Python List Index( ) Method – Explained with Practical Examples

What is Index() In Python?

The index () method in Python is used to find the index position of the element or the desired element itself in a list, string, or data structure. 

For example, if you have a list of students in the commerce section in order of decreasing marks and want to find out who scored 3rd highest, you can use the index () function in the list. 

It returns the position or name of the student who has scored the 3rd highest marks. So, instead of going through the entire list to manually find out who has scored the 3rd highest marks, you just use a simple index function to get results in seconds.

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Example of Python list Index:

# List of students in order of decreasing marks

students = [“Alice”, “Bob”, “Charlie”, “David”, “Eve”]

# To find the student who scored the 3rd highest marks

third_highest_student = students[2]

print(f”The student who scored the 3rd highest marks is: {third_highest_student}”)

Output:

The student who scored the 3rd highest marks is: Charlie

Please note that you can access the index value, which is in this case, 3 or the element, which is Charlie, using the index function.

Also, index () is used to find the first occurrence of the specified value in the data structure.

For example:

# List of students in order of decreasing marks

students = [“Alice”, “Bob”, “Charlie”, “David”, “Alice”]

# Use the index() function to find the position of the first occurrence of ‘Alice’

first_occurrence = students.index(“Alice”)

print(f”The first occurrence of ‘Alice’ is at index: {first_occurrence}”)

Output:

The first occurrence of ‘Alice’ is at index: 0

In the above example, you can see that Alice occurs two times in the list. But when we apply the index () function, we get the lowest index value (0) as the result. Why?

This is because the index () in a string returns the lowest index position possible. If you want to find out the second index position, you can first find the lowest index and then use it as the starting point to begin your search from there. 

This way you will have the index position of the 2nd occurrence of Alice in the list.

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Basic Indexing Concepts

The index function in Python follows the same index across all data structures. Whether you want to use index in a list or string, the syntax remains the same.

Let’s have a look at the Python index syntax:

list_or_string.index(element, start, end)

Here element is the value you are searching for, and the start and end parameters are the index position ranges within which the search will happen.

Python List Index()

In Python, lists are a type of data structure that store an ordered collection of items with a unique index. This means that the first item has an index 0, the next element is 1 and so on.

The index allows you to modify, access and manage data in lists easily.

Python List Index Method

You can find the index position of any element in Python using the index function. 

For example:

fruits = [‘apple’, ‘banana’, ‘cherry’, ‘banana’]

print(fruits.index(‘banana’))  # Output: 1 (first occurrence of ‘banana’)

Here the index position of banana is 1 because this is the first time the word “banana” has occurred in the list. 

Negative Indexing

If you want to access the elements at the end of the list, you can use negative indexing. It is handy for cases when you have to use the elements at the end of the list but have no idea of its length, negative indexing returns the elements at -1, -2 and so on index positions.

For example:

my_list = [‘apple’, ‘banana’, ‘cherry’]

print(my_list[-1])  # Output: cherry

print(my_list[-2])  # Output: banana

Modifying Elements Using Indexing

In Python, you can also modify elements in a list with the index function. This is because lists in Python are mutable and can be easily modified with the right method.

Example of Python list index for modifying elements:

my_list = [‘apple’, ‘banana’, ‘cherry’]

my_list[1] = ‘blueberry’

print(my_list)  # Output: [‘apple’, ‘blueberry’, ‘cherry’]

Indexing in Strings

In Python, strings are data structures similar to lists bit contain data in a sequence of characters within closed quotations.

Similar to lists, you can use the index function to access the index position of a substring.

Index in Python String

The index () function in strings is similar to lists with the same syntax. You can use the same syntax with specified values and start and end parameters to find the first occurrence of that value or substring in the string.

For example:

# String of names

names = “Joey Chandler Ronald Monica Rachel Ross Ronald”

# Index of Ronald

index = names.index(“Ronald”)

# Print the result

print(“The index of Ronald is: “, index)

Output:

The index of Ronald is: 14

Here the name “Ronald” appears twice. But its first occurrence starts at the 14th index. So the result prints the index as 14th. This is because the index () in a string returns the lowest index possible.

Immutability Of Strings

Unlike lists, strings in Python are immutable. This means that you cannot modify or make any changes in the strings once it’s created. To modify a string, you will have to create a new string. 

In other words, every modification in strings gives birth to a new string in the memory, and the original string remains unchanged.

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Indexing in Tuples

Tuples in Python are similar to lists and strings. Each element in the tuple has a unique index and you can access these elements using the index function.

Similar To the Python List Index

In Tuples, the index of an element starts at 0 and continues in the increasing numerical order. Similar to the list index method, you can use the same syntax for searching index position of a specified value.

For example:

my_tuple = (10, 20, 30, 40)

print(my_tuple.index(30))  # Output: 2

Tuples also support negative indexing, where you can access the elements from the end of a tuple. 

For example:

print(my_tuple[-1])  # Output: 40

print(my_tuple[-3])  # Output: 20

Immutability Of Tuples

Tuples are similar to lists in many ways but there is one major difference. Unlike lists, tuples are immutable. This means that you cannot change, add or remove the elements in a Tuple.

Because of their immutability, tuples are often used when you want to store a collection of values that should not change during program execution.

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Index In Python For Loop

Index in Python for loop does not provide the index position of the specified value. Instead, it gives out the element itself.

However, there are several methods to access the index position and element within a loop. The most common methods are:

• range() and len() method
• enumerate() method

Example of range() and len() method for index in Python for Loop

fruits = [‘apple’, ‘banana’, ‘cherry’]

for i in range(len(fruits)):

   print(f”Index {i}: {fruits[i]}”)

Output:

• Index 0: apple
• Index 1: banana
• Index 2: cherry

Example of enumerate() method for index in Python for Loop

fruits = [‘apple’, ‘banana’, ‘cherry’]

for index, fruit in enumerate(fruits):

    print(f”Index {index}: {fruit}”)

Output:

• Index 0: apple
• Index 1: banana
• Index 2: cherry

Example of enumerate() index with start

fruits = [‘apple’, ‘banana’, ‘cherry’]

for index, fruit in enumerate(fruits, start=1):

    print(f”Index {index}: {fruit}”)

Output:

• Index 1: apple
• Index 2: banana
• Index 3: cherry

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Advanced Indexing Techniques

The above-mentioned examples are very common and straightforward indexing methods in Python. But when you are handling complex data structures, multi-dimensional lists, or filtering elements with conditions, you will have to employ advanced indexing techniques.

Multi-Dimensional Lists (Matrices)

In Python, multi-dimensional lists are also called nested lists. Here you will have to use two indices for the matrix, one for the column and one for the row.

For example:

matrix = [

    [1, 2, 3],

    [4, 5, 6]

]

print(matrix[0][1])  # Output: 2 (element in the first row, second column)

print(matrix[1][2])  # Output: 6 (element in the second row, third column)

Indexing With Conditions

Now, There are times when you have to filter out elements from the list that satisfy certain conditions. In such cases, you have to use list comprehension or filter() functions.

Example of Python list index using filter() :

numbers = [1, 2, 3, 4, 5, 6]

even_numbers = [num for num in numbers if num % 2 == 0]

print(even_numbers)  # Output: [2, 4, 6]

Conclusion

Python list index method is simple to use and handy to access and modify data in Python. The best thing about indexing in Python is that the same syntax is followed in lists, strings, and tuples. So, Python beginners don’t have to struggle to get their hands on the index function.

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However, when it comes to advanced techniques and complex data structures, you will have to be careful with the functions you use. 

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