class StringLooper {

  /** Return how many occurrences of `target` occur in `s`.
   * @param s the string to search through
   * @param target the char to count.
   * @return how many occurrences of `target` occur in `s`.
   */
  int countOccurences( String s, char target ) {
    int seenSoFar = 0;
    int i=0;

    while (i < s.length()) {
      if (s.charAt(i) == target) {
         seenSoFar = seenSoFar + 1;
      }
      // else do nothing.

      i = i + 1;
    }
    return seenSoFar;
  }

  int charToInt( char digit ) {
    return Integer.parseInt( Character.toString(digit) );
  }

  boolean isValidISBN10( String isbn ) {
    int checkSoFar = 0;
    for (int i=0;  i<isbn.length();  ++i) {
        int charValue = (isbn.charAt(i) == 'X') ? 10 : charToInt(isbn.charAt(i));
        checkSoFar += charValue*(i+1);
    }
    checkSoFar %= 11;
    return checkSoFar == 0;
/*    char lastChar = isbn.charAt(isbn.length()-1);
    if (lastChar=='X') {
      return checkSoFar==10;
    }
    else {
      return checkSoFar == charToInt( isbn.charAt(isbn.length()-1) );
    }
    */
  }


}