Construct two dimensional numpy array from indices and values of a one dimensional array

Question

Say I\'ve got

Y = np.array([2, 0, 1, 1])

From this I want to obtain a matrix X with shape (len(Y), 3). In this particular cas

Answer 1

Maybe:

>>> Y = np.array([2, 0, 1, 1])
>>> X = np.zeros((len(Y), 3))
>>> X[np.arange(len(Y)), Y] = 1
>>> X
array([[ 0.,  0.,  1.],
       [ 1.,  0.,  0.],
       [ 0.,  1.,  0.],
       [ 0.,  1.,  0.]])

Answer 2

Y = np.array([2, 0, 1, 1])
new_array = np.zeros((len(Y),3))
for i in range(len(Y)):
    new_array[i,Y[i]] = 1

I think ... i dont think there is an easier way (but i might be wrong)

Answer 3

To give a one-liner alternative to DSM's perfectly good answer:

>>> Y = np.array([2, 0, 1, 1])
>>> np.arange(3) == Y[:, np.newaxis]
array([[False, False,  True],
       [ True, False, False],
       [False,  True, False],
       [False,  True, False]], dtype=bool)