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Tags : PythonPython List

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A list is used in Python to store multiple elements under a single name. Each element can be accessed using its position in the list. An element can be present at multiple positions in a list.

In this tutorial, we will introduce how to find the indices of all the occurrences of a specific element in a list. We will work with the following list and find all the indices of the element `1`

.

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

`for`

Loop to Find the Indices of All the Occurrences of an ElementWe can easily iterate over the list and compare each element to the required element and find its indices. We can store the final result in a new list. In the following example, we iterate over the list using the `range()`

function:

`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 of implementing the above code is to use the list comprehension below.

`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 the `enumerate()`

function, which returns the index and the 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()`

Function to Find the Indices of All the Occurrences of an Element in PythonThe `NumPy`

library has the `where()`

function, which is used to return the indices of an element in an array based on some condition. For this method, we have to pass the list as an array. The final result is also in 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] `

`more_itertools.locate()`

Function to Find the Indices of All the Occurrences of an ElementThe `more_itertools`

is a third party and handy module. It has many functions that can create efficient and compact code when working with iterables. The `locate()`

function in this module returns the indices of the elements which are `True`

for the condition. It returns an `itertools`

object. 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 the `list()`

function to ensure that the final result is in the form of a list.