🌱 Beginner

Loops (for & while)

📖 Lesson 10 ⏱ 30 min 🧪 5 questions 💻 3 exercises

🎯 Learning Objectives

Use for loops to iterate over sequences, ranges, and dictionaries
Use while loops for condition-based repetition
Control loop execution with break, continue, and else
Understand range() and its parameters
Avoid common loop pitfalls (infinite loops, modifying during iteration)
Use nested loops and know when to flatten them

The `for` Loop

Python's for loop iterates over any iterable — lists, strings, tuples, sets, dictionaries, files, and more. It doesn't use a counter like C-style for loops; it directly gives you each item.

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

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

# Iterate over a dictionary
scores = {"Alice": 85, "Bob": 92}
for name, score in scores.items():
    print(f"{name}: {score}")

# Iterate over a set (order not guaranteed)
for n in {3, 1, 4, 1, 5}:
    print(n)

for_basics.py

Python's for is a for-each loop. It doesn't count — it asks the iterable for the next item each iteration. This is why it works with any iterable, not just indexed collections.

The `range()` Function

range() generates a sequence of numbers. It's commonly used when you need to loop a specific number of times or need numeric indices.

Call	Produces	Notes
`range(5)`	0, 1, 2, 3, 4	Start defaults to 0; stop is exclusive
`range(2, 6)`	2, 3, 4, 5	Start inclusive, stop exclusive
`range(0, 10, 2)`	0, 2, 4, 6, 8	Step of 2
`range(10, 0, -1)`	10, 9, 8, …, 1	Count down

# Repeat 5 times
for i in range(5):
    print(f"Iteration {i}")

# Loop with specific start/stop
for n in range(1, 11):
    print(n, end=" ")  # 1 2 3 4 5 6 7 8 9 10

# Count backwards
for n in range(5, 0, -1):
    print(n, end=" ")  # 5 4 3 2 1

# Use with len() to iterate by index (prefer enumerate instead!)
colors = ["red", "green", "blue"]
for i in range(len(colors)):
    print(f"{i}: {colors[i]}")

# Better — use enumerate
for i, color in enumerate(colors):
    print(f"{i}: {color}")

range_demo.py

range() is lazy — it doesn't create a list in memory. range(1_000_000_000) uses almost no memory because it generates numbers on demand.

The `while` Loop

A while loop repeats as long as its condition is True. Use it when you don't know in advance how many iterations you need.

# Basic while
count = 0
while count < 5:
    print(count)
    count += 1
# 0, 1, 2, 3, 4

# User input loop
while True:
    answer = input("Type 'quit' to exit: ")
    if answer == "quit":
        break
    print(f"You said: {answer}")

# Countdown
n = 10
while n > 0:
    print(n, end=" ")
    n -= 1
print("Liftoff!")

while_basics.py

A while loop with a condition that never becomes False creates an infinite loop. Always ensure something inside the loop will eventually make the condition False (or use break).

for vs while — when to use which

Use `for` when…	Use `while` when…
You have a definite collection to iterate	You don't know how many iterations ahead of time
You want to loop a known number of times	You're waiting for a condition to change
Processing each item in a sequence	User input loops, polling, convergence algorithms

`break` and `continue`

Statement	Effect
`break`	Exit the loop immediately (skip remaining iterations)
`continue`	Skip the rest of this iteration, jump to the next one

# break — stop searching once found
numbers = [4, 7, 2, 9, 1, 8]
for n in numbers:
    if n == 9:
        print("Found 9!")
        break
    print(f"Checked {n}")
# Checked 4, Checked 7, Checked 2, Found 9!

# continue — skip odd numbers
for n in range(1, 11):
    if n % 2 != 0:
        continue  # skip to next iteration
    print(n, end=" ")
# 2 4 6 8 10

# break in a while loop
attempts = 0
while True:
    password = input("Password: ")
    attempts += 1
    if password == "secret":
        print("Access granted!")
        break
    if attempts >= 3:
        print("Too many attempts!")
        break

break_continue.py

Use break to exit early when you've found what you're looking for. Use continue to skip items that don't meet a criterion — it keeps the main loop body at a lower indentation level.

The `else` Clause on Loops

Python uniquely allows an else block on loops. It runs only if the loop completed normally (was NOT terminated by break):

# Search with else — clean "not found" handling
targets = [2, 4, 6, 8, 10]
search = 7

for n in targets:
    if n == search:
        print(f"Found {search}!")
        break
else:
    # This runs only if break was NEVER hit
    print(f"{search} not found in list.")
# Output: 7 not found in list.

# With while
n = 0
while n < 100:
    if is_prime(n) and n > 50:
        print(f"First prime > 50: {n}")
        break
    n += 1
else:
    print("No prime > 50 found under 100")

loop_else.py

Think of loop else as "no-break" — it executes when the loop finishes without hitting break. It's most useful for search patterns where you need to know if nothing was found.

Nested Loops

# Multiplication table
for i in range(1, 6):
    for j in range(1, 6):
        print(f"{i*j:3}", end=" ")
    print()  # newline after each row
#   1   2   3   4   5
#   2   4   6   8  10
#   3   6   9  12  15
#   ...

# Iterate over a 2D grid
matrix = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
for row in matrix:
    for cell in row:
        print(cell, end=" ")
    print()

# Finding pairs
names = ["Alice", "Bob", "Charlie"]
for i, name1 in enumerate(names):
    for name2 in names[i+1:]:
        print(f"{name1} & {name2}")

nested_loops.py

break only exits the innermost loop. To break out of multiple nested loops, use a flag variable, a function with return, or restructure with itertools.product().

Common Loop Patterns

# Accumulator pattern
total = 0
for n in [10, 20, 30, 40]:
    total += n
print(total)  # 100

# Building a list
squares = []
for n in range(1, 6):
    squares.append(n ** 2)
print(squares)  # [1, 4, 9, 16, 25]
# Better: list comprehension → [n**2 for n in range(1, 6)]

# Filter pattern
words = ["hello", "world", "hi", "hey", "howdy"]
h_words = []
for w in words:
    if w.startswith("h"):
        h_words.append(w)
print(h_words)  # ['hello', 'hi', 'hey', 'howdy']

# zip — parallel iteration
names = ["Alice", "Bob", "Charlie"]
scores = [85, 92, 78]
for name, score in zip(names, scores):
    print(f"{name}: {score}")

# enumerate with start
for rank, name in enumerate(sorted(names), start=1):
    print(f"{rank}. {name}")

patterns.py

Common Pitfalls

# ❌ Modifying a list while iterating over it
items = [1, 2, 3, 4, 5]
# for item in items:
#     if item % 2 == 0:
#         items.remove(item)  # DANGEROUS — skips elements!

# ✓ Solution 1: iterate over a copy
for item in items[:]:       # slice creates a copy
    if item % 2 == 0:
        items.remove(item)

# ✓ Solution 2: build a new list (preferred)
items = [1, 2, 3, 4, 5]
items = [x for x in items if x % 2 != 0]  # [1, 3, 5]

# ❌ Forgetting to update the condition variable in while
# count = 0
# while count < 5:
#     print(count)
#     # forgot count += 1 → infinite loop!

# ❌ Off-by-one errors with range
# Want 1-10: range(1, 11) not range(1, 10)
for i in range(1, 11):    # 1 through 10 inclusive
    print(i)

pitfalls.py

Primary sources: Python Docs — for Statements · Python Docs — range()

🤖

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💻 Exercises

01 FizzBuzz (Full)

Print numbers from 1 to 100. For multiples of 3 print "Fizz", for multiples of 5 print "Buzz", for multiples of both print "FizzBuzz", otherwise print the number.

Show solution

for n in range(1, 101):
    if n % 3 == 0 and n % 5 == 0:
        print("FizzBuzz")
    elif n % 3 == 0:
        print("Fizz")
    elif n % 5 == 0:
        print("Buzz")
    else:
        print(n)

02 Number Guessing Game

Write a number guessing game. Pick a secret number (e.g. 42). Use a while loop to repeatedly ask the user for a guess. Print "Too high" or "Too low" as hints. When they guess correctly, print how many attempts it took. Limit to 7 attempts.

Show solution

secret = 42
attempts = 0
max_attempts = 7

while attempts < max_attempts:
    guess = int(input("Guess (1-100): "))
    attempts += 1

    if guess == secret:
        print(f"Correct! It took {attempts} attempt(s).")
        break
    elif guess < secret:
        print("Too low!")
    else:
        print("Too high!")
else:
    print(f"Out of attempts! The number was {secret}.")

03 Prime Numbers

Write a script that finds all prime numbers between 2 and 50. A number is prime if it's only divisible by 1 and itself. Use a nested loop and break/else.

Show solution

primes = []

for n in range(2, 51):
    for i in range(2, int(n ** 0.5) + 1):
        if n % i == 0:
            break   # not prime
    else:
        # Loop completed without break → n is prime
        primes.append(n)

print(f"Primes up to 50: {primes}")
# [2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47]

Loops (for & while)

🎯 Learning Objectives

The for Loop

The range() Function

The while Loop

for vs while — when to use which

break and continue

The else Clause on Loops

Nested Loops

Common Loop Patterns

Common Pitfalls

💻 Exercises

The `for` Loop

The `range()` Function

The `while` Loop

`break` and `continue`

The `else` Clause on Loops