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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.


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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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