frequency_analysis

# Crypto Analysis: Frequency Analysis
#
# To analyze encrypted messages, to find out information about the possible 
# algorithm or even language of the clear text message, one could perform 
# frequency analysis. This process could be described as simply counting 
# the number of times a certain symbol occurs in the given text. 
# For example:
# For the text "test" the frequency of 'e' is 1, 's' is 1 and 't' is 2.
#
# The input to the function will be an encrypted body of text that only contains 
# the lowercase letters a-z. 
# As output you should return a list of the normalized frequency 
# for each of the letters a-z. 
# The normalized frequency is simply the number of occurrences, i, 
# divided by the total number of characters in the message, n.

def freq_analysis(message):
    """
    Takes an input string and returns the frequency occurance 
    of each letter a-z in the string
    """
    # define a message length and a pre-build list
    length = len(message)
    freq_list = [0 for i in range(26)]
    
    # first count each occurance of each letter and store in list
    for each in message:
        freq_list[ord(each)-97] = freq_list[ord(each)-97] + 1
    # then analyze the frequency by dividing through the message length
    for each in freq_list:
        if each != 0:
            freq_list[freq_list.index(each)] = each * 1.0 / length
    return freq_list
    



#Tests

print freq_analysis("abcd")
#>>> [0.25, 0.25, 0.25, 0.25, 0.0, ..., 0.0]

print freq_analysis("adca")
#>>> [0.5, 0.0, 0.25, 0.25, 0.0, ..., 0.0]

print freq_analysis('bewarethebunnies')
#>>> [0.0625, 0.125, 0.0, 0.0, ..., 0.0]