better way to do this in Numeric
Todd Miller
jmiller at stsci.edu
Mon Aug 4 20:16:37 CEST 2003
John Hunter wrote:
> I have a list of indices and want to assign the number of times an
> index shows up in the list to an array with the count specified at the
> index; ie,
>
> from Numeric import *
>
> a = zeros((10,), Int)
> ind = [1,1,4,4,4,4,7,7,9,9,9,9,9]
>
> for i in ind:
> a[i] += 1
>
> I'm wondering if there is a way to use some combination of Numeric
> array functions to make this speedy.
>
> Thanks,
> John Hunter
>
There's a simple histogram function defined in the Numeric manual, based
on the searchsorted function.
>>> def histogram(a, bins):
... n = searchsorted(sort(a), bins)
... n = concatenate([n, [len(a)]])
... return n[1:]-n[:-1]
If you set bins:
>>> bins = arange(0.5,10.5,1)
I think it does what you want:
>>> ind = [1,1,4,4,4,4,7,7,9,9,9,9,9]
>>> histogram(ind, bins)
array([0, 0, 2, 0, 0, 4, 0, 0, 2, 0, 5])
In numarray CVS, histogram improves performance by a factor of ~7x
versus the 2 lines of code you defined the problem with. (I tried this
with test cases of 10**5 and 10**6 elements.)
Sadly, Numeric has much slower sort functions than numarray so
histogram winds up being a net loss of ~2x.
I think you probably need an extension function to get really good
performance.
Todd
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