How to reverse sklearn.OneHotEncoder transform to recover original data?

Question

I encoded my categorical data using sklearn.OneHotEncoder and fed them to a random forest classifier. Everything seems to work and I got my predicted output back.

Is there a way to reverse the encoding and convert my output back to its original state?

Answer 1

Just compute the dot-product of the encoded values with enc.active_features_. It would work both for sparse and dense representation. 

For example:

from sklearn.preprocessing import OneHotEncoder

import numpy as np

orig = np.array([6, 9, 8, 2, 5, 4, 5, 3, 3, 6])

enc = OneHotEncoder()

encoded = enc.fit_transform(orig.reshape(-1, 1)) 

decoded = encoded.dot(ohe.active_features_).astype(int)

assert np.allclose(orig, decoded)

The key insight is that the active_features_ attribute of the OHE model, that represents the original values for each binary column. Thus we can decode the binary-encoded number by simply computing a dot-product with active_features_. For each data point, there's just a single 1 the position of the original value.

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