Tough sorting problem: or, I'm confusing myself

david jensen dmj.ccc at gmail.com
Tue Apr 13 13:31:31 CEST 2010


On Apr 12, 1:22 am, Paul McGuire <pt... at austin.rr.com> wrote:
> On Apr 9, 10:03 am, david jensen <dmj.... at gmail.com> wrote:
>
>
>
> > Hi all,
>
> > I'm trying to find a good way of doing the following:
>
> > Each n-tuple in combinations( range( 2 ** m ), n ) has a corresponding
> > value n-tuple (call them "scores" for clarity later). I'm currently
> > storing them in a dictionary, by doing:
>
> > ----
> > res={}
> > for i in itertools.combinations( range( 2**m ) , n):
> >     res[ i ] = getValues( i )    # getValues() is computationally
> > expensive
> > ----
>
> > For each (n-1)-tuple, I need to find the two numbers that have the
> > highest scores versus them. I know this isn't crystal clear, but
> > hopefully an example will help: with m=n=3:
>
> > Looking at only the (1, 3) case, assuming:
> > getValues( (1, 2, 3) ) == ( -200, 125, 75 )    # this contains the
> > highest "other" score, where 2 scores 125
> > getValues( (1, 3, 4) ) == ( 50, -50, 0 )
> > getValues( (1, 3, 5) ) == ( 25, 300, -325 )
> > getValues( (1, 3, 6) ) == ( -100, 0, 100 )    # this contains the
> > second-highest, where 6 scores 100
> > getValues( (1, 3, 7) ) == ( 80, -90, 10  )
> > getValues( (1, 3, 8) ) == ( 10, -5, -5 )
>
> > I'd like to return ( (2, 125), (6, 100) ).
>
> > The most obvious (to me) way to do this would be not to generate the
> > res dictionary at the beginning, but just to go through each
> > combinations( range( 2**m), n-1) and try every possibility... this
> > will test each combination n times, however, and generating those
> > values is expensive. [e.g. (1,2,3)'s scores will be generated when
> > finding the best possibilities for (1,2), (1,3) and (2,3)]
>
> Add a memoizing decorator to getValues, so that repeated calls will do
> lookups instead of repeated calculations.
>
> -- Paul

Thanks for the idea... I'd thought of that, but didn't use it because
I don't think it improves complexity or readability: just makes it a
little more intuitive.

But thanks for your time!

David



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