John Jaques - 1 year ago 112
Python Question

# From list of indices to one-hot matrix

What is the best (elegant and efficient) way in Theano to convert a vector of indices to a matrix of zeros and ones, in which every row is the one-of-N representation of an index?

``````v = t.ivector()  # the vector of indices
n = t.scalar()   # the width of the matrix
convert = <your code here>
f = theano.function(inputs=[v, n], outputs=convert)
``````

Example:

``````n_val = 4
v_val = [1,0,3]
f(v_val, n_val) = [[0,1,0,0],[1,0,0,0],[0,0,0,1]]
``````

Answer Source

I didn't compare the different option, but you can also do it like this. It don't request extra memory.

``````import numpy as np
import theano

n_val = 4
v_val = np.asarray([1,0,3])
idx = theano.tensor.lvector()
z = theano.tensor.zeros((idx.shape[0], n_val))
one_hot = theano.tensor.set_subtensor(z[theano.tensor.arange(idx.shape[0]), idx], 1)
f = theano.function([idx], one_hot)
print f(v_val)[[ 0.  1.  0.  0.]
[ 1.  0.  0.  0.]
[ 0.  0.  0.  1.]]
``````
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