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Find the union of two NumPy arrays

  • Last Updated : 05 Sep, 2020

To find union of two 1-dimensional arrays we can use function numpy.union1d() of Python Numpy library. It returns unique, sorted array with values that are in either of the two input arrays.


numpy.union1d(array1, array2)

Note The arrays given in input are flattened if they are not 1-dimensional.

Let’s see examples of how to find union of two arrays.

Example 1:

# import libraries
import numpy as np
arr1 = np.array([10, 20, 30, 40])
print("array1 ", arr1)
arr2 = np.array([20, 40, 60, 80])
print("array2 ", arr2)
# print union of the two arrays
print("Union of two arrays :", np.union1d(arr1, arr2))


Example 2:
Let’s see example of finding union of a 2-d and a 1-d array. As discussed earlier, if array passed as arguments to function numpy.union1d is 2-dimensional, then they are flattened to 1-dimension.

# import libraries
import numpy as np
# 2-d array
arr1 = np.array([[1, 2, 3], [4, 5, 6]])
print("array1 ")
arr2 = np.array([0, 5, 10])
print("array2 ", arr2)
# print union of 2-d array and 1-d array
print("Union of two arrays", np.union1d(arr1, arr2))


If we want to find union of more than two arrays, then we can find that by using functools.reduce function.

# code to find union of more than two arrays
# import libraries
import numpy as np
from functools import reduce
array = reduce(np.union1d, ([1, 2, 3], [1, 3, 5],
                            [2, 4, 6], [0, 0, 0]))
print("Union ", array)


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