text: #!/usr/bin/python -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/
  
  """Wordcount exercise
  Google's Python class
  
  The main() below is already defined and complete. It calls print_words()
  and print_top() functions which you write.
  
  1. For the --count flag, implement a print_words(filename) function that counts
  how often each word appears in the text and prints:
  word1 count1
  word2 count2
  ...
  
  Print the above list in order sorted by word (python will sort punctuation to
  come before letters -- that's fine). Store all the words as lowercase,
  so 'The' and 'the' count as the same word.
  
  2. For the --topcount flag, implement a print_top(filename) which is similar
  to print_words() but which prints just the top 20 most common words sorted
  so the most common word is first, then the next most common, and so on.
  
  Use str.split() (no arguments) to split on all whitespace.
  
  Workflow: don't build the whole program at once. Get it to an intermediate
  milestone and print your data structure and sys.exit(0).
  When that's working, try for the next milestone.
  
  Optional: define a helper function to avoid code duplication inside
  print_words() and print_top().
  
  """
  
  import sys
  
  # +++your code here+++
  # Define print_words(filename) and print_top(filename) functions.
  # You could write a helper utility function that reads a file
  # and builds and returns a word/count dict for it.
  # Then print_words() and print_top() can just call the utility function.
  
  def print_words(filename):
    word_count = parse_file(filename)
    words = sorted(word_count.keys())
    for word in words:
      print (word + " -> " + str(word_count[word]))
  
  
  def print_top(filename):
    word_count = parse_file(filename)
    items = sorted(word_count.items(), key = sort_count, reverse = True)
  
    for item in items[:20]:
      print (str(item[0]) + " -> " + str(item[1]))
  
  def parse_file(filename):
    word_count = {}
    input_file = open(filename, "r")
    for line in input_file:
      words = line.split()
      for word in words:
        word = word.lower()
        if not word in word_count:
          word_count[word] = 1  # Sets the value for that word to 1
        else:
          word_count[word] += 1 # Increases the count for that word by 1
    input_file.close()
    return word_count
  
  def sort_count(word_count_tuple):
    return word_count_tuple[1]  # return the value for custom sorting
  
  
  
  # This basic command line argument parsing code is provided and
  # calls the print_words() and print_top() functions which you must define.
  def main():
    if len(sys.argv) != 3:
      print ('usage: ./wordcount.py {--count | --topcount} file')
      sys.exit(1)
  
    option = sys.argv[1]
    filename = sys.argv[2]
    if option == '--count':
      print_words(filename)
    elif option == '--topcount':
      print_top(filename)
    else:
      print ('unknown option: ' + option)
      sys.exit(1)
  
  if __name__ == '__main__':
    main()
  