What is the numpy.intersect1d() function in Python?

What is the numpy.intersect1d() function in Python? Answer


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.

Return value

  • 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 12, and 3 at indices 01, and 2.
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