Thread: What's wrong with my code?

Strings in C need to be null terminated. Since you overwrite the first null terminator, you get to add on to your string; so that's fine. BUT, you need to put the null terminator back on the end of your string when you are finished.

Also, don't ever use 'gets', it's a very common cause of buffer overflows - use 'fgets' instead. And you can probably do away with the stdafx.h include.

strlen() returns the number of char's in the string *NOT COUNTING* the end of string char ('\0').

So your program is overwriting the end of string char, and now you don't have a string (or two or three), you have just a long bunch of char's, with no strings.

Try (i+k+1), and see how that does.

That should take care of the first and second strings. But it won't take care of the third one. Your loop resets i to zero, and k is still set at the strlen(), so the third string will overwrite, the second string.

You can do what you want with one loop, but you need to introduce another variable which will set up the third string's write to the char array s. I prefer to use two nested loops. Something like:

Code:

for(i = 0; i < 3; i++) {  
   n = i * k + 1;
   for(j = n; j < 3 * n; j++) {
      //etc.

Code:

int main()
{
   	FILE *f;
	char s[1000];
	int k, i;
		
	gets(s);
	k = strlen(s);
	
	f=fopen("output.txt","w");
	if (!f)
		return 1;
	for (i=0; i<=k; i++)
	{
		s[i+k]=s[i];
	}
	fprintf(f,"%s\n",s);
	fclose(f);
	return 0;
}

This is all off the cuff, and not meant as a running program snippet. I'm sure you'll find a better way to do it, in a bit.

Welcome to the forum - if you're still stumped, answer back and I'll take it farther.

I think tabstop's suggestion should work in this case. Just keep the loop as is and null terminate when done. Incidentally, this should really be a function (think: 'strcat'). That clarifies the intent of the program, besides making it more modular. Just seek to the null terminator of the string, write the new characters, and then add a new terminator. Making sure that there is enough room beforehand, of course.