Added symetric array (can be improved)

README.rst

@@ -527,6 +527,26 @@ Expert
       C = stride_tricks.as_strided(Z, shape=(i, j, n, n), strides=Z.strides + Z.strides)
 
 
+3. Create a 2D array subclass such that Z[i,j] == Z[j,i]
+
+   .. code:: python
+
+      # Eric O. Lebigot
+      # Note: only works for 2d array and value setting using indices
+
+      class Symetric(np.ndarray):
+          def __setitem__(self, (i,j), value):
+              super(Symetric, self).__setitem__((i,j), value)
+              super(Symetric, self).__setitem__((j,i), value)
+
+      def symetric(Z):
+          return np.asarray(Z + Z.T - np.diag(Z.diagonal())).view(Symetric)
+
+      S = symetric(np.random.randint(0,10,(5,5))
+      S[2,3] = 42
+      print S
+
+
 Master
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