`numpy:tril_indices_from`

Signature

def tril_indices_from(arr, k = 0)

Return the indices for the lower-triangle of arr.

See tril_indices for full details.

arr : array_like: The indices will be valid for square arrays whose dimensions are the same as arr.
k : int, optional: Diagonal offset (see tril for details).

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 lower triangular elements.

trili = np.tril_indices_from(a)
trili

✓

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

a[trili]

✓

array([ 0,  4,  5,  8,  9, 10, 12, 13, 14, 15])

This is syntactic sugar for tril_indices().

np.tril_indices(a.shape[0])

✓

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

Use the `k` parameter to return the indices for the lower triangular array up to the k-th diagonal.

trili1 = np.tril_indices_from(a, k=1)
a[trili1]

✓

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