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Index in Python

Updated on 22/05/20244,481 Views

Introduction

The index in Python is used to find the position or index of an element within a list or a string. It is a valuable function when you need to locate the first occurrence of an element in a sequence. Understanding Python indexing is vital for anyone who wants to modify components in a sequence efficiently. This article will teach you the basic concepts of indexing in Python and how to use it with different elements.

Overview

The Python index() method is used to locate an element within a list and retrieve its position or index. By default, if there are multiple occurrences of the index of element in list python, index() will return the first occurrence position. In this article, we will explore the index() method, which provides a concise solution for finding the index of an element in a list.

How to Find Index of Element in List Python

The Python find index of all occurrences in list is used to retrieve the index of an element in a list.

  1. Create a list that contains the elements you want to work with.
  2. Use the index() method on the list to find the index of the specific element you're interested in.
  3. The index() method takes one argument to find the index of within the list.
  4. If the index of element in list python is found, the method returns its index. Python uses 0-based indexing, so the first element has an index of 0, the second element has an index of 1 and so on.

Suppose we have a list of fruits, and we want to find the indexing in Python of the element 'banana' within the list. Here's how you can do it:

fruits = ['apple', 'banana', 'cherry', 'date', 'elderberry']

# Using the index() method to find the index of 'banana'
index = fruits.index('banana')

# How to print value of index in Python
print("The index of 'banana' is:", index)

Output:

The index of 'banana' is: 1

In this example:

  • We have a list called fruits that contains various fruit names.
  • We use the index() method to find python the index of 'banana' within the list.
  • The result is printed, showing the index of 'banana' is 1. Keep in mind Python uses 0-based indexing, so 'banana' is the second element in the list but its index is 1.

Syntax of List Index () Method

Before we dive into practical examples, let's dissect the syntax of the index in Python for lists and strings.

Syntax of the index() Method for Lists

The syntax of the index() method for lists is straightforward:

Code

list.index(element)

Here's what each part of the syntax means:

  • list: The name of the list you want to search within.
  • .index(): The method itself.
  • element: The element you want to find the index of within the list.

Syntax of the index() Method for Strings

For python index string:

Code

string.index(substring)

In this case:

  • string: The string you want to search within.
  • .index(): The method for strings.
  • substring: The substring you want to find the index of within the string.

Find the index of the element

Locating the index of an element within a list is a fundamental task in Python. It involves using the index() method, a built-in function to find the position of a specific item in a sequence. Here are the practical examples to illustrate its functionality.

Example 1:

Suppose you have a list of colors:

colors = ['red', 'green', 'blue', 'yellow', 'red']

You want to find the indexing in Python of 'blue' within this list. You can use the index() method to do this:

Code

# Using the index() method to find the .index python of 'blue'

index = colors.index('blue')

# How to print value of index in Python

print("The index of 'blue' is:", index)

Output:

The index of 'blue' is: 2

In this example, the index Python method returns 2 because 'blue' is the third element in the list, and Python uses 0-based indexing.

Example 2

To find the index of a specific element in a list, Python offers the index() method. For example, if you have a list of numbers and want to find the index of the number 42:

Code

numbers = [10, 20, 30, 40, 50]
index = numbers.index(42)
print("The index of 42 is:", index)

Output:

The index of 42 is: 3

In this case, the index() method returns 3 because 42 is the fourth element in the list, considering 0-based indexing.

Handling Element Not Found:

If the index of element in list Python you're searching for is not in the list, the index() method raises a ValueError exception.

Example: 1

Code

list1 = [1, 2, 3, 4, 1, 1, 1, 4, 5]
# Attempting to find the index python of 10 which is not in the list
index = list1.index(10)

# This line will not be reached due to the ValueError
print("The index of 10 is:", index)

Output:

Traceback (most recent call last):

File "/home/b910d8dcbc0f4f4b61499668654450d2.py", line 8, in

print(list1.index(10))

ValueError: 10 is not in the list

Example 2:

fruits = ['apple', 'banana', 'cherry', 'date']

# Attempting to find the index of grapes which is not in the list
index = fruits.index('grape')

# The line will not be reached due to the ValueError
print("The index of 'grape' is:", index)

Output:

ValueError: 'grape' is not in the list.

Playing With Parameters

The index() method can be used with optional parameters, start and end, to specify a range for the search. Here's how it works:

Working on the index() With Start and End Parameters

The index() method in Python can accept two additional parameters, start and end, to specify a range within which the method searches for the element. It searches for the element only within the specified range and returns its index if found.

Here's the syntax:

list.index(element, start, end)

  • list: The name of the list you want to search within.
  • element: The element you want to find the index of within the list.
  • start: The index at which the search should begin (inclusive).
  • end: The index at which the search should end (exclusive).

Let's explore this with an example:

Code

numbers = [10, 20, 30, 20, 40, 50]
index = numbers.index(20, 1, 4)

# Printing the result
print("The index of 20 within the range (1, 4) is:", index)

Output:

The index of 20 within the range (1, 4) is: 1

In this example:

  • The index() method is used to find the index of '20' within the list numbers.
  • The search starts at index 1 (inclusive) and ends at index 4 (exclusive), meaning it looks for '20' only between indices 1 and 3.
  • The result is 1, indicating that '20' is found at index 1 within the specified range.

Working of the index() With two Parameters only

When you use the index() method with only the element parameter, it works like the index() method. It searches the entire Python select list elements by index for the first occurrence of the specified element and returns its index.

Example 1:

fruits = ['apple', 'banana', 'cherry', 'banana', 'date']

# Using the index() method
index = fruits.index('banana', 1)

# Printing the result
print("The index of the first occurrence of 'banana' starting from index 1 is:", index)

Output:

The index of the first occurrence of 'banana' starting from index 1 is: 3

In this example, the index() method searches for the index of the first occurrence of 'banana' starting from index 1 within the fruits list. It returns 3 because 'banana' is found at index 3 within the sublist.

Example 2:

Code

numbers = [10, 20, 30, 20, 40, 50]

# Using the index() method
index = numbers.index(20, 1)

# Printing the result
print("The index of '20' starting from index 1 is:", index)

Output:

The index of '20' starting from index 1 is: 1

In this example, the index() method searches for the index of '20' starting from index 1 within the numbers list. It returns 1 because '20' is found at index 1 within the sublist.

How to fix list index out of range using Index()

The index() method in Python is used to find the position or index of an element within a list or a Python index string. However, it does not directly fix a "list index out of range" error. This error occurs when you try to access an index that does not exist in the list.

Code

my_list = [10, 20, 30, 40, 50]

# Attempting to iterate using a constant range greater than the list length
for i in range(6):
    print(my_list[i])

Output:

10

20

30

40

50

IndexError: list index out of range

Solving the error

Code

my_list = [10, 20, 30, 40, 50]

# Counting the number of elements in the list
count = 0
for _ in my_list:
    count = 1

# Iterating through the list using the count as the range limit
for i in range(count):
    print(my_list[i])

Output:

10

20

30

40

50

List Index in Python

The list index() Python method returns the index number of a certain element in a list. If there are numerous occurrences of the item, index() will return the first index position where the item occurs.

Example:

Code:

participants = ['Leslie Tucker', 'Peter Free', 'Luke Oliver', 'Ernie Bolton']
participant_name = 'Ernie Bolton'
index_of_participant = participants.index(participant_name)

print(inxdex_of_participant)

Output:

3

This output indicates that "Ernie Bolton" is found at index 3 in the participants list.

String Index in Python

In Python, strings are sequences of characters, and each character within a string can be accessed using an index. String indexing is zero-based, the first character has an index of 0, the second character has an index of 1, and so on.

Example:

Code:

student_names = 'Judith Joey Fred Luke Linda'
index_position_of_joey = student_names.index('Joey')
print(index_position_of_joey)

Output:

7

This output indicates the substring "Joey" starts at index 7 (0-based indexing) within the student_names string.

Conclusion

The index in Python is a versatile and essential tool for locating elements within lists and strings. This extensive guide has equipped you with the knowledge and practical examples to utilize this method effectively. You can now confidently work with lists, strings and indices in Python.

FAQs

1. Where is indexing in Python not possible?

The set's elements are immutable (cannot be changed), but the set as a whole is changeable. Any element in a Python set does not have an index. As a result, no indexing or slicing in Python operations are supported.

2. Can you use the index() method with a substring within a string?

Yes, you can use the index() method with strings to find the index of a substring within the string. It works similarly to finding elements in a list.

3. What data types support indexing in Python?

Lists, strings, tuples, and dictionaries support indexing in Python to retrieve individual elements or characters based on their position or key.

4. How does Python find the index of all occurrences in a list or string?

To find the indices of all occurrences of an element, you can use a loop or list comprehension. Iterate through the list or string and record the indices where the element is found.

Pavan

PAVAN VADAPALLI

Director of Engineering

Director of Engineering @ upGrad. Motivated to leverage technology to solve problems. Seasoned leader for startups and fast moving orgs. Working … Read More

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