np.in1d() for membership testing in NumPy

Introduction

We use np.in1d() to quickly check if elements from one list or array appear in another. It helps us find matches easily.

Checking which products in a shopping list are available in the store's inventory.

Finding which students from one class attended a school event.

Filtering emails to see which ones are from known contacts.

Identifying common items between two datasets.

Marking which data points belong to a specific group.

Syntax

NumPy

np.in1d(ar1, ar2, assume_unique=False, invert=False)

ar1 is the array of elements to test.

ar2 is the array of elements to check against.

Examples

Checks which elements of [1, 2, 3] are in [2, 3, 4].

NumPy

import numpy as np

result = np.in1d([1, 2, 3], [2, 3, 4])
print(result)

Checks which fruits from the first list appear in the second list.

NumPy

import numpy as np

result = np.in1d(['apple', 'banana'], ['banana', 'cherry'])
print(result)

Returns True for elements in the first array NOT in the second array because of invert=True.

NumPy

import numpy as np

result = np.in1d([10, 20, 30], [15, 20, 25], invert=True)
print(result)

Sample Program

This program checks which students from the full class list submitted their homework. It prints each student's name with their submission status.

NumPy

import numpy as np

# List of students who submitted homework
submitted = np.array(['Alice', 'Bob', 'Charlie', 'David'])

# List of all students in class
all_students = np.array(['Alice', 'Eve', 'Bob', 'Frank', 'Charlie'])

# Check who submitted homework
submitted_mask = np.in1d(all_students, submitted)

# Print results
for student, did_submit in zip(all_students, submitted_mask):
    print(f"{student}: {'Submitted' if did_submit else 'Not Submitted'}")

Important Notes

The output is a boolean array showing True where elements of ar1 are in ar2.

Use invert=True to find elements NOT in the second array.

Setting assume_unique=True can speed up the function if you know inputs have no duplicates.

Summary

np.in1d() helps check membership of elements between arrays.

It returns a boolean array matching the first array's shape.

Useful for filtering, matching, and comparing data quickly.