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Discussion on: Custom endsWith and startsWith Functions

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Replies for: You should be passing char const* around; you should be using size_t, not int. Calculating strlen() in advance is more work that necessary: just it...

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rivea0 profile image
Eda •

Thank you for pointing these out. I only have a very basic understanding of C from a general introductory course, so I should've probably not even attempted writing about it in the article.

I guess ends_with() could also work like this:

bool ends_with(char const *string, char const *target) {
  size_t diff = strlen(string) - strlen(target);
  return strcmp(string + diff, target) == 0;
}
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But, I'm not sure how I could avoid using strlen() here, and do it in a similar way to your starts_with().

I'll update the article later on, and also would like to apologize for being quick to write a C example while still being quite the beginner, but, lesson learned. Thank you.

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pauljlucas profile image
Paul J. Lucas • • Edited

size_t diff = strlen(string) - strlen(target);

Nope. size_t is an unsigned type. If target is longer than string, you'll end up with a very large positive number.

For ends_with(), the simplest solution does use strlen():

bool ends_with( char const *s, char const *t ) {
    size_t const s_len = strlen( s );
    size_t const t_len = strlen( t );
    return t_len <= s_len && strcmp( s + s_len - t_len, t ) == 0;
}
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However, that's slightly inefficient. If t_len is much longer than s_len, then you've wasted time traversing to the end of t. This version is more efficient:

bool ends_with( char const *s, char const *t ) {
    char const *const t0 = t;
    for ( ; *t != '\0'; ++s, ++t ) {
        if ( *s == '\0' )
            return false;
    }
    for ( ; *s != '\0'; ++s )
        ;
    size_t const t_len = (size_t)(t - t0);
    return strcmp( s - t_len, t0 ) == 0;
}
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You're determining t_len while checking to see if you've gone past the end of s: if you have, then s can't end with t and you can just stop immediately. After, you scan for the end of s. Once you find it, then finally check for a match.

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rivea0 profile image
Eda •

The first example made perfect sense, but I struggle with for ( ; s[ t_len ] != '\0'; ++s ); line from the second one.

So, in the second version, with an example string being "hey" and target being "ey", t_len would be 2, if I understand it right. Since "ey" is not longer than "hey", we don't return false immediately. But, doesn't incrementing s until the length of target in the first for loop mean that s is now only at the last character "y". So, s[t_len] confused me.

Also, is s0 needed since it's not used here?

Sorry for asking noob questions, I'm surely missing a lot and confusing myself but trying to understand it, now even regret writing the example in the first place. Thank you for your time and patience.

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pauljlucas profile image
Paul J. Lucas •

You are correct: s0 is not needed. (It was left over from an earlier version.) You've also found a bug. (Even "simple" code like this can be tricky!) I've edited the code with a corrected version.

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rivea0 profile image
Eda •

Thank you, again, for your help. I updated the article with the correct versions.