COMP 1917 Computing 1
Session 2, 2015

Tutorial Solutions - Week 7

1. int len_str(char *str){
       int count = 0;
       while(str[count]!='\0'){
           count++;
       }
       return count;
   }
   

2. argc     4
   
   argv[0] "./myprog"
   argv[1] "-a"
   argv[2] "-bc"
   argv[3] "junk"
   

3. void reverse_string( char s[] )
   {
     char ch;
     int  lo=0,hi=0;
   
     while( s[hi] != '\0' ) {
       hi++;
     }
     hi--;            // last character before '\0'
   
     while( lo < hi ) {
       ch    = s[lo]; // swap s[lo] and s[hi]
       s[lo] = s[hi];
       s[hi] = ch;
       lo++;          // lo,hi both move toward middle
       hi--;
     }
   }
   

4. void substr( char s[], char d[], int lo, int hi )
   {
     int i=0, j=0;
   
     // scan to s[lo], or end of s[]
     while( i < lo && s[i] != '\0' ) {
        i++;
     }
     if( i == lo ) {
        // keep copying until s[hi], or end of s[]
        while( i <= hi && s[i] != '\0' ) {
           d[j++] = s[i++];
        }
     }
     // finally, add the end-of-string marker
     d[j] = '\0';
   }
   

5. char *strstr( char s[], char d[] )
   {
      char *return_pointer = NULL;
      int i=0, j;
   
      while( s[i] != '\0' && return_pointer == NULL ) {
         j=0;
         // compare d[] to substring of s[] starting at i
         while( s[i+j] == d[j] && d[j] != '\0' ) {
            j++;
         }
         if( d[j] == '\0' ) {       // substring found
            return_pointer = &s[i]; // pointer to start of substring
         }
         i++; // increment start index
      }
      // if return_pointer is still NULL, the substring was not found
      return( return_pointer );
   }
   

   Note: Later in your studies, you might learn more sophisticated ways of implementing the strstr() function which are faster, such as the Boyer-Moore algorithm and the Knuth-Morris-Pratt algorithm (but these are beyond the scope of this course).