Find position in 3D numpy array where value of 2D array is first exceeded

Question

I want to find the position in a 3D numpy array where a 2D array first exceeds the value in the 3D array. E.g.

import numpy as np
array_3D = np.random.rand(10, 3, 3)
array_2D = np.random.rand(3, 3)

np.argmax(array_2D > array_3D)

I get the result 0, but I want a 2D array i.e. essentially argmax results for each grid cell.

kabanus · Accepted Answer · 2018-07-12 18:17:29Z

2

If I understand you correctly, that is what the axis parameter is for:

>>>np.argmax(array_2D > array_3D,axis=0)
array([[1, 0, 0],
       [0, 2, 1],
       [0, 2, 0]])

otherwise it runs across the flat array.

answered Jul 12, 2018 at 18:17

kabanus

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Bacon · Accepted Answer · 2018-07-12 18:09:27Z

0

np.where(array_2D > array_3D) will give you 3 arrays with the coordinates of the points you are looking for. If you are looking for the first point:

x =  np.where(array_2D > array_3D)[0][0]
y =  np.where(array_2D > array_3D)[1][0]
z =  np.where(array_2D > array_3D)[2][0]

Certainly not the most elegant, thought, as you won't have directly the first point.

answered Jul 12, 2018 at 18:09

Bacon

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