Shift letters by a certain value in python

Question

I had a coded text file that was coded by simple letter shifting. I have now gotten the information from it into two lists. In this format:

list_1 =['fjsir', 'vnjk', 'eioafnvjf', 'einbvfbj']
list_2 =[3,4,7,1]

The second list is by how many places in the alphabet should it move over. For eg. if index 0 in list one had 'fjsir' and it's corresponding index in list_2 is 3 then it would decode to 'cfpeo'. I'm not sure how I would match these in python.

Answer 1

To shift elements to the left:

chr(ord(char) - n)

Which uses ord() to get an integer representation of char, and substracts n from this number. It then uses chr() to convert it back to a character.

The function could look like this:

def shift(s, n):
    return ''.join(chr(ord(char) - n) for char in s)

Which can get called nicely with zip():

list_1 =['fjsir', 'vnjk', 'eioafnvjf', 'einbvfbj']
list_2 =[3,4,7,1]

for x, y in zip(list_1, list_2):
    print(shift(x, y))

And gives back the decodings:

cgpfo
rjfg
^bhZ_goc_
dhmaueai

Additionally, if you only want decodings to only consist of letters from the English alphabet, you can use a modulus function % instead:

 def shift(s, n):
     return ''.join(chr((ord(char) - 97 - n) % 26 + 97) for char in s)

Which gives back decodings with only letters:

cgpfo
rjfg
xbhtygocy
dhmaueai

Answer 2

Here is a simple solution:

alphabets = 'abcdefghijklmnopqrstuvwxyz'
list_1 = ['fjsir', 'vnjk', 'eioafnvjf', 'einbvfbj']
list_2 = [3,4,7,1]
final_list = []
for index,original_word in enumerate(list_1):
    new_word = ''
    for letter in original_word:
        if letter in alphabets:
            index_val = alphabets.index(letter) - list_2[index]
            new_word += alphabets[index_val]
    final_list.append(new_word)
print final_list

Output: ['cgpfo', 'rjfg', 'xbhtygocy', 'dhmaueai']

Answer 3

You can convert characters to number with ord() and convert numbers to characters with chr(). A shift function may look like:

def shift_string(inString, inOffset):
    return "".join([chr(ord(x) + inOffset) for x in inString])

Accessing array elements from list is left to the reader. ;)

Note: This simple version will probably produce unwanted characters. You may simply use a modulo function to remain in the ASCII range.

def shift_string(inString, inOffset):
    return "".join([chr(((ord(x) - ord(' ') + inOffset) % 95) + ord(' ')) for x in inString])

If your set of valid characters is more complex (e.g. letters), you should write a special function for the character transformation.