Can I create a view from a boolean selection of a numpy array?

Question

If I create a numpy array, and another to serve as a selective index into it:

>>> x
array([[ 2,  3,  4],
       [ 5,  6,  7],
       [ 6,  7,  8],
       [11, 12, 13]])

>>> nz
array([ True,  True, False,  True], dtype=bool)

then direct use of nz returns a view of the original array:

>>> x[nz,:]
array([[ 2,  3,  4],
       [ 5,  6,  7],
       [11, 12, 13]])

>>> x[nz,:] += 2
>>> x
array([[ 4,  5,  6],
       [ 7,  8,  9],
       [ 6,  7,  8],
       [13, 14, 15]])

however, naturally, an assignment makes a copy:

>>> v = x[nz,:]

Any operation on v is on the copy, and has no effect on the original array.

Is there any way to create a named view, from x[nz,:], simply to abbreviate code, or which I can pass around, so operations on the named view will affect only the selected elements of x?

The last assignment uses x.__getitem__(idx_tuple). x[nz,:]=value uses x.__setitem__(idx_tuple, value). So while both use [nz,:] the underlying indexing operation is different. The distinction between view and copy only applies to the get operation. You could pass a idx_tuple=(nz,slice(None)) around, but you still have to use the x[idx_tuple]=... syntax. — hpaulj
– hpaulj, Commented Apr 27, 2019 at 16:14
Thank you. I would have thought that qualifies as an answer.. but I'm new here. — Robin Betts
– Robin Betts, Commented Apr 27, 2019 at 16:48

Ian · Accepted Answer · 2019-05-14 08:37:44Z

1

Numpy has masked_array, which might be what you are looking for:

import numpy as np

x = np.asarray([[ 2,  3,  4],[ 5,  6,  7],[ 6,  7,  8],[11, 12, 13]])
nz = np.asarray([ True,  True, False,  True], dtype=bool)

mx = np.ma.masked_array(x, ~nz.repeat(3)) # True means masked, so "~" is needed
mx += 2

# x changed as well because it is the base of mx 
print(x)
print(x is mx.base)

answered May 14, 2019 at 8:37

Ian

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1 Comment

Robin Betts Over a year ago

This is pretty much what I've been looking for There are some warnings..but my operations are nearly as simple as in the OP, not nested, shape-changing, or anything... I'll test first before accepting. (There may be room for further commentary)

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