Thread: strlen function help

Code:

int mystrlen( char *s )
{
    char *p = s; /* make p point to s */
    while( p && *p ) /* while p is not null, and *p is not \0 */
        p++; /* increment p */
    return p - s; /* return the difference between p and s */
}

Pointers are values. If you have a pointer pointing to the start of an array (s) and you have a pointer pointing further along in that same array (p), then the difference between (p) and (s) is how far along the array you are.

Code:

p = s;
p++;

if( p - s == 1 )
    printf( "p is 1 element along the array" );

So, if the loop fails right off, then p and s will be pointing to the same. So, p - s will be zero. If we advance P one spot, then p - s will be 1. And so on.

As for the recursive function:

Code:

check to see if s is NULL, and not '\0'
    if it's not
        return 1 (counting that it's a valid non-nul character) + whatever further recursive calls return

So, if we have a string that is 2 characters long, not counting the nul:

Code:

if s and *s
    return 1 (the first character) + (recursive call, points to the next character)
        if s and * s
            return 1 + recursive call now will be \0
            if s and *s ..fails because it's now \0, so it returns 0

So, 0 + 1 + 1 = 2.


Quzah.

I was just illustrating that it was recursing there. That wouldn't actually be in the function itself. Here is the difference between NULL and a nul character:

Code:

char *p = NULL;
char c = '\0';

The pointer p is set to "point nowhere". The character c contains a nul character, meaning the value is zero. If we do this:

Code:

p = &c;

Then the pointer p is actually pointing some place (it's pointing at the character c, whose value is nul). So, if we did:

Code:

if( p == NULL )
    printf( "this won\'t print because the pointer actually points some place" );
else
if( *p == '\0' )
    printf( "this will print because the value of c is nul" );
else
    printf( "this won\'t print because the value of c is nul" );

Edit: The nul character (or terminator) is generally used to indicate the end of strings.


Quzah.