[GitHub] [incubator-mxnet] BenjaminCHEN2016 commented on a change in pull request #17904: [Numpy] add: numpy op trilindices

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BenjaminCHEN2016 commented on a change in pull request #17904: [Numpy] add: numpy op trilindices
URL: https://github.com/apache/incubator-mxnet/pull/17904#discussion_r400737486
 
 

 ##########
 File path: python/mxnet/numpy/multiarray.py
 ##########
 @@ -5533,6 +5533,87 @@ def tril(m, k=0):
     """
     return _mx_nd_np.tril(m, k)
 
+@set_module('mxnet.numpy')
+def tril_indices(n, k=0, m=None):
+    """
+    Return the indices for the lower-triangle of an (n, m) array.
+
+    Parameters
+    ----------
+    n : int
+        The row dimension of the arrays for which the returned
+        indices will be valid.
+    k : int, optional
+        Diagonal offset (see `tril` for details).
+    m : int, optional
+        .. versionadded:: 1.9.0
+
+        The column dimension of the arrays for which the returned
+        arrays will be valid.
+        By default `m` is taken equal to `n`.
+
+
+    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
+    -----
+    .. versionadded:: 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],
 
 Review comment:
   I have added more tests.

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