[Tutor] implementing rot 13 problems

[RJ Ewing]

Thank you all for the help. I really appreciated the suggestions. Some of
the things you pointed out, I originally used, but started changing thing
when it wasn't working. I got it to work, but if you could let me know if
there is anything I should do to make this code more pythonesque that would
be great.

def rot_text(self, s):
ls = list(s)
for position, char in enumerate(ls):
if char.isupper() or char.islower():
test = 'M' if char.isupper() else 'm'
if char <= test:
ls[position] = chr(ord(char) + 13)
else:
ls[position] = chr(ord(char) - 13)
return "".join(ls)



On Tue, Mar 12, 2013 at 4:11 AM, Karim <kliateni at gmail.com> wrote:

>
> What can be said after this detailed lesson...Bravo!
>
>
> On 12/03/2013 11:58, Steven D'Aprano wrote:
>
>> On 12/03/13 16:57, RJ Ewing wrote:
>>
>>> I am trying to implement rot 13 and am having troubles. My code is as
>>> follows:
>>>
>>
>> A few comments follow.
>>
>>
>>    def rot_text(self, s):
>>>       ls = [i for i in s]
>>>       for i in ls:
>>>           if i.isalpha():
>>>               if i.isupper():
>>>                   if i <= 'M':
>>>                       x = ls.index(i)
>>>                       ls[ls.index(i)] = chr(ord(i) + 13)
>>>
>>
>>
>> This is unnecessarily verbose and complicated.
>>
>> First problem, you use the name "i" to represent a character. Generally
>> you should avoid single character names, and prefer meaningful names that
>> explain what the variable represents. If you must use a single character,
>> then the conventional names include:
>>
>> x, y, z: numbers, particularly floats
>> x: (rarely) an arbitrary value of no particular type
>> i, j, k: integer loop variables (rare in Python)
>> n, m: other integers
>> s: string
>> c: character
>>
>> Notice that "c for character" makes much more sense than "i for
>> character".
>>
>>
>> You can, and should, create a list from the string using the list()
>> function, rather than a list comprehension that just walks over the
>> characters:
>>
>> ls = [c for c in s]  # No, too verbose.
>> ls = list(s)  # Much better.
>>
>>
>> You then loop over the list, again using "i for character". You can
>> combine all those multiple if statements into a single clause:
>>
>>       for i in ls:
>>           if i.isalpha() and i.isupper() and i <= 'M':
>>
>> But wait! The call to isalpha() is not necessary, because if a character
>> is *not* alphabetical, isupper() will always return False:
>>
>> py> '9'.isupper()
>> False
>>
>> So we can throw out the call to isalpha().
>>
>> Your code to rot13 the letter is, sadly, broken:
>>
>> x = ls.index(i)
>> ls[ls.index(i)] = chr(ord(i) + 13)
>>
>>
>> This might make more sense if I re-write it using more meaningful names:
>>
>> position = ls.index(char)  # This variable never gets used :-(
>> ls[ls.index(char)] = chr(ord(char) + 13)
>>
>> The bit with chr() and ord() is fine. But what you do with it is wrong.
>> The problem is that index always returns the position of the *first* copy
>> of the item. So if your string is:
>>
>> 'ABA'
>>
>> the first time around the loop, your character is 'A'. ls.index('A') will
>> return 0, and the rot13ed version, 'N', will be shoved into position 0. The
>> second time around the loop, the character is 'B' and the rot13ed version
>> gets shoved into position 1. But the third time, the character is 'A'
>> again, index() will return 0 again, and the rot13ed version gets shoved
>> into position 0 instead of position 2, giving you:
>>
>> 'NOA'
>>
>>
>> The best way to fix this is to ignore index, and instead count which
>> character we're at each time:
>>
>>
>> position = -1
>> for char in ls:
>>     position = position + 1  # Advance the count.
>>     ... blah blah blah
>>     ls[position] = chr(ord(char) + 13)
>>
>>
>> But wait! Python has a function to do exactly this for us:
>>
>>
>> for position, char in enumerate(ls):
>>     ... blah blah blah
>>     ls[position] = chr(ord(char) + 13)
>>
>>
>>
>>
>> Have a go with that, and see how far you get.
>>
>>
>>
>>
>>
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