Mask a 3d array with a 2d mask in numpy

python arrays numpy mask
Chiel picture Chiel · Jun 7, 2016 · Viewed 9.2k times · Source

I have a 3-dimensional array that I want to mask using a 2-dimensional array that has the same dimensions as the two rightmost of the 3-dimensional array. Is there a way to do this without writing the following loop?

import numpy as np

nx = 2
nt = 4

field3d = np.random.rand(nt, nx, nx)
field2d = np.random.rand(nx, nx)

field3d_mask = np.zeros(field3d.shape, dtype=bool)

for t in range(nt):
    field3d_mask[t,:,:] = field2d > 0.3

field3d = np.ma.array(field3d, mask=field3d_mask)

print field2d
print field3d

Answer

user2379410 picture user2379410 · Jun 8, 2016

There's numpy.broadcast_to (new in Numpy 1.10.0):

field3d_mask = np.broadcast_to(field2d > 0.3, field3d.shape)

Related questions

How to apply a disc shaped mask to a NumPy array?

I have an array like this: >>> np.ones((8,8)) array([[ 1., 1., 1., 1., 1., 1., 1., 1.], [ 1., 1., 1., 1., 1., 1., 1., 1.], [ 1., 1., 1., 1., 1., 1., 1., 1.], [ 1., 1., 1., 1., 1., 1., 1., 1.], [ 1., 1., 1., 1., 1., 1., 1., 1.], [ 1., 1., 1., 1., 1., 1., 1., 1.], [ 1., 1., 1., 1., 1., 1., 1., 1.], [ 1., 1., 1., 1., 1., 1., 1., 1.]]) I'm creating a disc shaped mask with radius 3 thus: y,x = np.ogrid[-3: 3+1, -3: 3+1] mask = x**2+y**2 <= 3**2 This gives: >> mask array([[False, False, False, …

Read More
Numpy inverse mask

I want to inverse the true/false value in my numpy masked array. So in the example below i don't want to mask out the second value in the data array, I want to mask out the first and third …

Read More
How can you turn an index array into a mask array in Numpy?

Is it possible to convert an array of indices to an array of ones and zeros, given the range? i.e. [2,3] -> [0, 0, 1, 1, 0], in range of 5 I'm trying to automate something like this: >>> index_array = np.arange(200,300) …

Read More