text: #!/usr/bin/python2.4 -tt
  # Copyright 2010 Google Inc.
  # Licensed under the Apache License, Version 2.0
  # http://www.apache.org/licenses/LICENSE-2.0
  
  # Google's Python Class
  # http://code.google.com/edu/languages/google-python-class/
  
  # Additional basic string exercises
  
  # D. verbing
  # Given a string, if its length is at least 3,
  # add 'ing' to its end.
  # Unless it already ends in 'ing', in which case
  # add 'ly' instead.
  # If the string length is less than 3, leave it unchanged.
  # Return the resulting string.
  def verbing(s):
    length_of_string = len(s)
    if length_of_string < 3:
      return s
    elif s[-3:] == 'ing':
      return s + 'ly'
    else:
      return s + 'ing'
    # +++your code here+++
  
  
  # E. not_bad
  # Given a string, find the first appearance of the
  # substring 'not' and 'bad'. If the 'bad' follows
  # the 'not', replace the whole 'not'...'bad' substring
  # with 'good'.
  # Return the resulting string.
  # So 'This dinner is not that bad!' yields:
  # This dinner is good!
  def not_bad(s):
    n = s.find('not')
    b = s.find('bad')
    if n != -1 and b != -1 and b > n: # if could not find the word in the string, .find() returns a -1 value.
      s = s[:n] + 'good' + s[b+3:] # just modify the original string is fine here~~~~~
    # +++your code here+++
    return s
  
  
  # F. front_back
  # Consider dividing a string into two halves.
  # If the length is even, the front and back halves are the same length.
  # If the length is odd, we'll say that the extra char goes in the front half.
  # e.g. 'abcde', the front half is 'abc', the back half 'de'.
  # Given 2 strings, a and b, return a string of the form
  #  a-front + b-front + a-back + b-back
  def define_front_back(s):
    if len(s)%2 == 0:
      return len(s)/2
    else:
      return int(len(s)/2) + 1 # because the index starts from 0 in string... but why????????
  
  def front_back(a, b):
    a_front = a[:define_front_back(a)]
    a_back = a[define_front_back(a) :]
    b_front = b[:define_front_back(b)]
    b_back = b[define_front_back(b):]
    # +++your code here+++
    return a_front + b_front + a_back + b_back
  
  
  # Simple provided test() function used in main() to print
  # what each function returns vs. what it's supposed to return.
  def test(got, expected):
    if got == expected:
      prefix = ' OK '
    else:
      prefix = '  X '
    print '%s got: %s expected: %s' % (prefix, repr(got), repr(expected))
  
  
  # main() calls the above functions with interesting inputs,
  # using the above test() to check if the result is correct or not.
  def main():
    print 'verbing'
    test(verbing('hail'), 'hailing')
    test(verbing('swiming'), 'swimingly')
    test(verbing('do'), 'do')
  
    print
    print 'not_bad'
    test(not_bad('This movie is not so bad'), 'This movie is good')
    test(not_bad('This dinner is not that bad!'), 'This dinner is good!')
    test(not_bad('This tea is not hot'), 'This tea is not hot')
    test(not_bad("It's bad yet not"), "It's bad yet not")
  
    print
    print 'front_back'
    test(front_back('abcd', 'xy'), 'abxcdy')
    test(front_back('abcde', 'xyz'), 'abcxydez')
    test(front_back('Kitten', 'Donut'), 'KitDontenut')
  
  if __name__ == '__main__':
    main()
  