I created 2D array and I did boolean indexing with 2 bool index arrays. first one is for axis 0, next one is for axis 1.
I expected that values on cross True and True from each axis are selected like Pandas. but the result is not.
I wonder how it works that code below. and I want to get the link from official numpy site describing this question.
Thanks in advance.
a = np.arange(9).reshape(3,3)
a
----------------------------
array([[0, 1, 2],
[3, 4, 5],
[6, 7, 8]])
a[ [True, False, True], [True, False, True] ]
--------------------------
array([0, 8])
My expectation is [0, 6, 2, 8]
.
(I know how to get the result that I expect.)
CodePudding user response:
In [20]: a = np.arange(9).reshape(3,3)
If the lists are passed to ix_
, the result is 2 arrays that can be used, with broadcasting
to index the desired block:
In [21]: np.ix_([True, False, True], [True, False, True] )
Out[21]:
(array([[0],
[2]]),
array([[0, 2]]))
In [22]: a[_]
Out[22]:
array([[0, 2],
[6, 8]])
This isn't 1d, but can be easily raveled.
Trying to make equivalent boolean arrays does not work:
In [23]: a[[[True], [False], [True]], [True, False, True]]
Traceback (most recent call last):
File "<ipython-input-23-26bc93cfc53a>", line 1, in <module>
a[[[True], [False], [True]], [True, False, True]]
IndexError: too many indices for array: array is 2-dimensional, but 3 were indexed
Boolean indexes must be either 1d, or nd matching the target, here (3,3).
In [26]: np.array([True, False, True])[:,None]& np.array([True, False, True])
Out[26]:
array([[ True, False, True],
[False, False, False],
[ True, False, True]])
CodePudding user response:
What you want is consecutive slices: a[[True, False, True]][:,[True, False, True]]
a = np.arange(9).reshape(3,3)
x = [True, False, True]
y = [True, False, True]
a[x][:,y]
as flat array
a[[True, False, True]][:,[True, False, True]].flatten(order='F')
output: array([0, 6, 2, 8])
alternative
NB. this requires arrays for slicing
a = np.arange(9).reshape(3,3)
x = np.array([False, False, True])
y = np.array([True, False, True])
a.T[x&y[:,None]]
output: array([0, 6, 2, 8])