查找字符串中给定子字符串的出现次数

Question

The below code always returns number of matching sub strings as zero.There are no errors in the code and i am not sure where have i gone wrong logically.

#include<stdio.h>
#include<string.h>

int main()
{ 
    int i,j,len ,k ,count, num ;
    char str[100],sub[100],comp[100] ; 
    // sub is the sub string .
    printf("Please enter the string") ; 
    gets(str) ;
    printf("Enter the substring to be searched for") ;
    gets(sub) ;
    len=strlen(sub) ;
    for ( i=0 ; i < strlen(str) - len ; i++ ) 
    //Goes till length of string - length of sub string so that all characters can be compared.
      {  
         num = i + len ;
         for ( j=i,k=0 ; j<num ; j++, k++ )
         //Loop to store each sub string in an array comp.
           {
             comp[k]=str[j] ;
           }
         if ( strcmp(comp,sub) == 0 )
            { count++ ; }
     }
    printf("no of occurances is:%d",count) ;
    return 0 ;
}  

Answer 1

As mentioned in the comments, when constructing comp , you're not adding a terminating null byte at the end. Because the rest of comp is not initialized, you invoke undefined behavior when calling strcmp .

Add the null byte at the end of the inner for loop will fix the problem:

     for ( j=i,k=0 ; j<num ; j++, k++ )
     //Loop to store each sub string in an array comp.
       {
         comp[k]=str[j] ;
       }
     comp[k] = '\0';

Actually, rather than creating a separate substring, just use strncmp , which compares up to a certain number of characters:

for ( i=0 ; i < strlen(str) - len ; i++ ) 
//Goes till length of string - length of sub string so that all characters can be compared.
  {  
     if ( strncmp(&str[i],sub,strlen(sub)) == 0 )
        { count++ ; }
 }

Also, don't use gets , as that is prone to buffer overflows. Use fgets instead.

Answer 2

• Try changing your for loop from this :

   for ( i=0 ; i < strlen(str) - len ; i++ ) 

  to

   for ( i=0 ; i <= strlen(str) - len ; i++ )