[Tutor] Better structure?

[Orri Ganel]

Orri Ganel wrote:

> Kent Johnson wrote:
>
>> Jacob S. wrote:
>>
>>>> Try writing the code to do what lstrip actually does
>>>> - its much harder. So the library includes the more
>>>> difficult function and lets you code the easy ones.
>>>
>>>
>>>
>>>
>>> def lstrip(string,chars=' ')
>>>    string = list(string)
>>>    t = 0
>>>    for x in string:
>>>        if x in chars:
>>>            string.remove(t)
>>>        else:
>>>            break
>>>        t = t+1
>>>
>>>
>>> Okay, so it's not that difficult, 
>>
>>
>>
>> Well, after fixing the obvious syntax error I tried
>> print lstrip('abcd', 'abcd')
>>
>> and got
>> Traceback (most recent call last):
>>   File "D:\Personal\Tutor\LStrip.py", line 11, in ?
>>     print lstrip('abcd', 'abcd')
>>   File "D:\Personal\Tutor\LStrip.py", line 6, in lstrip
>>     string.remove(t)
>> ValueError: list.remove(x): x not in list
>>
>> so maybe it's not that easy either. Don't forget the unit tests! :-)
>>
>> Kent
>>
>> _______________________________________________
>> Tutor maillist  -  Tutor at python.org
>> http://mail.python.org/mailman/listinfo/tutor
>>
> Actually, its not all that difficult. Instead of removing characters 
> from the list, just replace them with an empty string and return 
> ''.join(string):
>
> >>> def lstrip(string,chars=' '):
>    string = list(string)
>    i = 0
>    for x in string:
>        if x in chars:
>            string[i] = ''
>        else:
>            break
>        i+=1
>    return ''.join(string)
>
> >>> lstrip('abcd','abcd')
> ''
> >>> 'abcd'.lstrip('abcd')
> ''
> >>> 'go on long log buddy'.lstrip('gonl ')
> 'buddy'
> >>> lstrip('go on long log buddy', 'gonl ')
> 'buddy'
> >>> 'go on long log buddy'.lstrip('gonl')
> ' on long log buddy'
> >>> lstrip('go on long log buddy', 'gonl')
> ' on long log buddy'
>
> So its not a brute force thing so much as a change in how you look at 
> it that makes it difficult.
>
By the same token, to do rstrip just [::-1] - ify string when you make 
it into a list and when you join it to be returned:

 >>> def rstrip(string,chars=' '):
    string = list(string)[::-1]
    i = 0
    for x in string:
        if x in chars:
            string[i] = ''
        else:
            break
        i+=1
    return ''.join(string)[::-1]

 >>> 'abcde'.rstrip('cde')
'ab'
 >>> 'abcde'.rstrip('ecd')
'ab'
 >>> rstrip('abcde','cde')
'ab'
 >>> rstrip('abcde','ecd')
'ab'

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