`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

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Create a 4 by 4 array

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

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

trili = np.tril_indices_from(a)
trili

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a[trili]

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This is syntactic sugar for tril_indices().

np.tril_indices(a.shape[0])

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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]

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