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

The predicate code in it's entirety will be executed on every iteration of the for loop. In order to memoize the result of the strlen(ss) call the compiler would need to know that at least

1. The function strlen was side effect free
2. The memory pointed to by ss doesn't change for the duration of the loop

The compiler doesn't know either of these things and hence can't safely memoize the result of the first call

Answer 2

Yes. The test doesn't know that ss doesn't get changed inside the loop. If you know that it won't change then I would write:

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

Answer 3

Yes, every time you use the loop. Then it will every time calculate the length of the string. so use it like this:

char str[30];
for ( int i = 0; str[i] != '\0'; i++)
{
//Something;
}

In the above code str[i] only verifies one particular character in the string at location i each time the loop starts a cycle, thus it will take less memory and is more efficient.

See this Link for more information.

In the code below every time the loop runs strlen will count the length of the whole string which is less efficient, takes more time and takes more memory.

char str[];
for ( int i = 0; i < strlen(str); i++)
{
//Something;
}

Answer 4

A good compiler may not calculate it every time, but I don't think you can be sure, that every compiler does it.

In addition to that, the compiler has to know, that strlen(ss) does not change. This is only true if ss is not changed in for loop.

For example, if you use a read-only function on ss in for loop but don't declare the ss-parameter as const, the compiler cannot even know that ss is not changed in the loop and has to calculate strlen(ss) in every iteration.

Answer 5

Formally yes, strlen() is expected to be called for every iteration.

Anyway I do not want to negate the possibility of the existance of some clever compiler optimisation, that will optimise away any successive call to strlen() after the first one.

Answer 6

Yes.

strlen() calculated everytime when i increases and does not optimized.

Below code shows why the compiler should not optimize strlen().

for ( int i = 0; i < strlen(ss); ++i )
{
   // Change ss string.
   ss[i] = 'a'; // Compiler should not optimize strlen().
}