Translate integers in a numpy array to a contiguous range 0…n

Question

I would like to translate arbitrary integers in a numpy array to a contiguous range 0...n, like this:

source: [2 3 4 5 4 3]
translating [2 3 4 5] -> [0 1          


        

Answer 1

IIUC you can simply use np.unique's optional argument return_inverse, like so -

np.unique(source,return_inverse=True)[1]

Sample run -

In [44]: source
Out[44]: array([2, 3, 4, 5, 4, 3])

In [45]: np.unique(source,return_inverse=True)[1]
Out[45]: array([0, 1, 2, 3, 2, 1])

Answer 2

pandas.factorize is one method:

import pandas as pd

lst = [2, 3, 4, 5, 4, 3]
res = pd.factorize(lst, sort=True)[0]

# [0 1 2 3 2 1]

Note: this returns a list, while np.unique will always return an np.ndarray.