Python Ways to find indices of value in list (original) (raw)

Last Updated : 11 Jul, 2025

In Python, it is common to locate the index of a particular value in a list. The built-in index() method can find the first occurrence of a value. However, there are scenarios where multiple occurrences of the value exist and we need to retrieve all the indices. Python offers various methods to achieve this efficiently. Let’s explore them.

**Using List Comprehension

List comprehension is an efficient way to find all the indices of a particular value in a list. It iterates through the list using enumerate() checking each element against the target value.

Python `

a = [4, 7, 9, 7, 2, 7]

Find all indices of the value 7

indices = [i for i, x in enumerate(a) if x == 7] print("Indices", indices)

`

Explanation of Code:

• enumerate(a) generates pairs of index and value from the list.
• The condition if x == 7 filters indices where the value is 7.

Let's explore some other methods on ways to find indices of value in list.

Table of Content

• Using the index() Method
• Using NumPy
• Using a Loop

**Using index() Method

The index() method can be used iteratively to find all occurrences of a value by updating the search start position. This is particularly useful if only a few occurrences need to be located.

Example:

Python `

a = [4, 7, 9, 7, 2, 7]

Find all indices of the value 7

indices = [] start = 0

while True: try: index = a.index(7, start) indices.append(index) start = index + 1 except ValueError: break

print("Indices", indices)

`

Explanation:

• a.index(7, start) begins searching for 7 starting from the start index.
• The ValueError exception ends the loop when no more occurrences are found.

**Using NumPy

NumPy is a popular library for numerical computations. Converting the list to a NumPy array and using the where() function makes locating indices of a value efficient and concise.

Python `

import numpy as np

a = [4, 7, 9, 7, 2, 7]

Convert list to NumPy array

arr = np.array(a)

Find all indices of the value 7

indices = np.where(arr == 7)[0] print("Indices", indices.tolist())

`

Explanation:

• np.where(arr == 7) identifies positions where the array's value matches 7.

Using For Loop

A loop provides a straightforward way to iterate through the list and collect indices of matching values. This is useful when conditions beyond equality are required.

Python `

a = [4, 7, 9, 7, 2, 7]

Find all indices of the value 7

indices = [] for i in range(len(a)): if a[i] == 7: indices.append(i)

print("Indices", indices)

`

Explanation:

• The loop iterates over the indices of the list.
• The condition a[i] == 7 filters indices with the target value.