Brute force string matching problem

[sunilr245]

im supposed to write a code for fast string matching for class, i wrote the function find_string, and it worked for random strings i tested on my own but the test code doesnt output the correct answer, can anyone help as to why its not working? thank you

#include <iostream>
#include <stdlib.h>
#include <stdio.h>
#define POSITION 89765


using namespace std;

int length(char const*s){
    int i = 0;
    while(s[i] != '\0'){++i;}
    return i;}

int find_string(char *s, char *t)
{
    int x = length(s);
    int y = length(t);
    int i;

    for(i=0;i<=(x-y);i++){
        int j=0;
        while((j<y) && (s[i+j] == t[j])){
            j++;
        }
        if(j==y){return i;}
    }
    return -1;
}




int main(void)

   {  char sequence[100001];
   char pattern[1000];
   int i, j;
   for(i=0; i<100000; i++)
      sequence[i] = 'a';
   sequence[100000]='\0';
   for(i=0; i<4000; i++)
   {  j = rand()%100000;
      sequence[j]='b';
   }
   for(j=0; j< 1000; j++)
      pattern[j] = sequence[POSITION+j];
   pattern[1000] = '\0';
   if(find_string(sequence, pattern) == POSITION )
     printf("accepted\n");
   else
     printf("needs check?\n");
}

[weaknessforcats]

You don't need to know the lengths of the strings in order to compare them.

Just start off comparing the first character of each string and make the following test (assumes you have a pointer to each string):

if ((*strA == *strB) && (*strA != '\0')) then advance to compare the next character of each string.

Otherwise,

if (*strA < *str2) then string strA is less then string strB
if (*strA > *strB) then strA is greater than string strB.

[sunilr245]

so what would my end condition for my for loop be if i dont know the length of the strings?

[weaknessforcats]

The loop runs until either *strA or *strB is \0.

while ((*strA == *strB) && (*strA != '\0'))
{
    ++strA;
    ++strB;
}
//At this point you evaluate
//One or both of strA and strB is \0

if (*strA == *strB) return 0;  //the strings are equal
if (*strA < *str2)  return -1; //strA is less then string strB
return 1;  //strA s greater than strB

[sunilr245]

i need the funtion to return the first position at which the substring starts. for example: if s = 'example' and t = 'ample'
the function should return 2 and it should return -1 if there are no substrings of s

[weaknessforcats]

OK. So use the loop in the example to locate a single matching character between the two strings. If there is one, then inside the loop compare starting at that location for the length of the substring (the mask in this case):

while (*strA == *strB)
 {
     /* potential substring starts at strA  */
     if (!strcmp(strA, strB))
     {
         /*then your substring starts a strA */ 
     }
     ++strA;
     }
/* if you get here no sub string was found  */