Pointer version of strcat

Write a pointer version of the function strcat that we showed in Chapter 2: strcat(s, t) copies the string t to the end of s.

This is the version of strcat from Chapter 2:

void strcat(char s[], char t[]) {
    int i, j;

    i = j = 0;
    while(s[i] != '\0') /* find the end of s */
        i++;

    while((s[i++] = t[j++]) != '\0') /*  copy t */
        ;
}

Here is my solution:

void custom_strcat(char *s, char *t) {
    while(*s) /* finding the end of the string */
        s++;

    while((*s++ = *t++)) /* copy t */
        ;
}

There are 2 while loops in my function, first loop will run until *s is equal to '\0'. At each iteration the value of s is incremented - thus, it will point to the next element;

The second loop will copy the elements of the string t at the and of s. The incrementation is done in the test part - even though after the loop stops s and t will point to other objects.This technique couldn't be applied in the first loop because when the loop stops s will point to an irrelevant value.

The exercise can be found at page 121 in K&R second edition.

It looks idiomatic and bug-free.

s++;

Perhaps ++s as a habit unless postfix s++ is required (as it is during the copy).

while((*s++ = *t++))

Could have just a single pair of parentheses.

void custom_strcat(char *s, char *t)

The signature of strcat is usually char strcat(char destination, const char* source). The const means that the source buffer is not modified. The return code is supposed to be a pointer to the original (unmodified) source value, so to implement this the implementation would need to increment a local variable copy of destination.


This technique couldn't be applied in the first loop because when the loop stops s will point to an irrelevant value.

You could write that as, if (s) while ((++s));.

StackExchange Code Review Q#40616, answer score: 10