I am trying to use array slicing to reverse part of a NumPy array. If my array is, for example,
a = np.array([1,2,3,4,5,6]) then I can get a slice b
b = a[::-1] Which is a view on the original array. What I would like is a view that is partially reversed, for example
1,4,3,2,5,6 I have encountered performance problems with NumPy if you don't play along exactly with how it is designed, so I would like to avoid "fancy" indexing if it is possible.
3 Answers
If you don't like the off by one indices
>>> a = np.array([1,2,3,4,5,6]) >>> a[1:4] = a[1:4][::-1] >>> a array([1, 4, 3, 2, 5, 6]) >>> a = np.array([1,2,3,4,5,6]) >>> a[1:4] = a[3:0:-1] >>> a array([1, 4, 3, 2, 5, 6]) You can use the permutation matrices (that's the numpiest way to partially reverse an array).
a = np.array([1,2,3,4,5,6]) new_order_for_index = [1,4,3,2,5,6] # Careful: index from 1 to n ! # Permutation matrix m = np.zeros( (len(a),len(a)) ) for index , new_index in enumerate(new_order_for_index ): m[index ,new_index -1] = 1 print np.dot(m,a) # np.array([1,4,3,2,5,6]) 
