K&R C Programs Exercise 5-4 - C Program Examples

Exercise 5-4. Write the function strend(s,t), which returns 1 if the string t occurs at the end of the string s, and zero otherwise.

Advance both pointers to their respective null terminators, then walk backwards together comparing characters. If t runs out first (pointer reaches the start of t), every character matched — t is a suffix of s. If s runs out first, s is shorter than t and the answer is no.

Solution

/* K&R Exercise 5-4 — strend: does t occur at the end of s?
 * Compile: gcc -ansi -Wall ex5-4.c -o ex5-4 */
#include <stdio.h>

int strend(const char *s, const char *t)
{
    const char *ps = s, *pt = t;

    while (*ps) ps++;   /* advance to end of s */
    while (*pt) pt++;   /* advance to end of t */

    /* Walk backwards together while characters match */
    while (pt > t && ps > s && *--ps == *--pt)
        ;

    return *ps == *pt && pt == t;
}

int main(void)
{
    printf("%d\n", strend("hello world", "world"));  /* 1 */
    printf("%d\n", strend("hello world", "hello"));  /* 0 */
    printf("%d\n", strend("hello", "hello"));        /* 1 */
    printf("%d\n", strend("hello", ""));             /* 1 — empty string */
    printf("%d\n", strend("hi", "hello"));           /* 0 — t longer than s */
    printf("%d\n", strend("", ""));                  /* 1 */
    return 0;
}

Why the Final Test Is `pt == t`

When the loop exits, either:

pt == t — all of t was matched; return 1 if the last compared characters also matched (*ps == *pt)
ps == s — ran out of s before exhausting t; t is longer than s, return 0
Characters differed — return 0

The condition *ps == *pt && pt == t handles all three cases correctly, including the empty string edge case (both pointers at their respective starts with matching null terminators).

Compile and Run

gcc -ansi -Wall ex5-4.c -o ex5-4 && ./ex5-4

Sample Output

What This Exercise Teaches

Pointer comparison for bounds checking — pt > t and ps > s detect when a pointer has walked back to the start; pointer comparison within the same array is well-defined
Walking backwards — advance to end, then decrement; a common pattern for suffix/palindrome problems
Empty string edge case — every string ends with the empty string; the condition handles this naturally because both pointers start and end at t

Set Up Your C Environment

← Exercise 5-3 |
Chapter 5 Solutions |
Exercise 5-5 →

Book: The C Programming Language, 2nd Ed — Kernighan & Ritchie