Re: [PLUG] python, list comprehensions

[Dan Crosta <dcrosta@sccs.swarthmore.edu>]

Jeff Abrahamson wrote:
> I just wrote the following loop in python, which sums those values of
> the matrix W that have the property that their distinct coordinates
> are both in a set called H:
>
>         for i in range(len(k)):
>             for j in range(len(k)):
>                 if(i != j and i in H and j in H):
>                     w += W[i,j]
>
> I feel I ought to be able to do this with a list comprehension, but
> it's not clear how to get the double loop.  Any thoughts?
>
>         w = sum([W[i,j] for ?? ])

I don't think so -- or at least, I can't figure out a syntax to make it 
work. However, especially if your matrix is large, it probably makes 
more sense to iterate over the set H rather than over the matrix:

for i in H:
    for j in H:
        if i is not j:
            sum += W[i, j]

plus this saves the creation of the two range() arrays (actually -- 
someone might know if python optimizes this somehow, I've never looked 
at how it actually executes the code). I'm pretty certain python is 
optimized for iteration, so

for item in H:

is better than

for i in range(len(H)):
    item = H[i]

dsc
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