Concatenate two string program not giving correct output

Question

#include<stdio.h>
char * sstrcat(char*,char*);
void main() {
    char *c;
    char s[100] = "abcde";
    char t[] = "fghi";
    c = sstrcat(s,t);
    printf("%s",c);
}

char* sstrcat(char *s,char *t) {
    char* temp = s;
    while(*s++ != '\0');
    while((*s++ = *t++) != '\0');

    return temp;
}

Above written code I am getting output abcde but expected output is concatenation of string s and t .

Please help me to figure out what mistake I am doing? thanks.

Answer 1

This line

while(*s++ != '\0');

will increment s after the comparison has been made, leaving '\0' , which is a string terminator in your array.

If you can use a debugger you will find that all values are in your array, its just that printf will stop at '\0'

Answer 2

Just move your string iterator back one char due to '\0' (after you execute while(*s++;= '\0'); ) and that fixes your code.

---

Explanation:

Your s string is "abcde\0" . After the first while loop, the iterator will be at '\0' . If you leave it there you will concatenate both strings obtaining the result "abcde\0fghi\0" which prints "abcde" due to the first '\0' .

Instead, if you move back the s string iterator one position with ( s-- ) you will have this string as result "abcdefghi\0" which prints the string as you expect.

Fixed Code:

#include<stdio.h>

char * sstrcat(char*,char*);

void main() {
    char *c;
    char s[100] = "abcde";
    char t[] = "fghi";
    c = sstrcat(s,t);
    printf("%s\n",c);
}

char* sstrcat(char *s,char *t) {
    char* temp = s;
    while(*s++ != '\0');
    s--;
    while((*s++ = *t++) != '\0');

    return temp;
}