Finding unique attributes in python numpy array

03 May, 2019

np.unique() function is used to find the unique elements of an array. It returns the sorted unique elements of an array. It can also return three other optional outputs-

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

Example: Return unique values of an array

import numpy as np
np.unique([1,2,2,3,3,4,5,5,5,6,6,7])

Output:

array([1, 2, 3, 4, 5, 6, 7])

Example: Defining an array and getting unique values of it

a = np.array([[1, 1], [2, 3],[3,3])
np.unique(a)

Output:

array([1, 2, 3])

Example: Return unique rows of a 2D array

a = np.array([[1, 0, 0], [1, 0, 0], [2, 3, 4]])
np.unique(a,axis=0)

Output:

array([[1, 0, 0],
[2, 3, 4]])

Example: Return indices of original array which gives unique values

a = np.array(['a', 'b', 'b', 'c', 'a'])
u, indices = np.unique(a, return_index=True)
u

Output:

array(['a', 'b', 'c'], dtype='<U1')

Example: : Return indices of original array which gives unique values

indices

Output:

array([0, 1, 3], dtype=int64)

Example: Return indices of original array which gives unique values

a[indices]

Output:

array(['a', 'b', 'c'], dtype='<U1')