In this tutorial, we will look at how to get the union of two numpy arrays with the help of some examples.

## Numpy `union1d()`

Function

You can use the numpy `union1d()`

function to find the union of two arrays. Pass the two arrays as arguments to the function. The following is the syntax:

import numpy as np union_ar = np.union1d(ar1, ar2)

It returns the unique, sorted array of values that are in either of the two input arrays. Note that if the input arrays are not 1d then they are flattened before applying the union operation.

Let’s look at some examples of using the numpy union1d() function.

### Union of two 1d numpy arrays

Pass the two arrays as arguments to the numpy union1d() function.

import numpy as np # create two 1d arrays ar1 = np.array([1, 3, 3, -2, 0, 0, 1]) ar2 = np.array([4, 1, 2, 7]) # union of ar1 and ar2 ar_union = np.union1d(ar1, ar2) # display the union array print(ar_union) # display the type of ar_union print(type(ar_union))

Output:

[-2 0 1 2 3 4 7] <class 'numpy.ndarray'>

Here, we created two 1d numpy arrays and then applied the union1d() function. Note that the returned array contains the unique values present in both the arrays in a sorted manner.

### Union of two 2d numpy arrays

If the arrays are not 1d, they are flattened first, and then the union operation is applied. For example, if you apply the union1d() function on two 2d arrays:

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# create two 2d arrays ar1 = np.array([[1, 3], [3, 2]]) ar2 = np.array([[2, 3], [0, 5]]) # union of ar1 and ar2 ar_union = np.union1d(ar1, ar2) # display the union array print(ar_union)

Output:

[0 1 2 3 5]

You can see that we get a 1d numpy array containing the unique values in both the arrays as output.

### Union of more than two arrays in python

You can only get the union of two numpy arrays at a time with the np.union1d() function.

To get the union of more than two arrays, you can use the `np.union1d()`

function with the `functools`

`reduce()`

function. See the example below:

from functools import reduce # create three 1d numpy arrays ar1 = np.array([1, 2, 2]) ar2 = np.array([2, 3]) ar3 = np.array([5, 1, 7]) # union of three arrays ar_union = reduce(np.union1d, (ar1, ar2, ar3)) # display the union array print(ar_union)

Output:

[1 2 3 5 7]

We get the unique values from the three arrays as a numpy array.

For more on the numpy union1d() function, refer to its documentation.

With this, we come to the end of this tutorial. The code examples and results presented in this tutorial have been implemented in a Jupyter Notebook with a python (version 3.8.3) kernel having numpy version 1.18.5

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