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# numpy.tril_indices

`numpy.``tril_indices`(n, k=0)

  Return the indices for the lower-triangle of an (n, n) array.

  Parameters : | n : int The row dimension of the square arrays for which the returned indices will be valid. k : int, optional Diagonal offset (see `tril` for details).
  Returns : | inds : tuple of arrays The indices for the triangle. The returned tuple contains two arrays, each with the indices along one dimension of the array.

  See also

  `triu_indices`
    similar function, for upper-triangular.
  `mask_indices`
    generic function accepting an arbitrary mask function.

  `tril`, `triu`

  Notes

  New in version 1.4.0.

  Examples

  Compute two different sets of indices to access 4x4 arrays, one for the lower triangular part starting at the main diagonal, and one starting two diagonals further right:

      >>> il1 = np.tril_indices(4)
      >>> il2 = np.tril_indices(4, 2)

  Here is how they can be used with a sample 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]])

  Both for indexing:

      >>> a[il1]
      array([ 0,  4,  5,  8,  9, 10, 12, 13, 14, 15])

  And for assigning values:

      >>> a[il1] = -1
      >>> a
      array([[-1,  1,  2,  3],
             [-1, -1,  6,  7],
             [-1, -1, -1, 11],
             [-1, -1, -1, -1]])

  These cover almost the whole array (two diagonals right of the main one):

      >>> a[il2] = -10
      >>> a
      array([[-10, -10, -10,   3],
             [-10, -10, -10, -10],
             [-10, -10, -10, -10],
             [-10, -10, -10, -10]])

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