We can easily test it :

char nums[] = "0123456789";
size_t end;
int i;
for( i=0, end=strlen(nums); i<strlen(nums); i++ ) {
    putchar( nums[i] );
    num[--end] = 0;
}

Loop condition evaluates after each repetition, before restarting the loop .

Also be careful about the type you use to handle length of strings . it should be size_t which has been defined as unsigned int in stdio. comparing and casting it to int might cause some serious vulnerability issue.

Answer 6

Yes, the strlen(ss) will calculate the length at each iteration. If you are increasing the ss by some way and also increasing the i; there would be infinite loop.