seemingly simple list indexing problem

Tobiah toby at tobiah.org
Wed Jul 30 21:06:59 CEST 2008


On Mon, 28 Jul 2008 23:41:51 -0700, iu2 wrote:

> On Jul 29, 3:59 am, John Machin <sjmac... at lexicon.net> wrote:
>> On Jul 29, 8:10 am, John Krukoff <jkruk... at ltgc.com> wrote:
>>
>>
>>
>>
>>
>> > On Mon, 2008-07-28 at 16:24 -0500, Ervan Ensis wrote:
>> > > My programming skills are pretty rusty and I'm just learning Python so
>> > > this problem is giving me trouble.
>>
>> > > I have a list like [108, 58, 68].  I want to return the sorted indices
>> > > of these items in the same order as the original list.  So I should
>> > > return [2, 0, 1]
>>
>> > > For a list that's already in order, I'll just return the indices, i.e.
>> > > [56, 66, 76] should return [0, 1, 2]
>>
>> > > Any help would be appreciated.
>>
>> > > --
>> > >http://mail.python.org/mailman/listinfo/python-list
>>
>> > If your lists aren't so large that memory is an issue, this might be a
>> > good place for a variation of decorate, sort, undecorate.
>>
>> > >>> listToSort = [ 108, 58, 68 ]
>> > >>> decorated = [ ( data, index ) for index, data in
>>
>> > enumerate( listToSort ) ]>>> decorated
>>
>> > [(108, 0), (58, 1), (68, 2)]>>> result = [ None, ] * len( listToSort )
>> > >>> for sortedIndex, ( ignoredValue, originalIndex ) in
>>
>> > enumerate( sorted( decorated ) ):
>> > ...     result[ originalIndex ] = sortedIndex
>> > ...>>> result
>>
>> > [2, 0, 1]
>>
>> Simpliciter:
>>
>>
>>
>> >>> data = [99, 88, 77, 88, 66]
>> >>> [x[1] for x in sorted(zip(data, xrange(len(data))))]
>> [4, 2, 1, 3, 0]
>>
>> Use case? Think data == database table, result == index ...- Hide quoted text -
>>
>> - Show quoted text -
> 
> I think it is wrong, using this on my data returns the wrong result
>  data = [108, 58, 68, 108]
>>>> [x[1] for x in sorted(zip(data, xrange(len(data))))]
> [1, 2, 0, 3]

It looked wrong to me at first, but I think this is correct.
The two largest numbers were at positions 0 and 3.  They come
last in the result.  58 is the smallest, and was at position
1, which is the first in the result.



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