Using a dictionary to create Caesar Cipher, outputting only one item-CodePudding

For class I am creating a function that converts each letter in the alphabet into its corresponding letter after a given shift, in a way that a caesar cipher would shift each letter. So far I have:

alphabet = ['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z']


# Problem 1.

def build_cipher(shift):
    '''
    Description: takes in shift (an integer representing the amount the letter key in the dictionary is shifted from its corresponding letter) and returns a dictionary containing all letters and their corresponding letters after the shift. This is acheived through subtracting the shift from the number corresponding to the letter, and using modulo 26.
    
    >>> build_cipher(3)
    
    '''
    
    for i in range(0, 26):
        letter = alphabet[i]
        shiftedletter = alphabet[(i - shift) % 26]
      
        return {letter : shiftedletter}

This is mostly correct, but it only outputs:

{'a': 'x'}

How can I get it to output for each letter? I want my output to be:

{‘a’: ‘x’,
‘b’: ‘y’,
‘c’: ‘z’,
‘d’: ‘a’,
‘e’: ‘b’,
‘f’: ‘c’,
‘g’: ‘d’,
‘h’: ‘e’,
‘i’: ‘f’,
‘j’: ‘g’,
‘k’: ‘h’,
‘l’: ‘i’,
‘m’: ‘j’,
‘n’: ‘k’,
‘o’: ‘l’,
‘p’: ‘m’,
‘q’: ‘n’,
‘r’: ‘o’,
‘s’: ‘p’,
‘t’: ‘q’,
‘u’: ‘r’,
‘v’: ‘s’,
‘w’: ‘t’,
‘x’: ‘u’,
‘y’: ‘v’,
‘z’: ‘w’}

I think the problem is that it isn't looping for each letter in the alphabet, but I am not exactly sure how to fix that. Any help is appreciated, thank you!

CodePudding user response：

You want to build the dictionary first. When you're done, return the dictionary you just built.

def build_cipher(shift):
    cipher = dict()
    for i in range(0, 26):
        letter = alphabet[i]
        shiftedletter = alphabet[(i - shift) % 26]
        cipher[letter] = shiftedletter
    return cipher

Or, as a comprehension:

def build_cipher(shift):
    return {alphabet[i]: alphabet[(i - shift) % 26] for i in range(0, 26)}

CodePudding user response：

As an alternative, you can use str.maketrans() to make the translation table t, and then str.translate(t) to translate a string:

In [1]: alpha = "abcdefghijklmnopqrstuvwxyz"

In [2]: shift = 13

In [3]: t = str.maketrans(alpha, alpha[shift:]   alpha[:shift])

In [4]: "hello world".translate(t)
Out[4]: 'uryyb jbeyq'

In [5]: "hello world".translate(t).translate(t)
Out[5]: 'hello world'

Note that str.translate(t).translate(t) only works for a shift of 13.

CodePudding user response：

This is one way to solve it.

alphabet = ['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z']

# Problem 1.

def build_cipher(shift):
result = {}
for i in range(0, 26):
    letter = alphabet[i]
    shiftedletter = alphabet[(i - shift) % 26]
  
    result.update({letter : shiftedletter})
return result

CodePudding user response：

A simple solution.

def build_cipher(shift):
    '''
    Description: takes in shift (an integer representing the amount the letter
    key in the dictionary is shifted from its corresponding letter) and returns
    a dictionary containing all letters and their corresponding letters after
    the shift. This is acheived through subtracting the shift from the number
    corresponding to the letter, and using modulo 26.

    >>> build_cipher(3)
    '''

    alphabet = [
        'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n',
        'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z'
    ]

    cipher = {}
    for i in range(0, 26):
        cipher[alphabet[i]] = alphabet[(i - shift) % 26]

    return cipher

cipher = build_cipher(3)
for k,v in cipher.items():
    print("{}: {}".format(k, v))

Output:

a: x
b: y
c: z
d: a
e: b
f: c
g: d
h: e
i: f
j: g
k: h
l: i
m: j
n: k
o: l
p: m
q: n
r: o
s: p
t: q
u: r
v: s
w: t
x: u
y: v
z: w

CodePudding user response：

The moment you first call return it will jump out of the function. You could either change the return statement into a print-statement, or you could have a return string, that you partially put together in the loop. Something like this:

ret = ""
for i in range(0, 26):
    letter = alphabet[i]
    shiftedletter = alphabet[(i - shift) % 26]
    ret  = "{"   letter    " : "   shiftedletter   "}\n"
return ret