Reverse part of an array using NumPy

Question

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

Answer 1

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])

Answer 2

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])

Answer 3

>>> a = np.array([1,2,3,4,5,6])
>>> a[1:4] = a[3:0:-1]
>>> a
array([1, 4, 3, 2, 5, 6])