`enumerate()`

Introduction

In many programming languages, iterating over an array requires a traditional for loop with an index variable i.

While Python supports for i in range(len(nums)):, it is considered un-Pythonic. When you need both the index and the value from a list, always use the built-in enumerate() function.

How `enumerate()` Works

enumerate() takes an iterable and returns an iterator that yields tuples of (index, value).

fruits = ["apple", "banana", "cherry"]

for index, value in enumerate(fruits):
    print(f"Index {index}: {value}")

Output:

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

Specifying a Start Index

You can provide an optional start parameter if you want the index to start counting from a number other than 0.

for count, value in enumerate(fruits, start=1):
    print(f"Item {count}: {value}")

When to use `enumerate()`

Use enumerate() whenever you need to update an element in place, or when the index itself is part of the problem's logic.

Example: Modifying a List In-Place

If you need to double every element in a list, you must know the index to overwrite the original value.

nums = [1, 2, 3]

# The Pythonic way
for i, num in enumerate(nums):
    nums[i] = num * 2

print(nums) # [2, 4, 6]

Example: Finding Indices of a Target

In a Two Sum variation, you might need to store the indices of elements in a hash map.

nums = [2, 7, 11, 15]
num_map = {}

for i, num in enumerate(nums):
    num_map[num] = i

Common Interview Mistakes

Mistake: `range(len())` vs `enumerate()`

While for i in range(len(nums)): works, it requires you to extract the value manually via val = nums[i]. It is slower and less readable. Interviewers familiar with Python expect you to use enumerate().

Avoid:

for i in range(len(nums)):
    print(i, nums[i])

Prefer:

for i, val in enumerate(nums):
    print(i, val)

Time and Space Complexity

Time Complexity: \(O(N)\) because it iterates through the list exactly once.
Space Complexity: \(O(1)\) extra space, as it returns an iterator that yields elements lazily without copying the list.

Summary

Use enumerate() when you need both the index and the value while iterating.
It returns an iterator of (index, value) tuples.
It is faster, cleaner, and more Pythonic than range(len()).

enumerate()