C programming related problem(beginner)

The following line of code checks to see if the current character is the null terminator. A null terminator indicated the End Of String.

str[j] != '\0'

This is usually used in a while loop to check if the last charecter of the string has been reached. If it is the last char, the loop will terminate.

Check this resource: http://www.cprogramming.com/tutorial/c/lesson9.html

I assume you want to remove characters common to both strings. You could do this:

int i=0, j=0, index=0;
char tmp;

while(str1[i] != '\0')
{
tmp = str[i];
    while(str2[j] != '\0')
    { 
         if(  tmp != str2[j] ) 
             { 
                  
                result[pointer] = tmp;     // Copy the non common charecter to result array
                pointer++;                 // Increment pointer

             }   
     j++;
    }
i++;
}