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string comparison

summary

    This subchapter looks at string comparison.

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string comparison

    This subchapter looks at string comparison.

are two strings the same?

    This example comes from a discussion on Google+ Programming Circle. The discusssion was about how to implement the standard C string fucntions without using the standard C library. Stéphane Zuckerman provided the following C code from an actual standard C library implementation.

    Note that there are still two corrections needed to this routine, but the corrections are left to the student as an exercise.

int strcmp(const char* l, const char * r) {
    while(*l && *l++ == *r++)
        ;
    return (const unsigned char)l - *(const unsigned char)r;
}

    The variables l and r are pointers to the beginning of the two strings (left and right). The official C standard uses the names str1 and str2.

    The first short circuit test for the while loop is to determine if the current character pointed to by the pointer variable l equals zero or not. The standard C end of string marker is zero (all zero bits). We stop the test if the first string has ended. What happens if the second (r) string is longer? This code will claim that the two strings are equal. How do you fix this?

    There is no corresponding test for the end of the second strng (r). The author of this code pointed out that “The beauty of this is also that you don’t need to test for both *l and *r in the while : either *l is done, and the comparison should stop; or it’s not, and you should continue comparing the strings — if *r is shorter than *l, then *r = '\0', which is necessarily different from the current value of *l”

    The second part of the while test is to see if the strings have the same characters (equaiity) at each position of the strings (character arrays).

    Notice that there is no work inside the while loop. All of the work is done by the while condition tests.

    The standard for the official C routine is to return zero if the strings are equal, return a value greater than zero (positive number) if the first string is longer and return a dvalue less than zero (negative number) if the second string is longer. The normal case is to return one, zero, or negative one. Some implementations will return an integer that indicates not just which string is longer, but how much it is longer. This is a non-standard implementation and any software that depends on this extra information will break horribly when ported to another system.

    There is an error in the return value of the sample code. Can you spot the error and correct it?

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