Python for Research - HarvardX - Case Study 1 Solution

Introduction to Problem from HarvardX

A cipher is a secret code for a language. In this case study, we will explore a cipher that is reported by contemporary Greek historians to have been used by Julius Caesar to send secret messages to generals during times of war.

What is a Ceaser Cipher?

The Caesar cipher shifts each letter of a message to another letter in the alphabet located a fixed distance from the original letter. If our encryption key were 1, we would shift h to the next letter i, i to the next letter j, and so on. If we reach the end of the alphabet, which for us is the space character, we simply loop back to a. To decode the message, we make a similar shift, except we move the same number of steps backwards in the alphabet.

So, we now know about our topic and what is a Ceaser Cipher. We should read the exercises and go on with solutions.

Exercises

Exercise 1

In this exercise, we will define the alphabet used in the cipher.

• The string library has been imported. Create a string called alphabet consisting of the space character ' ' followed by (concatenated with) the lowercase letters. Note that we’re only using the lowercase letters in this exercise.

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import string
# write your code here!
alphabet = " " + string.ascii_lowercase

The above code creates a string called alphabet as intended.

What is the string.ascii_lowercase part and why does it work?

string.ascii_lowercase allows us to create a string that has English lowercase letters. There are also string.ascii_letters, which returns all English letters, and string.ascii_uppercase, which returns all letters in uppercase.

Exercise 2

In this exercise, we will define a dictionary that specifies the index of each character in alphabet.

• alphabet has already defined in the last exercise. Create a dictionary with keys consisting of the characters in alphabet and values consisting of the numbers from 0 to 26. Store this as positions.

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# write your code here!
positions = {}

for idx, letter in enumerate(alphabet):
  positions[letter] = idx

Works easily. However, how should we interpret this solution?

What is enumerate() and what does it do?

As you know, we need both indices and values, which leads to the question: How to do this in Python? And here comes the enumerate() built-in function. When enumerate() function returns too loopable (?) variables:

1. The index (place of the letter)
2. The value (letter)

Exercise 3

For simplicity, I will skip Exercise 3. Since it asks us to encode a sentence and I want to generalize the problem, let’s skip to Exercise 4, in which we have to create an encoder function. Just know that, later on, our will be encoded string is message = "hi my name is caesar".

Exercise 4

In this exercise, we will define a function that encodes a message with any given encryption key.

• alphabet, position and message remain defined from previous exercises. Define a function encoding that takes a message as input as well as an int encryption key key to encode a message with the Caesar cipher by shifting each letter in message by key positions. Your function should return a string consisting of these encoded letters. Use encoding to encode message using key = 3 and save the result as encoded_message. Print encoded_message.

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# write your code here
def encoding(message, key):
  encoded_message = ""
  for i in message:
    new_letter = positions[i]+key
    if new_letter > 26:
      new_letter %= 27
    encoded_message += alphabet[new_letter]
  return encoded_message

print(encoded_message=encoding(message, key=3))

Observe that we had to take the modulus of new_letter if it increases more than 26.

This problem has one more exercise; however, I won’t be explaining it since the decoding process is just backward, negative, of the encoding.