for Loops

Iterating over sequences, ranges, and collections with Python's for loop.

Overview

The for loop in Python iterates over any iterable a sequence, collection, or generator executing the loop body for each item. Unlike C-style languages where you typically write a counter-based loop, Python's for loop works directly on the items themselves. The built-in range() function generates a sequence of integers and is the standard way to run a loop a specific number of times. range(n) generates 0 through n-1; range(start, stop) generates start through stop-1; range(start, stop, step) lets you control the step size. Two useful built-ins that pair with for loops: enumerate() gives you both the index and value, and zip() lets you iterate over two sequences in parallel. Both are commonly used in real Python code.

Key Points

•for item in iterable: iterates over any sequence, list, string, tuple, dict, or range
•range(n) generates 0 to n-1; range(5, 10) generates 5 to 9; range(0, 10, 2) generates even numbers
•break exits the loop immediately; continue skips to the next iteration
•enumerate(seq) yields (index, value) pairs avoids manual counter tracking
•zip(a, b) iterates two sequences in parallel, pairing items by position
•for loops can have an else clause it runs when the loop completes normally (not via break)

Examples

Iterating over a list and a string

fruits = ["apple", "banana", "cherry"]
for fruit in fruits:
    print(fruit)
# apple
# banana
# cherry

for char in "Python":
    print(char, end=" ")   # P y t h o n

range() and enumerate()

# Count from 0 to 4
for i in range(5):
    print(i, end=" ")    # 0 1 2 3 4

# Count from 1 to 10, step 2
for i in range(1, 11, 2):
    print(i, end=" ")    # 1 3 5 7 9

# enumerate: index + value together
names = ["Alice", "Bob", "Carol"]
for i, name in enumerate(names):
    print(f"{i}: {name}")
# 0: Alice  1: Bob  2: Carol

break, continue, and zip()

# break exits the loop
for n in range(10):
    if n == 5:
        break
    print(n, end=" ")   # 0 1 2 3 4

# continue skips the current iteration
for n in range(10):
    if n % 2 == 0:
        continue
    print(n, end=" ")   # 1 3 5 7 9

# zip: iterate two lists in parallel
names = ["Alice", "Bob"]
scores = [95, 82]
for name, score in zip(names, scores):
    print(f"{name}: {score}")

Common Mistakes

Modifying a list while iterating over it iterate over a copy: `for item in items[:]:`
Using range(len(items)) when enumerate(items) is cleaner and more Pythonic
Off-by-one errors: range(1, 10) gives 1–9, not 1–10; use range(1, 11) for 1–10

Note

If you find yourself writing `for i in range(len(my_list))` to access elements by index, switch to `for item in my_list` or `for i, item in enumerate(my_list)` it's cleaner and less error-prone.

for Loops

Overview

Key Points

Examples

Common Mistakes

Note

Exercises

Knowledge Check

What does `range(2, 10, 3)` generate?