Unique

The goal

Questions to David Rotermund

numpy.unique

numpy.unique(ar, return_index=False, return_inverse=False, return_counts=False, axis=None, *, equal_nan=True)

Find the unique elements of an array.

Returns the sorted unique elements of an array. There are three optional outputs in addition to the unique elements:

• the indices of the input array that give the unique values
• the indices of the unique array that reconstruct the input array
• the number of times each unique value comes up in the input array

unique can be used on multi-dimensional arrays. However, the results are strange since empty places need to be filled for shaping the results into one common matrix.

import numpy as np


a = np.arange(10, 21)
print(a)  # -> [10 11 12 13 14 15 16 17 18 19 20]
idx = np.r_[0:5, 3:8]
print(idx)  # -> [0 1 2 3 4 3 4 5 6 7]
print(a[idx])  # -> [10 11 12 13 14 13 14 15 16 17]

print(np.unique(idx))  # -> [0 1 2 3 4 5 6 7]
print(np.unique(a[idx]))  # -> [10 11 12 13 14 15 16 17]

There are more return arguments available

import numpy as np


a = np.r_[0:5, 3:8]
print(a)  # -> [0 1 2 3 4 3 4 5 6 7]

values, unique_index = np.unique(a, return_index=True)
_, unique_inverse = np.unique(a, return_inverse=True)
_, unique_counts = np.unique(a, return_counts=True)

print(values)  # -> [0 1 2 3 4 5 6 7]
print(unique_index)  # -> [0 1 2 3 4 7 8 9]
print(unique_inverse)  # -> [0 1 2 3 4 3 4 5 6 7]
print(unique_counts)  # -> [1 1 1 2 2 1 1 1]