String concatenation using pointers in C

Question

I am concatenating a string (t) at the end of another string (s) in this code but stuck in the use of operators and their precedence in the loop ( while() )

1.while(*s++); doesn't do the work

2.while(*s) ++s; does

But what is the difference between them ?

#include<stdio.h>
void strcat(char *, const char *);

int main(void){
    char s[100] = "Aditya ";
    char t[100] = "Kumar";
    strcat(s, t);
    printf("%s ", s);
    return 0;
}

void strcat(char * s, const char * t){
    while(*s)
        s++;

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

Why in the strcat() function in first while while(*s++); doesn't concatenate the string but while(*s) s++; does I guess they work the same way ?

@JohnnyMopp no, it's not a duplicate of that. Here the issue is incrementing the pointer no matter what, failing the next loop because s if 1 too much. — Jean-François Fabre
– Jean-François Fabre ♦, Commented Oct 11, 2017 at 19:48

Jean-François Fabre · Accepted Answer · 2017-10-11 19:42:51Z

2

they aren't equivalent.

while(*s++);

increments the pointer no matter what, so when *s is \0, the pointer is incremented once more. And the concatenation happens after the nul-terminator. The rest of the buffer contains zeroes so it doesn't crash but it doesn't do the job.

while(*s) { s++; }

stops when *s is \0. So no extra incrementation is performed.

answered Oct 11, 2017 at 19:42

Jean-François Fabre♦

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9 Comments

Jean-François Fabre Over a year ago

To be honest, I answered because it took me a while to figure that out :)

Eugene Sh. Over a year ago

@Jean-FrançoisFabre Your "a while" was approximately 20 seconds to my estimation :P

Aditya kumar Over a year ago

Does it mean that it is a nice question ?

Eugene Sh. Over a year ago

@Adityakumar Let the score speak for it.

wra Over a year ago

It‘s an interesting brain-twister but not a suitable real-world solution for string concatenation because of the missing boundary checking. Only strings that are guaranteed to be null-terminated and not to exceed 100 characters (together!) are allowed. Otherwise memory overrun occurs.

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