[Numpy-discussion] k maximal elements
Keith Goodman
kwgoodman@gmail....
Mon Jun 6 08:44:38 CDT 2011
On Sun, Jun 5, 2011 at 11:15 PM, Alex Ter-Sarkissov <ater1980@gmail.com> wrote:
> I have a vector of positive integers length n. Is there a simple (i.e.
> without sorting/ranking) of 'pulling out' k larrgest (or smallest) values.
> Something like
>
> sum(x[sum(x,1)>(max(sum(x,1)+min(sum(x,1))))/2,])
>
> but smarter
You could use bottleneck [1]. It has a partial sort written in cython:
>> a = np.arange(10)
>> np.random.shuffle(a)
>> a
array([6, 3, 8, 5, 4, 2, 0, 1, 9, 7])
>> import bottleneck as bn
>> k = 3
>> b = bn.partsort(a, a.size-k)
>> b[-k:]
array([8, 9, 7])
Or you could get the indices:
>> idx = bn.argpartsort(a, a.size-k)
>> idx[-k:]
array([2, 8, 9])
[1] The partial sort will be in Bottleneck 0.5.0. There is a beta2 at
https://github.com/kwgoodman/bottleneck/downloads. Release version
will be posted at http://pypi.python.org/pypi/Bottleneck
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