How to efficiently use a index array as a mask to turn a numpy array into a boolean array?

Question

I have a numpy array like this:

>>> I
array([[ 1.,  0.,  2.,  1.,  0.],
       [ 0.,  2.,  1.,  0.,  2.]])

And an array A like this:

>>> A = np.ones((2,5,3))

I'd like to obtain the following matrix:

>>> result
array([[[ False,  False,  True],
        [ False,  True,  True],
        [ False,  False,  False],
        [ False,  False,  True],
        [ False,  True,  True]],

       [[ False,  True,  True],
        [ False,  False,  False],
        [ False,  False,  True],
        [ False,  True,  True],
        [ False,  False,  False]]], dtype=bool)

It is better to explain with an example:
I[0,0] = 1 -> result[0,0,:2] = False and result[1,1,2:] = True
I[1,0] = 0 -> result[1,1,0] = False and result[1,1,1:] = True

Here is my current implementation (correct):

result = np.empty((A.shape[0], A.shape[1], A.shape[2]))
r = np.arange(A.shape[2])
for i in xrange(A.shape[0]):
    result[i] = r > np.vstack(I[i])

print result.astype(np.bool)

Is there a way to implement in a faster way (avoiding the for loop)?

Thanks!

I tried to run your code, but it does not work for me. Could you revise your question and make it clearer? — mrcl
– mrcl, Commented Oct 27, 2014 at 7:24
Sorry, my fault. A is a (2,5,3) matrix. I added also a print with the bool conversion — Rowandish
– Rowandish, Commented Oct 27, 2014 at 7:29

ebarr · Accepted Answer · 2014-10-27 07:37:53Z

1

You just need to add another dimension on to I, such that you can broadcast r properly:

result = r > I.reshape(I.shape[0],I.shape[1],1)

e.g.

In [41]: r>I.reshape(2,5,1)
Out[41]: 
array([[[False, False,  True],
        [False,  True,  True],
        [False, False, False],
        [False, False,  True],
        [False,  True,  True]],

       [[False,  True,  True],
        [False, False, False],
        [False, False,  True],
        [False,  True,  True],
        [False, False, False]]], dtype=bool)

answered Oct 27, 2014 at 7:37

ebarr

7,8521 gold badge32 silver badges43 bronze badges

Sign up to request clarification or add additional context in comments.

Collectives™ on Stack Overflow

How to efficiently use a index array as a mask to turn a numpy array into a boolean array?

1 Answer 1

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Your Answer

Sign up or log in

Post as a guest

Related