python - 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 case, the first row of X should have a one on the second index and zero otherwhise. The second row of X should have a one on the 0 index and zero otherwise. To be explicit:

X = np.array([[0, 0, 1], [1, 0, 0], [0, 1, 0], [0, 1, 0]])

How do I produce this matrix? I started with

X = np.zeros((Y.shape[0], 3))

but then couldn't figure out how to populate/fill in the ones from the list of indices

As always, thanks for your time!

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.]])