Python

Python Fundamentals

Loops & Iteration Basics

Python's for loop iterates directly over items in a collection — there's no manual index management like in C-style languages.

JrCodex·6 min read

Jr Codex Python Notes

Level: Beginner Prerequisites: Chapter 10 Time to complete: ~25 minutes

Table of Contents

  1. The for Loop
  2. The range() Function
  3. The while Loop
  4. break and continue
  5. The else Clause on Loops
  6. Looping with enumerate() and zip()
  7. Common Pitfalls
  8. Summary & Next Steps

1. The for Loop

Python's for loop iterates directly over items in a collection — there's no manual index management like in C-style languages.

fruits = ["apple", "banana", "cherry"]
 
for fruit in fruits:
    print(fruit)
 
# Also works directly on strings, dicts, sets, tuples — anything "iterable"
for char in "abc":
    print(char)      # a  b  c
for item in collection:
─────────────────────────────────────
  Python pulls items ONE AT A TIME from the collection
  and binds each to `item` for the duration of that pass.
─────────────────────────────────────

2. The range() Function

When you need a sequence of numbers (not an existing collection), use range():

for i in range(5):          # 0, 1, 2, 3, 4  — stop is EXCLUSIVE
    print(i)
 
for i in range(2, 6):        # 2, 3, 4, 5 — start, stop
    print(i)
 
for i in range(0, 10, 2):     # 0, 2, 4, 6, 8 — start, stop, step
    print(i)
 
for i in range(10, 0, -1):     # 10, 9, 8, ..., 1 — counting down
    print(i)
CallProduces
range(5)0, 1, 2, 3, 4
range(2, 6)2, 3, 4, 5
range(0, 10, 2)0, 2, 4, 6, 8
range(5, 0, -1)5, 4, 3, 2, 1

range() is lazy — it doesn't build a full list in memory, just generates numbers as needed (the same underlying idea behind generators, covered in Module 5).

3. The while Loop

Repeats as long as a condition remains true. Use this when you don't know the number of iterations in advance.

count = 0
while count < 5:
    print(count)
    count += 1     # without this, the loop runs forever!
 
# Common pattern: loop until a condition is met
import random
 
target = random.randint(1, 10)
guess = None
attempts = 0
 
while guess != target:
    guess = random.randint(1, 10)   # simulate a guess
    attempts += 1
 
print(f"Found {target} after {attempts} attempts")

The #1 while-loop bug: forgetting to update the condition variable, causing an infinite loop. Always double-check that something inside the loop moves you toward the exit condition.

4. break and continue

# break — exit the loop immediately
for num in range(1, 20):
    if num == 5:
        break
    print(num)          # prints 1, 2, 3, 4 — then stops
 
# continue — skip to the next iteration
for num in range(1, 10):
    if num % 2 == 0:
        continue          # skip even numbers
    print(num)             # prints 1, 3, 5, 7, 9
break vs continue
─────────────────────────────────────
  break     → exits the loop entirely, no more iterations
  continue  → skips the REST of the current iteration only
─────────────────────────────────────

5. The else Clause on Loops

An often-overlooked Python feature: loops can have an else block that runs only if the loop completes without hitting break.

def find_prime_factor(n):
    for i in range(2, n):
        if n % i == 0:
            print(f"{n} is divisible by {i}")
            break
    else:
        print(f"{n} is prime")   # only runs if the loop never broke
 
find_prime_factor(15)    # "15 is divisible by 3"
find_prime_factor(13)    # "13 is prime"

This is genuinely useful for "search and confirm not found" logic — no flag variable needed.

6. Looping with enumerate() and zip()

Two of the most useful loop helpers in everyday Python.

enumerate() — get index and value together

fruits = ["apple", "banana", "cherry"]
 
# Instead of manual index tracking:
for i in range(len(fruits)):
    print(i, fruits[i])
 
# Idiomatic:
for i, fruit in enumerate(fruits):
    print(i, fruit)
 
for i, fruit in enumerate(fruits, start=1):   # start counting from 1
    print(f"{i}. {fruit}")

zip() — loop over multiple sequences together

names = ["Alice", "Bob", "Charlie"]
ages = [25, 30, 35]
 
for name, age in zip(names, ages):
    print(f"{name} is {age} years old")
 
# zip stops at the SHORTEST iterable
a = [1, 2, 3]
b = ["x", "y"]
print(list(zip(a, b)))    # [(1, 'x'), (2, 'y')] — no error, just stops early

7. Common Pitfalls

# Pitfall 1: modifying a list while iterating over it
numbers = [1, 2, 3, 4, 5]
for n in numbers:
    if n % 2 == 0:
        numbers.remove(n)     # ✗ skips elements — indices shift as you remove!
# Result is unpredictable: [1, 3, 5] is NOT guaranteed here
 
# Fix: iterate over a copy, or build a new list
numbers = [1, 2, 3, 4, 5]
numbers = [n for n in numbers if n % 2 != 0]   # list comprehension (Module 2)
print(numbers)                                   # [1, 3, 5]
 
 
# Pitfall 2: off-by-one with range()
for i in range(1, 10):     # Does this include 10? NO — range stop is exclusive
    pass
for i in range(1, 11):      # ✓ this includes 10
    pass
 
 
# Pitfall 3: infinite while loop
count = 0
# while count < 5:
#     print(count)           # forgot count += 1 — runs forever!

8. Summary & Next Steps

Key Takeaways

  • for loops iterate directly over collection items — no manual indexing needed.
  • range(start, stop, step) generates numbers lazily; stop is always exclusive.
  • while loops run until a condition becomes false — always ensure the loop body moves toward that condition.
  • break exits a loop entirely; continue skips to the next iteration; a loop's else runs only if break was never hit.
  • enumerate() and zip() are the idiomatic ways to get indices and to loop over multiple sequences in parallel.

Concept Check

  1. What does range(2, 10, 3) produce?
  2. Why is modifying a list while iterating over it dangerous?
  3. When does a for/else block's else clause execute?

Module 1 Complete — Next Module

You've now covered every core data type and control structure in Python. Module 2 builds on this foundation with functions — how to package logic into reusable, named blocks.

→ Module 2: Functions & Functional Basics

Jr Codex — 1-on-1 Personalized Coaching | Back to Module Index