[Numpy-discussion] picking elements with boolean masks
Neal Becker
ndbecker2@gmail....
Mon Mar 25 07:24:23 CDT 2013
starting with a NxM array, I want to select elements of the array using a set of
boolean masks. The masks are simply where the indexes have a 0 or 1 in the
corresponding bit position. For example, consider the case where M = 4.
all_syms = np.arange (4)
all_bits = np.arange (2)
bit_mask = (all_syms[:,np.newaxis] >> all_bits) & 1
mask0 = bit_mask == 0
mask1 = bit_mask == 1
Maybe there's a more straightforward way to generate these masks. That's not my
question.
In [331]: mask1
Out[331]:
array([[False, False],
[ True, False],
[False, True],
[ True, True]], dtype=bool)
OK, now I want to use this mask on D
In [333]: D.shape
Out[333]: (32400, 4)
Just to simplify, let's just try the first row of D
In [336]: D[0]
Out[336]: array([ 0., 2., 2., 4.])
In [335]: D[0][mask1[...,0]]
Out[335]: array([ 2., 4.])
that worked fine. But I want not just to apply one of the masks in the set
(mask1 is [4,2], it has 2 masks), I want the results of applying all the masks
(2 in this case)
In [334]: D[0][mask1]
---------------------------------------------------------------------------
ValueError Traceback (most recent call last)
<ipython-input-334-243c7a5e45a4> in <module>()
----> 1 D[0][mask1]
ValueError: boolean index array should have 1 dimension
Any ideas what's the best approach here?
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