Looping for every character in a string in Python decoder ring

Question

I'm trying to make a simple decoder ring in Python.

Example:

a=b, `b=c, c=d, etc.

I want the script to take an encoded message and output the decoded message.
For instance, I would input "ifmmp" and it would output "hello".

I've been thinking I need to split all the characters up and loop through them and change their chr() or ord() values.

There doesn't seem to be any documentation for this in python.

What documentation exactly do you think is missing? `for` loops are documented as are `chr()` and `ord()`. Remember that strings are immutable so you cannot just change one string into another, you have to create a new completely string with the new value. Also unless this is homework on how to do a simple for loop you probably want to forget loops entirely and just call `string.translate()`. — Duncan, Nov 24 '11 at 09:22

score 14 · Accepted Answer · answered Nov 24 '11 at 09:21

14

How about:

s = 'ifmmp'
new_s = ''
for c in s:
    n = ord(c)
    n = n - 1
    if n < ord('a'):
        # 'a' -> 'z'
        n = ord('z')
    new_s += chr(n)
# print('new_s = %r' % new_s)  -> new_s = 'hello'

Of course, this is only handling small letters, not capital.

answered Nov 24 '11 at 09:21

Some programmer dude

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Sorry, but this solution has some deficiencies. A minor one is that the function must obtain ord('a') for each character c in s: replacing with number 97 would be better. A bigger one is that ``new_s += chr(n)`` creates a new string for each character of s. If the text is 10000 long, you see the useless work... A major deficiency is that capital letters are treated indeed, as all the characters having an ord() < 97 that is to say digits, capital letters, characters !"#$%'()*+,-./:;<=>?@[\]^_` and \t \r \n and others... More in the edit of my answer – eyquem Nov 24 '11 at 11:36
@eyquem [Magic numbers](https://en.wikipedia.org/wiki/Magic_number_(programming)) are never a solution I would recommend. – Some programmer dude Feb 03 '17 at 06:32

score 4 · Answer 2 · answered Nov 24 '11 at 09:29

4

Forget loops, whenever possible use the features built in to Python:

from string import maketrans, translate, ascii_lowercase
import functools
translation = maketrans(ascii_lowercase, ascii_lowercase[-1]+ascii_lowercase[:-1])
decipher = functools.partial(string.translate, table=translation)

print(decipher("ifmmp")) # 'hello'

answered Nov 24 '11 at 09:29

Duncan

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For the record, `maketrans` is a static function of `bytes` and `bytearrays` in Python 3 http://stackoverflow.com/questions/3031045/how-come-string-maketrans-does-not-work-in-python-3-1 – Wtower Apr 29 '15 at 14:27

eyquem · Answer 3 · 2011-11-25T04:19:28.250

from string import maketrans

table = maketrans('abcdefghijklmnopqrstuvwxyz/','zabcdefghijklmnopqrstuvwxy ')

for x in ('pdfbo','qipophsbqi','cmvf/nppo/jo/b/sfe/tlz'):
    print x,' -> ',x.translate(table)

result

pdfbo  ->  ocean
qipophsbqi  ->  phonograph
cmvf/nppo/jo/b/sfe/tlz  ->  blue moon in a red sky

.

Edit

I rewrote the algorithm of Joachim (function better()) and I wrote my own solution not using maketrans() ( yop() ) :

s = '{ifmmp}\t\tcmvf-nppo \n SUNNY ~ ajhabh 14568'

def bof(s):
    new_s = ''
    for c in s:
        n = ord(c)
        n = n - 1
        if n < ord('a'):
            # 'a' -> 'z'
            n = ord('z')
        new_s += chr(n)
    return new_s


def better(s):
    li = []
    for c in s:
        n = ord(c)-1
        if n == 96:
            li.append('z')
        elif 96<n<122:
            li.append(chr(n))
        else:
            li.append(c)
    return ''.join(li)

def yop(s):
    gen = ((c,ord(c)-1) for c in s)
    return ''.join('z' if y==96 else chr(y) if 96<y<122 else x for x,y in gen)


def better_yop(s):
    def gen(h):
        for c in h:
            n = ord(c)-1
            if n == 96:
                yield 'z'
            elif 96<n<122:
                yield chr(n)
            else:
                yield c
    return ''.join(gen(s))

for truc in (s,bof(s),better(s),yop(s),better_yop(s)):
    print '%r\n%s\n' % (truc,truc)

result

'{ifmmp}\t\tcmvf-nppo \n SUNNY ~ ajhabh 14568'
{ifmmp}     cmvf-nppo 
 SUNNY ~ ajhabh 14568

'zhello|zzbluezmoonzzzzzzzzz}zzigzagzzzzzz'
zhello|zzbluezmoonzzzzzzzzz}zzigzagzzzzzz

'{hello}\t\tblue-moon \n SUNNY ~ zigzag 14568'
{hello}     blue-moon 
 SUNNY ~ zigzag 14568

'{hello}\t\tblue-moon \n SUNNY ~ zigzag 14568'
{hello}     blue-moon 
 SUNNY ~ zigzag 14568

'{hello}\t\tblue-moon \n SUNNY ~ zigzag 14568'
{hello}     blue-moon 
 SUNNY ~ zigzag 14568

However , my function yop() is slower than the function better()

.

Edit

Now the function better_yop() has a speed equivalent to the speed of better()
However, better() seems to be slightly faster than better_yop(). Since it is also simpler, better() is the best

score 1 · Answer 4 · answered Nov 24 '11 at 15:14

1

I would recommend making the decoder ring a dictionary. Then open the input file for reading, read the file, loop through a character at a time and use the dictionary to translate to an output string. Finally write that string to a file.

answered Nov 24 '11 at 15:14

ncmathsadist

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Yes, I ended up using the maketrans solution, but made a translation table for all 26 orders it could be in, then had the program check for outputted english words, and had the english words written to a file. – user1063543 Nov 27 '11 at 11:42

Looping for every character in a string in Python decoder ring

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