How to Use enumerate() vs range(len())

Table Of Contents

• Problem
• Solution
• Explanation

Problem

You need both the index and value when iterating over a list, but using range(len()) creates verbose and less readable code.

Solution

# Bad: Using range(len())
items = ['apple', 'banana', 'orange']

for i in range(len(items)):
    print(f"{i}: {items[i]}")

# Good: Using enumerate()
for i, item in enumerate(items):
    print(f"{i}: {item}")

# Start counting from different number
for i, item in enumerate(items, start=1):
    print(f"{i}: {item}")
# 1: apple
# 2: banana  
# 3: orange

# Find index of specific item
def find_index(items, target):
    for i, item in enumerate(items):
        if item == target:
            return i
    return -1

index = find_index(['a', 'b', 'c'], 'b')  # Returns 1

# Process with condition based on index
data = [10, 20, 30, 40, 50]
processed = []

for i, value in enumerate(data):
    if i % 2 == 0:  # Even indices
        processed.append(value * 2)
    else:  # Odd indices
        processed.append(value)

print(processed)  # [20, 20, 60, 40, 100]

# Enumerate with slicing
text = "hello world"
for i, char in enumerate(text[2:], start=2):
    print(f"Position {i}: {char}")

# Compare performance (enumerate is faster)
import timeit

items = list(range(1000))

def with_range_len():
    result = []
    for i in range(len(items)):
        result.append((i, items[i]))
    return result

def with_enumerate():
    result = []
    for i, item in enumerate(items):
        result.append((i, item))
    return result

# enumerate() is faster and more memory efficient
print("range(len()):", timeit.timeit(with_range_len, number=1000))
print("enumerate():", timeit.timeit(with_enumerate, number=1000))

# Convert enumerate to list for multiple uses
indexed_items = list(enumerate(['x', 'y', 'z']))
print(indexed_items)  # [(0, 'x'), (1, 'y'), (2, 'z')]

Explanation

enumerate() returns tuples of (index, value) for each item in an iterable. It's more readable, faster, and more Pythonic than range(len()).

Use start parameter to begin counting from a different number. enumerate() works with any iterable, not just lists, making your code more flexible.