flopscope.numpy.tril_indices

Return the indices for the lower-triangle of arr.

Adapted from NumPy docs np.tril_indices_from

Areacore

Typefree

Cost

Flopscope Context

Return lower-triangle indices for given array.

See tril_indices for full details.

Parameters

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 flopscope.numpy as fnp

Create a 4 by 4 array

>>> a = flops.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 = flops.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().

>>> flops.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 = flops.tril_indices_from(a, k=1)
>>> a[trili1]
array([ 0,  1,  4,  5,  6,  8,  9, 10, 11, 12, 13, 14, 15])