Chapter 20 – Frequency Analysis 305

5. 
   


6. 
   

   frequency taken from http://en.wikipedia.org/wiki/Letter_frequency

   
   


7. englishLetterFreq = {'E': 12.70, 'T': 9.06, 'A': 8.17, 'O': 7.51, 'I':
   6.97, 'N': 6.75, 'S': 6.33, 'H': 6.09, 'R': 5.99, 'D': 4.25, 'L': 4.03, 'C':
   2.78, 'U': 2.76, 'M': 2.41, 'W': 2.36, 'F': 2.23, 'G': 2.02, 'Y': 1.97, 'P':
   1.93, 'B': 1.29, 'V': 0.98, 'K': 0.77, 'J': 0.15, 'X': 0.15, 'Q': 0.10, 'Z':
   0.07}


8. ETAOIN = 'ETAOINSHRDLCUMWFGYPBVKJXQZ'


9. LETTERS = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ'


10. 
    


11. 
    


12. 
    


13. def getLetterCount(message):


14. 
    

    Returns a dictionary with keys of single letters and values of the

    
    


15. 
    

    count of how many times they appear in the message parameter.

    
    


16. letterCount = {'A': 0, 'B': 0, 'C': 0, 'D': 0, 'E': 0, 'F': 0, 'G': 0,
    'H': 0, 'I': 0, 'J': 0, 'K': 0, 'L': 0, 'M': 0, 'N': 0, 'O': 0, 'P': 0, 'Q': 0,
    'R': 0, 'S': 0, 'T': 0, 'U': 0, 'V': 0, 'W': 0, 'X': 0, 'Y': 0, 'Z': 0}


17. 
    


18. for letter in message.upper():


19. if letter in LETTERS:


20. letterCount[letter] += 1


21. 
    


22. return letterCount


23. 
    


24. 
    


25. def getItemAtIndexZero(x):


26. return x[0]


27. 
    


28. 
    


29. def getFrequencyOrder(message):


30. 
    

    Returns a string of the alphabet letters arranged in order of most

    
    


31. 
    

    frequently occurring in the message parameter.

    
    


32. 
    


33. 
    

    first, get a dictionary of each letter and its frequency count

    
    


34. letterToFreq = getLetterCount(message)


35. 
    


36. 
    

    second, make a dictionary of each frequency count to each letter(s)

    
    


37. 
    

    with that frequency

    
    


38. freqToLetter = {}


39. for letter in LETTERS:


40. if letterToFreq[letter] not in freqToLetter:


41. freqToLetter[letterToFreq[letter]] = [letter]


42. else:


43. freqToLetter[letterToFreq[letter]].append(letter)


44.