Python How to Find all Indexes of an Item in a List

Lists are similar to dynamically sized arrays, which are declared in other languages(e.g., vector in C++ and ArrayList in Java). Lists do not have to be homogeneous all of the time, which makes it a very useful tool in Python. DataTypes such as Integers, Strings, and Objects can all be included in a single list. Lists are mutable, which means they can be changed after they’ve been created.

An index refers to a position within an list.

Given a list and item, the task is to print all indexes of occurrences of that item in the list

Examples:

Input :

givenlist = ["this", "is", "the", "new", "way", "to" , "learn" , "python", "which" , "is" ,"easy" ,"is" ]

item = is

Output:

Indexes are : 1 9 11

Explanation:

Here "is" is present at the indices 1,9 and 11

Print the indices of occurrences of the item

There are several ways to find indices of item in list some of them are:

Using For loop and list
Using enumerate() function
Using list comprehension and range
Using count() function in itertools
Using Numpy

Method #1: Using For loop and list

Create an empty list. Iterate through each element in the original list using a for-loop.

Append the index of each occurrence of the given item to the empty list.

Below is the implementation:

# Given list
givenlist = ["this", "is", "the", "new", "way", "to", "learn", "python",
             "which", "is", "easy", "is"]

# Given item
item = "is"

# Taking empty list
indexes = []

# Loop over indices of elements in givenlist
for index in range(len(givenlist)):
  # if element is equal to given item then append to empty list
    if givenlist[index] == item:
        indexes.append(index)

# printing the result indexes
print("Indexes are : ", *indexes)

Note : Here * is used to print list with spaces

Output:

Indexes are :  1 9 11

Method #2 : Using enumerate() function

The built-in function enumerate can be used to get the index of all occurrences of an element in a list ().

It was created to solve the loop counter issue and can be used in these type of problems.

Below is the implementation:

# Given list
givenlist = ["this", "is", "the", "new", "way", "to", "learn", "python",
             "which", "is", "easy", "is"]

# Given item
item = "is"

print("Indexes are :", end=" ")

for index, element in enumerate(givenlist):
    # If the element is equal to item then print its index
    if element == item:
        print(index, end=" ")

Output:

Indexes are :  1 9 11

Method #3: Using list comprehension and range

You can get a list of all valid indices with the range() function and then compare the corresponding value

at each index with the given object.

Below is the implementation:

# Given list
givenlist = ["this", "is", "the", "new", "way", "to", "learn", "python",
             "which", "is", "easy", "is"]

# Given item
item = "is"

print("Indexes are :", end=" ")
# Using list comprehension to store indexes in indexes list
indexes = [index for index in range(
    len(givenlist)) if givenlist[index] == item]

# print the list
print(*indexes)

Output:

Indexes are :  1 9 11

Method #4 : Using count() function in itertools

We can use itertools count() function to build an iterator for efficient looping that returns evenly

spaced values beginning with the given amount.

You may use it in conjunction with the zip() function to add sequence numbers as shown below.

from itertools import count
# Given list
givenlist = ["this", "is", "the", "new", "way", "to", "learn", "python",
             "which", "is", "easy", "is"]

# Given item
item = "is"

print("Indexes are :", end=" ")
indexes = [(i, j) for i, j in zip(count(), givenlist) if j == item]


# Traverse the indexes list and print only index
for i in indexes:
    print(i[0], end=" ")

Note: Here we need only index so we print only index element in indexes list

Output:

Indexes are :  1 9 11

Method #5 : Using Numpy

We use numpy.where() function to get indexes of the occurrences of item in given list.

Below is the implementation

import numpy as np
# Given list
givenlist = ["this", "is", "the", "new", "way", "to", "learn", "python",
             "which", "is", "easy", "is"]

# Given item
item = "is"

indexes = np.where(np.array(givenlist) == item)[0]

print("Indexes are :",*indexes)

Output:

Indexes are :  1 9 11

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