Getting non-zero data and positions from 2D array in Python

Question

I have a 2D array, and I hope I can get all of the non-zero data and their position. But now I just can get the non-zero data position.

Is there any way that I can get the value and the position at the same time?

import numpy as np

groupMatrix = np.array([
[1, 1, 0, 0],
[1, 0, 0, 0],
[0, 0, 0, 2],
[3, 3, 0, 2]
])

res = zip(*np.nonzero(groupMatrix))

print(list(res))

The result I got is [(0, 0), (0, 1), (1, 0), (2, 3), (3, 0), (3, 1), (3, 3)]

But I hope the result can show the position with value.

For example, (x, y, z): x, y represent the position and z represents the value at the point.

(0, 0, 1), (0, 1, 1), (1, 0, 1), (2, 3, 2), (3, 0, 3), (3, 1, 3), (3, 3, 2).

Thank you for your help!!!

Why use a list comprehension with numpy objects? The whole class is built to avoid doing exactlhy that — rafaelc
– rafaelc, Commented Feb 15, 2020 at 16:04

rafaelc · Accepted Answer · 2020-02-14 20:01:55Z

3

Use np.vstack and numpy multidimensional indexing.

i   = np.nonzero(arr)

res = np.vstack([i, arr[i]])

array([[0, 0, 1, 2, 3, 3, 3],
       [0, 1, 0, 3, 0, 1, 3],
       [1, 1, 1, 2, 3, 3, 2]])

If you really need a list of tuples with the information, you can use list(zip(*...))

>>> list(zip(*res))
[(0, 0, 1), (0, 1, 1), (1, 0, 1), (2, 3, 2), (3, 0, 3), (3, 1, 3), (3, 3, 2)]

edited Feb 14, 2020 at 20:01

answered Feb 14, 2020 at 19:40

rafaelc

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Mark Snyder · Accepted Answer · 2020-02-14 18:56:14Z

2

print(list(zip(*np.nonzero(groupMatrix),groupMatrix[groupMatrix!=0])))

This is certainly not the most efficient solution (it replicates the search for nonzero elements), but it works just fine.

answered Feb 14, 2020 at 18:56

Mark Snyder

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user3483203 · Accepted Answer · 2020-02-14 20:23:18Z

2

If your arrays are small, like the one in your question, ndenumerate will be readable and fast. If your arrays are large, you should default to vectorized methods.

[(*i, j) for i, j in np.ndenumerate(arr) if j != 0]

[(0, 0, 1), (0, 1, 1), (1, 0, 1), (2, 3, 2), (3, 0, 3), (3, 1, 3), (3, 3, 2)]

answered Feb 14, 2020 at 20:23

user3483203

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