Update README.md

README.md

@@ -11,7 +11,7 @@ def step(grid):
     """
     computes one step of Conway's game of life on a toroidal 2D grid
     """
-    count = np.zeros(a.shape, dtype=np.uint8)
+    count = np.zeros(grid.shape, dtype=np.uint8)
     count[:, :] = grid
     # sum all 8 neighbors
     count += np.roll(grid, 1, axis=0) + np.roll(grid, -1, axis=0)