`numpy:triu_indices_from`

Signature

def triu_indices_from(arr, k = 0)

Return the indices for the upper-triangle of arr.

See triu_indices for full details.

triu_indices_from : tuple, shape(2) of ndarray, shape(N): Indices for the upper-triangle of arr.

import numpy as np

✓

Create a 4 by 4 array

a = np.arange(16).reshape(4, 4)
a

✓

array([[ 0,  1,  2,  3],
       [ 4,  5,  6,  7],
       [ 8,  9, 10, 11],
       [12, 13, 14, 15]])

Pass the array to get the indices of the upper triangular elements.

triui = np.triu_indices_from(a)
triui

✓

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

a[triui]

✓

array([ 0,  1,  2,  3,  5,  6,  7, 10, 11, 15])

This is syntactic sugar for triu_indices().

np.triu_indices(a.shape[0])

✓

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

Use the `k` parameter to return the indices for the upper triangular array from the k-th diagonal.

triuim1 = np.triu_indices_from(a, k=1)
a[triuim1]

✓

array([ 1,  2,  3,  6,  7, 11])