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In Python, lists are used to store multiple elements under one name. Each element can be accessed by its position in the list. An element can appear in multiple positions in a list.

In this tutorial, we will see how to find the index of all occurrences of a particular element in a list. We will find all the indices of the element in the following list 1.

l1 = [1, 5, 1, 8, 9, 15, 6, 2, 1]

Use forloop to find the indices of all occurrences of an element

We can easily iterate over a list and compare each element with the required element and find its index. We can store the final result in a new list. In the following example, we use range()the iterate function to iterate over a list.

l1 = [1, 5, 1, 8, 9, 15, 6, 2, 1]
pos = []
x = 1  # The required element

for i in range(len(l1)):
    if l1[i] == x:
        pos.append(i)
print(pos)

Output:

[0, 2, 8]

A more efficient and compact way to implement the above code is to use the following list comprehension.

l1 = [1, 5, 1, 8, 9, 15, 6, 2, 1]

pos = [i for i in range(len(l1)) if l1[i] == 1]
print(pos)

Output:

[0, 2, 8]

Similarly, we can also use enumerate()the function to return the index and value together. For example

l1 = [1, 5, 1, 8, 9, 15, 6, 2, 1]
pos = [i for i, x in enumerate(l1) if x == 1]
print(pos)

Output:

[0, 2, 8]

numpy.where()Find the index of all occurrences of an element in Python using the

NumPyThe library has where()a function that returns the index of an element in an array based on some condition. To this method, we have to pass the list as an array. The final result also comes in the form of an array. The following code snippet shows how we can use this method.

import numpy as np

l1 = [1, 5, 1, 8, 9, 15, 6, 2, 1]

pos = np.where(np.array(l1) == 1)[0]

print(pos)

Output:

[0 2 8]

Use more_itertools.locate()the function to find the indices of all occurrences of an element

more_itertoolsis a third-party convenience module. It has many functions to create efficient and compact code when processing iterable elements. locate()The function in this module returns Truethe index of the element where the condition is . It returns a itertoolsobject. The following code snippet explains how we can use this method.

from more_itertools import locate

l1 = [1, 5, 1, 8, 9, 15, 6, 2, 1]

pos = list(locate(l1, lambda x: x == 1))

print(pos)

Output:

[0, 2, 8]

We use list()the function to ensure that the final result is in the form of a list.