title: EDX set 5 - caesars cipher
body: <p>I am doing edx problem set 5 and I stumbled upon a problem in my code:</p>
  
  <pre><code># 6.00x Problem Set 5
  #
  # Part 1 - HAIL CAESAR!
  
  import string
  import random
  
  WORDLIST_FILENAME = "words.txt"
  
  # -----------------------------------
  # Helper code
  # (you don't need to understand this helper code)
  def loadWords():
      """
      Returns a list of valid words. Words are strings of lowercase letters.
  
      Depending on the size of the word list, this function may
      take a while to finish.
      """
      print "Loading word list from file..."
      inFile = open(WORDLIST_FILENAME, 'r')
      wordList = inFile.read().split()
      print "  ", len(wordList), "words loaded."
      return wordList
  
  def isWord(wordList, word):
      """
      Determines if word is a valid word.
  
      wordList: list of words in the dictionary.
      word: a possible word.
      returns True if word is in wordList.
  
      Example:
      &gt;&gt;&gt; isWord(wordList, 'bat') returns
      True
      &gt;&gt;&gt; isWord(wordList, 'asdf') returns
      False
      """
      word = word.lower()
      word = word.strip(" !@#$%^&amp;*()-_+={}[]|\\:;'&lt;&gt;?,./\"")
      return word in wordList
  
  def randomWord(wordList):
      """
      Returns a random word.
  
      wordList: list of words  
      returns: a word from wordList at random
      """
      return random.choice(wordList)
  
  def randomString(wordList, n):
      """
      Returns a string containing n random words from wordList
  
      wordList: list of words
      returns: a string of random words separated by spaces.
      """
      return " ".join([randomWord(wordList) for _ in range(n)])
  
  def randomScrambled(wordList, n):
      """
      Generates a test string by generating an n-word random string
      and encrypting it with a sequence of random shifts.
  
      wordList: list of words
      n: number of random words to generate and scamble
      returns: a scrambled string of n random words
  
      NOTE:
      This function will ONLY work once you have completed your
      implementation of applyShifts!
      """
      s = randomString(wordList, n) + " "
      shifts = [(i, random.randint(0, 25)) for i in range(len(s)) if s[i-1] == ' ']
      return applyShifts(s, shifts)[:-1]
  
  def getStoryString():
      """
      Returns a story in encrypted text.
      """
      return open("story.txt", "r").read()
  
  
  # (end of helper code)
  # -----------------------------------
  
  
  #
  # Problem 1: Encryption
  #
  def buildCoder(shift):
      """
      Returns a dict that can apply a Caesar cipher to a letter.
      The cipher is defined by the shift value. Ignores non-letter characters
      like punctuation, numbers and spaces.
  
      shift: 0 &lt;= int &lt; 26
      returns: dict
      """
      dict={}
      upper = string.ascii_uppercase
      lower = string.ascii_lowercase
      for l in range(len(upper)):
          dict[upper[l]] = upper[(l+shift)%len(upper)]
      for l in range(len(lower)):
          dict[lower[l]] = lower[(l+shift)%len(lower)]
      return dict
  
  
  def applyCoder(text, coder):
      """
      Applies the coder to the text. Returns the encoded text.
  
      text: string
      coder: dict with mappings of characters to shifted characters
      returns: text after mapping coder chars to original text
      """
      new_text=''
      for l in text:
          if not(l in string.punctuation or l == ' ' or l in str(range(10))):
             new_text += coder[l]
          else:
             new_text += l            
      return new_text   
  
  def applyShift(text, shift):
      """
      Given a text, returns a new text Caesar shifted by the given shift
      offset. Lower case letters should remain lower case, upper case
      letters should remain upper case, and all other punctuation should
      stay as it is.
  
      text: string to apply the shift to
      shift: amount to shift the text (0 &lt;= int &lt; 26)
      returns: text after being shifted by specified amount.
      """
      ### TODO.
      ### HINT: This is a wrapper function.
      coder=buildCoder(shift)
      return applyCoder(text,coder)
  
  #
  # Problem 2: Decryption
  #
  def findBestShift(wordList, text):
      """
      Finds a shift key that can decrypt the encoded text.
  
      text: string
      returns: 0 &lt;= int &lt; 26
      """
      ### TODO
      wordsFound=0
      bestShift=0
  
      for i in range(26):
          currentMatch=0
          encrypted=applyShift(text,i)
          lista=encrypted.split(' ')
          for w in lista: 
              if isWord(wordList,w):
                  currentMatch+=1
          if currentMatch&gt;wordsFound:
                  currentMatch=wordsFound
                  bestShift=i
      return bestShift
  
  def decryptStory():
      """
      Using the methods you created in this problem set,
      decrypt the story given by the function getStoryString().
      Use the functions getStoryString and loadWords to get the
      raw data you need.
  
      returns: string - story in plain text
      """
      text = getStoryString()
      bestMatch = findBestShift(loadWords(), text)
      return applyShift(text, bestMatch)
  
  #
  # Build data structures used for entire session and run encryption
  #
  
  if __name__ == '__main__':
      wordList = loadWords()
      decryptStory()
  
  s = 'Pmttw, ewztl!'
  print findBestShift(wordList, s)
  
  print decryptStory()
  </code></pre>
  
  <p>The problem is that the single module of the program works apart from decrypt story. What is the problem in that piece of code?</p>