Convert array of indices to 1-hot encoded numpy array

Question

Let's say I have a 1d numpy array

a = array([1,0,3])

I would like to encode this as a 2D one-hot array

b = array([[0,1,0,0], [1,0,0,0], [0,0,0,1]])

Is there a quick way to do this? Quicker than just looping over a to set elements of b, that is.

Answer 1

Your array a defines the columns of the nonzero elements in the output array. You need to also define the rows and then use fancy indexing:

>>> a = np.array([1, 0, 3])
>>> b = np.zeros((a.size, a.max()+1))
>>> b[np.arange(a.size),a] = 1
>>> b
array([[ 0.,  1.,  0.,  0.],
       [ 1.,  0.,  0.,  0.],
       [ 0.,  0.,  0.,  1.]])