How would you write a user defined function to compare two strings without using the library function?

Answer 1

/* ellipses, ..., used to emulate indentation */

int strcmp (const char* s1, const char* s2) {

... while (s1 != NULL && s2 != NULL) { /* scan both strings */

... ... if (*s1 < *s2) return -1; /* first string less */

... ... if (*s1 > *s2) return 1; /* first string greater */

... ... s1++;

... ... s2++;

... }

... if (s1 == s2) return 0; /* strings equal or both empty */

... if (s1 != NULL) return 1 else return -1; /* first string longer(1) / shorter(-1) */

}

A slightly less computation-intensive version is:

int strcmp(const char* s1, const char* s2)

{

... if (s1==s2)

... ... return 0;

... while (*s1)

... ... if (*s1++^*s2++)

... ... ... if (--s1<--s2)

... ... ... ... return -1;

... ... ... else

... ... ... ... return 1;

... return 0;

}

Answer 2

#include <stdio.h>

/* my_strcmp has functionality equivalent to standard library strcmp */

int my_strcmp(const char *str1, const char *str2)

{ do {

char diff = *str1 - *str2;

if (diff != 0) return diff;

} while (*str1++ && *str2++);

return 0;

}

int main(void)

{

printf("%d\n", my_strcmp("ALA", "ALA"));

printf("%d\n", my_strcmp("AL", "ALA"));

printf("%d\n", my_strcmp("ALB", "ALA"));

return 0;

}