DOC: Added anti-diagonal examples to np.diagonal and np.fill_diagonal

1 files changed, 20 insertions, 3 deletions

diff --git a/numpy/lib/index_tricks.py b/numpy/lib/index_tricks.py
index 8d4b1ce1e..40c1cda05 100644
--- a/numpy/lib/index_tricks.py
+++ b/numpy/lib/index_tricks.py
@@ -813,7 +813,7 @@ def fill_diagonal(a, val, wrap=False):
     The wrap option affects only tall matrices:
 
     >>> # tall matrices no wrap
-    >>> a = np.zeros((5, 3),int)
+    >>> a = np.zeros((5, 3), int)
     >>> np.fill_diagonal(a, 4)
     >>> a
     array([[4, 0, 0],
@@ -823,7 +823,7 @@ def fill_diagonal(a, val, wrap=False):
            [0, 0, 0]])
 
     >>> # tall matrices wrap
-    >>> a = np.zeros((5, 3),int)
+    >>> a = np.zeros((5, 3), int)
     >>> np.fill_diagonal(a, 4, wrap=True)
     >>> a
     array([[4, 0, 0],
@@ -833,13 +833,30 @@ def fill_diagonal(a, val, wrap=False):
            [4, 0, 0]])
 
     >>> # wide matrices
-    >>> a = np.zeros((3, 5),int)
+    >>> a = np.zeros((3, 5), int)
     >>> np.fill_diagonal(a, 4, wrap=True)
     >>> a
     array([[4, 0, 0, 0, 0],
            [0, 4, 0, 0, 0],
            [0, 0, 4, 0, 0]])
 
+    The anti-diagonal can be filled by reversing the order of elements
+    using either `numpy.flipud` or `numpy.fliplr`.
+
+    >>> a = np.zeros((3, 3), int);
+    >>> np.fill_diagonal(np.fliplr(a), [1,2,3])  # Horizontal flip
+    >>> a
+    array([[0, 0, 1],
+           [0, 2, 0],
+           [3, 0, 0]])
+    >>> np.fill_diagonal(np.flipud(a), [1,2,3])  # Vertical flip
+    >>> a
+    array([[0, 0, 3],
+           [0, 2, 0],
+           [1, 0, 0]])
+
+    Note that the order in which the diagonal is filled varies depending
+    on the flip function.
     """
     if a.ndim < 2:
         raise ValueError("array must be at least 2-d")