Convert string / character to integer in python

Question

I want to convert a single character of a string into an integer, add 2 to it, and then convert it back to a string. Hence, A becomes C, K becomes M, etc.

Solution 1

This is done through the chr and ord functions. Eg; chr(ord(ch)+2) does what you want. These are fully described here.

Solution 2

This sounds a lot like homework, so I'll give you a couple of pieces and let you fill in the rest.

To access a single character of string s, its s[x] where x is an integer index. Indices start at 0.

To get the integer value of a character it is ord(c) where c is the character. To cast an integer back to a character it is chr(x). Be careful of letters close to the end of the alphabet!

Edit: if you have trouble coming up with what to do for Y and Z, leave a comment and I'll give a hint.

Solution 3

Normally, Just ord and add 2 and chr back, (Y, Z will give you unexpected result ("[","\")

>>> chr(ord("A")+2)
'C'

If you want to change Y, Z to A, B, you could do like this.

>>> chr((ord("A")-0x41+2)%26+0x41)
'C'
>>> chr((ord("Y")-0x41+2)%26+0x41)
'A'
>>> chr((ord("Z")-0x41+2)%26+0x41)
'B'

Here is A to Z

>>> [chr((i-0x41+2)%26+0x41) for i in range(0x41,0x41+26)]
['C', 'D', 'E', 'F', 'G', 'H', 'I', 'J', 'K', 'L', 'M', 'N', 'O', 'P', 'Q', 'R', 'S', 'T', 'U', 'V', 'W', 'X', 'Y', 'Z', 'A', 'B']

Solution 4

http://docs.python.org/library/functions.html

ord(c)

Given a string of length one, return an integer representing the Unicode code point of the character when the argument is a unicode object, or the value of the byte when the argument is an 8-bit string. For example, ord('a') returns the integer 97, ord(u'\u2020') returns 8224. This is the inverse of chr() for 8-bit strings and of unichr() for unicode objects. If a unicode argument is given and Python was built with UCS2 Unicode, then the character’s code point must be in the range [0..65535] inclusive; otherwise the string length is two, and a TypeError will be raised.

Solution 5

"ord" is only part of the solution. The puzzle you mentioned there rotates, so that "X"+3 rotates to "A". The most famous of these is rot-13, which rotates 13 characters. Applying rot-13 twice (rotating 26 characters) brings the text back to itself.

The easiest way to handle this is with a translation table.

import string

def rotate(letters, n):
    return letters[n:] + letters[:n]

from_letters = string.ascii_lowercase + string.ascii_uppercase
to_letters = rotate(string.ascii_lowercase, 2) + rotate(string.ascii_uppercase, 2)

translation_table = string.maketrans(from_letters, to_letters)

message = "g fmnc wms bgblr"
print message.translate(translation_table)

Not a single ord() or chr() in here. That's because I'm answering a different question than what was asked. ;)

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Comments

[Kena]

+1 for good homework guidance without giving away the solution

[Nikwin]

''.join([chr(ord(i)+2) for i in s]) would do the same

[Luke]

Yeah, I was afraid that someone was going to think it was homework. I already have a B.S. in Computer Science, and I'm an ASP.NET programmer. But I've been picking up Django and wanted to sharpen my Python skills using this site: pythonchallenge.com/pc/def/map.html. Anyways, I think that 'ord' is the function I was looking for; I'll give it a shot after work. Thanks for everyone's input!

[Luke]

What exactly do the brackets do? Why not just another set of paranthesis?

[danben]

Ah, ok. My two cents on making stuff not sound like homework, if you're so inclined: only post the part of the problem you want answered (like "how do I convert a character to an int in python" rather than the full question which I actually remember as being one of the first questions in my cryptography class in school) and also, I think just saying "This might sound like homework, but it's not" will get us all to remove the condescension from our responses :) That looks like a cool site though, I'll have to check it out.

[Peter Hansen]

@dalke already knows this, but for those who don't, an even simpler way to do the above for rot-13 specifically is message.decode('rot13'). (The above is rot-n.)