filter2

Jeff Epler jepler at unpythonic.net
Thu Jun 13 18:06:02 CEST 2002


On Thu, Jun 13, 2002 at 03:00:28PM +0000, Michael Hudson wrote:
> Write it in Python.  Particularly if test is expensive the loop
> overhead will disappear into the noise.
> 
> (with-seriousness-level :low
>   If you insist on C loops, there's always:
> 
>       lt = map(lambda x:(test(x), x), alist)
>       l1 = filter(lambda (x,y):x, lt)
>       l2 = filter(lambda (x,y):not x, lt)
> 
>   But I doubt you need me to tell you why this solution sucks.)

If you're benchmarking, try a solution like this in the mix.  It only
has one map/filter call, not map+2*filter. (uses nested scopes and the
new bool() builtin, but this can be avoided if necessary)

def filter2(test, list):
    l1 = []
    l2 = []
    funcs = [l2.append, l1.append]
    map(lambda x: funcs[bool(test(x))](x), list)
    return l1, l2

>>> print filter2(lambda x: x%3==0, range(16))
([0, 3, 6, 9, 12, 15], [1, 2, 4, 5, 7, 8, 10, 11, 13, 14])

Jeff





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