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thouis / numpy-trac-migration

numpy Trac to github issues migration
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Speed up N-D Boolean indexing (migrated from Trac #274) #1827

Open thouis opened 12 years ago

thouis commented 12 years ago

Original ticket http://projects.scipy.org/numpy/ticket/274 Reported 2006-09-06 by atmention:teoliphant, assigned to unknown.

Currently N-D Boolean indexing works by creating intermediate integer index arrays for N>1. This should be avoided when possible so that Boolean indexing is faster for N>1.

thouis commented 12 years ago

Comment in Trac by atmention:alberts, 2007-05-12

thouis commented 12 years ago

Comment in Trac by atmention:cournape, 2008-11-20

thouis commented 12 years ago

Comment in Trac by atmention:cournape, 2009-03-02

thouis commented 12 years ago

Comment in Trac by atmention:mwiebe, 2011-03-23

Here's a small benchmark illustrating the problem:

In [1]: import numpy as np

In [2]: np.__version__
Out[2]: '2.0.0.dev-7219ac2'

In [3]: a = np.random.random(100000)

In [4]: amask = a > 0.9

In [5]: b = a.reshape(100,1000)

In [6]: bmask = amask.reshape(100,1000)

In [7]: timeit a[amask]
1000 loops, best of 3: 787 us per loop

In [8]: timeit b[bmask]
1000 loops, best of 3: 1.22 ms per loop