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Indexing and Slicing Lists

Introduction

Python's list indexing and slicing syntaxes are identical to those used for strings. The key difference is that because lists are mutable, you can use slicing to modify or replace parts of a list in-place.

Negative Indexing

Python supports negative indexing, which counts from the end of the list.

nums = [10, 20, 30, 40]

print(nums[-1]) # 40 (last element)
print(nums[-2]) # 30 (second to last)

This is highly preferred in interviews over nums[len(nums) - 1].

Slicing Syntax

The syntax is list[start:stop:step]. It returns a new list.

nums = [0, 1, 2, 3, 4, 5]

print(nums[1:4])  # [1, 2, 3]
print(nums[:3])   # [0, 1, 2]
print(nums[3:])   # [3, 4, 5]

Reversing a List

The most Pythonic way to reverse a list is using a step of -1.

nums = [1, 2, 3]
rev = nums[::-1]
print(rev) # [3, 2, 1]
This takes \(O(N)\) time and \(O(N)\) space.

(Note: If the problem asks for \(O(1)\) space, use the in-place method nums.reverse() instead).

Modifying Lists with Slices

Because lists are mutable, you can assign an iterable to a slice. This replaces the targeted slice with the new elements.

nums = [1, 2, 3, 4]

# Replace elements at index 1 and 2
nums[1:3] = [9, 9]
print(nums) # [1, 9, 9, 4]

You can even insert or delete elements by assigning iterables of different lengths.

nums = [1, 2, 3]

# Insert without replacing (insert at index 1)
nums[1:1] = [9, 9]
print(nums) # [1, 9, 9, 2, 3]

# Delete a slice (equivalent to del nums[1:3])
nums[1:3] = []
print(nums) # [1, 2, 3]

While neat, this is rarely required in standard DSA questions.

Making a Shallow Copy

The most common use of slicing in interviews is creating a quick shallow copy of a list.

nums = [1, 2, 3]
copy_nums = nums[:]
This is equivalent to nums.copy() and prevents you from accidentally modifying the original list.

Time and Space Complexity

  • Time Complexity: \(O(K)\) where \(K\) is the length of the slice.
  • Space Complexity: \(O(K)\) because slicing always allocates a new list in memory.

Summary

  • Use nums[-1] to get the last element.
  • Slicing (nums[a:b]) creates a new list object.
  • Use nums[::-1] to create a reversed copy.
  • Use nums[:] to create a shallow copy.