Question

I have to write fast right-trimming function in C. My first attempt to do it was:

void TrimRight( char *s )
{
    char* pS;
    if (!*s)
        return;
    if (!s)
        return;
    for ( pS = s + strlen(s) - 1; isspace(*pS) && (pS >= s); pS--)
    {  
        pS[1] = '\0';
    }
}

Second attempt:

void TrimRight( char *s )
{   
    if (!*s)
        return;
    if (!s)
        return;
    char* pS = s + strlen(s) - 1;
    while ((*pS == ' ') && (pS > s))
    {
        (*pS--) = '\0';
    }
}

The question is: Did I improve memory accesses here? How can I optimize this even more? How to optimize memory access in general?

Upd: Is it true that scanning memory in increasing sequential order is faster?

Answer 1

I wouldn’t bother with the tests at the beginning. It should be a requirement of the function that a valid string be passed, which obviates the !s test, and the !*s is redundant (Besides, placing the !s test second means you’ll crash first on the !*s). Also, you can avoid scanning in both directions (the strlen scans one way, the while loop goes the other way) by keeping track of the last non-whitespace character:

char* TrimRight(char *s)
{
    // ws tracks the char after the last non-whitespace char.
    char* ws = s;
    while (*s)
        if (s++ != ' ') // or !isspace(s++)
            ws = s;
    *ws = '\0';
    return ws; // Knowing where the trimming occurred might come in handy.
}

EDIT: Note that strlen might be faster than scanning for spaces (see comments).