Positive and negative indices in Python?

Python sequences like lists, tuples, and strings support two types of indexing: positive indexing (starting from 0) and negative indexing (starting from -1). This tutorial explains both approaches with practical examples.

What Are Sequence Indexes?

Indexing allows us to access individual elements in Python sequence data types. There are two types:

• Positive indexing Starts from 0 and increases to n-1 (where n is the total number of elements)

• Negative indexing Starts from -1 (last element) and moves backwards to -n

Positive Indexing Examples

Positive indexing starts from 0 for the first element ?

# Accessing elements with positive indices
numbers = [10, 20, 30, 40, 50]

print("First element:", numbers[0])
print("Second element:", numbers[1])
print("Third element:", numbers[2])

First element: 10
Second element: 20
Third element: 30

Index Out of Range Error

Accessing an index that doesn't exist raises an IndexError ?

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

try:
    print(numbers[10])
except IndexError as e:
    print("Error:", e)

Error: list index out of range

Negative Indexing Examples

Negative indexing starts from -1 for the last element ?

# Accessing elements with negative indices
numbers = [10, 20, 30, 40, 50]

print("Last element:", numbers[-1])
print("Second last:", numbers[-2])
print("Third last:", numbers[-3])

Last element: 50
Second last: 40
Third last: 30

Negative Indexing with Slicing

You can combine negative indices with slicing operations ?

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

# Get last three elements
print("Last 3 elements:", numbers[-3:])

# Get elements from index -4 to -1 (excluding -1)
print("From -4 to -2:", numbers[-4:-1])

Last 3 elements: [30, 40, 50]
From -4 to -2: [20, 30, 40]

Practical Applications

Removing Last Element

Use negative indexing with pop() to remove elements from the end ?

fruits = ['apple', 'banana', 'cherry', 'date']
print("Original list:", fruits)

# Remove last element
removed = fruits.pop(-1)
print("Removed:", removed)
print("Updated list:", fruits)

Original list: ['apple', 'banana', 'cherry', 'date']
Removed: date
Updated list: ['apple', 'banana', 'cherry']

Finding Element Index

Use the index() method to find the position of an element ?

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

print("Index of 'blue':", colors.index('blue'))
print("Index of 'yellow':", colors.index('yellow'))

# Access using found index
blue_index = colors.index('blue')
print("Element at index", blue_index, "is:", colors[blue_index])

Index of 'blue': 2
Index of 'yellow': 3
Element at index 2 is: blue

Comparison of Indexing Methods

Conclusion

Python's dual indexing system provides flexibility for accessing sequence elements. Use positive indexing (0, 1, 2...) when working from the beginning and negative indexing (-1, -2, -3...) when accessing elements from the end. This feature makes Python code more readable and efficient for common operations like getting the last element or removing items from the end.