Thread: algorithm not working like i want it to

Im crap at C++. Could someone tell me why this wont work? I checked it over and over and I still can't see the thing I did wrong. It compiles flawlessly, and does everything like I want it to but one feature.

Code:

#include <iostream>
#include <cstring>
using namespace std;
int search(char *str,char *sub);
int main() {
start:
	int i;
	char str[100],sub[100],exit;
	cout<<"Input data."<<endl;
	gets(str);
	cout<<"Input search."<<endl;
	gets(sub);
	i=search(str,sub);
	if(i) cout<<"Match found, starting from the "<<i<<"th letter of your input."<<endl;
	else cout<<"No match found."<<endl;
	cout<<"Repeat? y/n"<<endl;
	do {
		cin>>exit;
		if(exit=='y') goto start;
	} while(exit!='n');
	return 0;
}
int search(char *str,char *sub) {
	int len=strlen(str),i;
	char *a,*b;
	for(i=0;i<len;i++) {
		a=&str[i];
		b=&sub[i];
		while(*a==*b) {
			a++;
			b++;
		}
		if(!*b) return i+1;
	}
	return 0;
}

(search() is suppose to tell you whether a substring match exists or not) I want the returned 'int i' to provide information on where the match starts. i.e A successful run would be something like:

Input data.
ifureadthisthenurgay
Input search.
read
Match found, starting from the 4th letter of your input.
Repeat? y/n

But when I entered that exact data it gives me

Input data.
ifureadthisthenurgay
Input search.
read
Match found, starting from the 5th letter of your input.
Repeat? y/n

Thanx in advance

It should return 4 using gets() [unless gets() retrieved the newline from the input, in which case it would return 5], but it would be wrong anyway. What is happening is "i" moves through BOTH strings, when you really want it to just move through the "str".

So the idea is to start b back at sub[0] every run. But be sure to check "a" for null while looping, because *a == *b will be true when they both reach their null terminators, and the pointers will just happily fly through unowned memory...

Code:

int search(char *str,char *sub) {
	int len=strlen(str),i;
	char *a,*b;
	for(i=0;i<len;i++) {
		a=&str[ i ];
		b=&sub[ 0 ];
		while(*a && *a==*b) {
			a++;
			b++;
		}
		if(!*b) return i+1;
	}
	return 0;
}

Still, I would complain that first of all, strlen is just double work, it just loops through a string till it reaches a null anyway, so you can discard it completely. Finally, since pointers calculate indexes just as well as a counter, you can replace "i" too.

Code:

int search(char *str,char *sub) 
{
char *a, *b, *c;

    for(a = str; *a; a++) 
  {
    c = a;
    b = sub;
                                 
        while(*a && *a==*b) 
      {
        a++;
        b++;
      }
        if(!*b) 
          return (int)(c - str)+1;
  }
 return 0;
}

Ahhh! What's that goto doing in there?! You really don't need it.

Code:

do
{
//stuff
cout << "Enter 'y' to continue: ";
cin >> exit;
}while(exit == 'y');

See, no reason to use that dreaded goto.