[Tutor] Unintentionally manipulating a list

[Knacktus]

Am 25.02.2011 15:49, schrieb ranjan das:
>
> I am facing the following problem
>
>
> I have a list of the form
>
> INPUT= [ [ ['A1-N','A2-Y','A3-N' ],['B1-Y','B2-N','B3-N' ] ], [.........] ]
>
>
> and I want an output of the form (selecting only those elements which
> have a Y associated with them)
>
> OUTPUT=[ ['A2-Y', 'B1-Y'],[....] ]
>
>
> I wrote the following code. Although it gives me the desired output, it
> CHANGES the list INPUT
>
> now after i run the code I get INPUT as the same as OUTPUT (which i dont
> want to happen). I have used the copy function but it still is not
> working. Any help or pointers is appreciated
>
> _CODE_
>
> from copy import copy
>
> temp=copy( INPUT )
> OUTPUT=temp
>
>
>
> for i in range(len(temp)):
>
>      for j in range(len(temp[i])):
>
>          for k in range(len(temp[i][j])):
>
>              if temp[i][j][k][-1]=='Y':
>
>                  OUTPUT[i][j]=temp[i][j][k]
>
>
There's no need for an index. You can iterate over the elements directly 
and create the result on the fly:

results = []
for sub_list in INPUT:
     sub_results = []
     for sub_sub_list in sub_list:
         sub_sub_results = []
         for entry in sub_sub_list:
             if entry.endswith("Y"):
                 sub_sub_results.append(entry)
         if sub_sub_results:
             sub_results.append(sub_sub_results)
     if sub_results:
         results.append(sub_results)

Uuppsy daisy, not really readable code ;-). Your solution looks cleaner. 
I bet, some of the gurus can come up with a real pythonic solution.

Cheers,

Jan

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