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NumPy is a popular library for working with arrays. NumPy’s `intersect1d()`

function returns the intersection between two arrays. In other words, it returns the common elements for two given arrays.

numpy.intersect1d(ar1, ar2, assume_unique=False, return_indices=False)

This function accepts the following parameter values:

`ar1`

and`ar2`

: These two required parameters represent the input arrays for which`intersect1d()`

will return the intersection.

Note:`intersect1d()`

accepts any array-like objects; this includes NumPy arrays and`scalars`

.

Note:If any input array is not one-dimensional, the function will flatten them and convert them to a single dimensional array.

`assume_unique`

: An optional parameter, passed as`True`

if both input arrays are assumed to be unique and`False`

otherwise. If both input arrays are unique, passing`assume_unique`

as`True`

can speed up calculation.

**Note:** If the input arrays are not unique and the user passes `assume_unique`

as `True`

, the function could return an incorrect result or an out-of-bound exception.

`return_indices`

: An optional parameter, which determines if`intersect1d()`

will return two extra arrays containing indices of the elements of the intersection array in the two input arrays.

- The function always returns an array that includes the intersection elements found in both the input arrays; this is the intersection array from earlier.
- The function optionally returns two additional arrays, which contain the indices of intersection elements in the input arrays. Each of these two optionally returned arrays represents one input array.

**Note:** The optional arrays are only returned when the `return_indices`

input argument has been set to `True`

.

```
import numpy as np
# creating the input arrays
a = np.array([1,3,5,7,9])
b = np.array([2,4,6,8])
# finding the intersect of the two arrays
print(np.intersect1d(a, b))
# creating the input arrays
c = np.array([[1,2,3], [4,5,6]])
d = np.array([[1,2,3], [4,5,6]])
# finding the intersect of the two arrays
print(np.intersect1d(c, d))
# creating the input arrays
e = np.array([[1,2,3], [7,8,9]])
f = np.array([[1,2,3], [4,5,6]])
# finding the intersect of the two arrays
print(np.intersect1d(e, f, return_indices = True))
```

Hit run to see the results! Try changing input arguments and observe the results.

- Line 1: We import
`numpy`

as`np`

. - Lines 3–4: We create two input arrays,
`a`

and`b`

. - Line 7: We use
`intersect1d()`

to find the intersection of`a`

and`b`

, and print the results. - Lines 10–11: We create two input arrays,
`c`

and`d`

. These are two 2D arrays. - Line 14: We use
`intersect1d()`

to find the intersection of`c`

and`d`

and print the results. The`intersect1d()`

function returns a 1D array even though we input two 2D arrays. - Lines 17–18: We create two input arrays,
`e`

and`f`

. - Line 21: We use
`intersect1d()`

to find the intersection of`e`

and`f`

, and print the results. The`return_indices`

argument in`intersect1d()`

has been set to`True`

. As a result,`intersect1d()`

returns two extra arrays, which contain indices of the intersection elements in the two input arrays.`e`

and`f`

both contain the intersection elements`1`

,`2`

, and`3`

at indices`0`

,`1`

, and`2`

.

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