Loops in Python: while and for 🔄

Imagine: you need to print numbers from 1 to 1000. Write 1000 print() calls? 😱

Loops are the superpower of programming! They let you repeat actions as many times as you like.

🤔 Why do we need loops?

Without loops (100 lines):

print(1)
print(2)
print(3)
print(4)
print(5)
# ... 95 more lines
print(100)

With loops (2 lines):

for i in range(1, 101):
    print(i)

The difference is obvious! 🚀

🔁 Two types of loops in Python

Python provides two main loop types:

1. while — repeat while a condition holds (unknown count)
2. for — iterate over each element (known count)

Let’s look at both!

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1️⃣ The WHILE loop — “as long as”

Syntax:

while condition:
    code to execute

Logic: While the condition is True → run the code. When it’s False → stop.

Simple example — countdown:

count = 10

while count > 0:
    print(count)
    count -= 1  # count = count - 1

print("🚀 Liftoff!")

What happens:
1. count = 10 → 10 > 0 → True → prints 10, sets count to 9
2. count = 9 → 9 > 0 → True → prints 9, sets count to 8
3. … continues …
4. count = 0 → 0 > 0 → False → loop stops!

⚠️ Beware of infinite loops:

# ❌ NEVER DO THIS:
count = 10
while count > 0:
    print(count)
    # Forgot count -= 1 !
    # This loop runs forever! 😱

Stuck in an infinite loop? Press Ctrl+C to stop the program.

Example — keep asking until the correct password:

password = ""

while password != "python123":
    password = input("Enter password: ")
    if password != "python123":
        print("❌ Wrong, try again")

print("✅ Access granted!")

The loop continues while the password is wrong.

Example — game with while:

health = 100

while health > 0:
    action = input("Action (attack/defend/run): ")

    if action == "attack":
        print("⚔️ You attacked!")
        health -= 20
    elif action == "defend":
        print("🛡️ You're defending!")
    elif action == "run":
        print("🏃 You ran away!")
        break  # Exit the loop

    print(f"💚 Health: {health}")

if health <= 0:
    print("☠️ Game over")

---

2️⃣ The FOR loop — “for each”

Syntax:

for element in sequence:
    code to execute

for is more convenient when you know how many times to repeat or what to iterate over.

📍 Option 1: for with range()

range(n) generates a sequence of numbers from 0 to n-1

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

# Prints:
# Iteration 0
# Iteration 1
# Iteration 2
# Iteration 3
# Iteration 4

range() has 3 forms:

range(5)          # 0, 1, 2, 3, 4
range(1, 6)       # 1, 2, 3, 4, 5 (from start up to stop-1)
range(0, 10, 2)   # 0, 2, 4, 6, 8 (step of 2)
range(10, 0, -1)  # 10, 9, 8...1 (counting down)

Example — multiplication table:

number = int(input("Enter a number: "))

for i in range(1, 11):
    result = number * i
    print(f"{number} x {i} = {result}")

📍 Option 2: for with a list

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

for fruit in fruits:
    print(f"I love {fruit}! 🍎")

Python automatically picks each element from the list!

📍 Option 3: for with a string

word = "PYTHON"

for letter in word:
    print(letter)

# P
# Y
# T
# H
# O
# N

Each character is processed individually!

Example — count vowels:

text = input("Enter text: ")
vowels = "aeiou"
count = 0

for char in text.lower():
    if char in vowels:
        count += 1

print(f"Number of vowels: {count}")

---

🛑 BREAK — early exit

break immediately stops the loop.

Example — find a number:

numbers = [3, 7, 12, 5, 18, 9]

for num in numbers:
    if num > 10:
        print(f"Found a large number: {num}")
        break  # Found it, exit!
    print(f"Checking {num}...")

# Prints:
# Checking 3...
# Checking 7...
# Found a large number: 12

After break the loop terminates entirely, even if elements remain.

Example — exit an infinite loop:

while True:  # Infinite loop!
    answer = input("Continue? (yes/no): ")

    if answer == "no":
        break  # Exit

    print("Continuing...")

print("Program finished")

---

⏭️ CONTINUE — skip an iteration

continue skips the current iteration and moves to the next one.

Example — print only even numbers:

for i in range(10):
    if i % 2 != 0:  # If odd
        continue     # Skip it

    print(i)  # Only even numbers reach here

# Prints: 0, 2, 4, 6, 8

Difference between break and continue:

# BREAK — exits entirely:
for i in range(5):
    if i == 3:
        break
    print(i)
# Prints: 0, 1, 2 (stopped at 3)

# CONTINUE — skips one iteration:
for i in range(5):
    if i == 3:
        continue
    print(i)
# Prints: 0, 1, 2, 4 (skipped 3, kept going)

Example — filter a list:

names = ["Anna", "Bob", "Alice", "Boris", "Andrew"]

print("Names starting with A:")
for name in names:
    if not name.startswith("A"):
        continue  # Skip names that don't start with A

    print(name)

# Prints: Anna, Alice, Andrew

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🎯 Nested loops — a loop inside a loop

A loop can contain another loop!

Example — grid of stars:

size = 5

for row in range(size):
    for col in range(size):
        print("⭐", end="")
    print()  # New line

# ⭐⭐⭐⭐⭐
# ⭐⭐⭐⭐⭐
# ⭐⭐⭐⭐⭐
# ⭐⭐⭐⭐⭐
# ⭐⭐⭐⭐⭐

What happens:
- The outer loop (row) runs 5 times
- For each row → the inner loop (col) runs 5 times
- Total: 5 × 5 = 25 iterations!

Example — triangle:

for row in range(1, 6):
    for col in range(row):
        print("▲", end="")
    print()

# ▲
# ▲▲
# ▲▲▲
# ▲▲▲▲
# ▲▲▲▲▲

Example — Pythagorean multiplication table:

print("   ", end="")
for i in range(1, 6):
    print(f"{i:3}", end="")
print()

for i in range(1, 6):
    print(f"{i:2} |", end="")
    for j in range(1, 6):
        print(f"{i*j:3}", end="")
    print()

#      1  2  3  4  5
#  1 |  1  2  3  4  5
#  2 |  2  4  6  8 10
#  3 |  3  6  9 12 15
#  4 |  4  8 12 16 20
#  5 |  5 10 15 20 25

---

📊 while vs for comparison

Criterion	while	for
When to use	Unknown iteration count	Known count
Condition	Checked each iteration	Iterates over a sequence
Risk	Infinite loop	Safer
Examples	Game loops, user input	Iterating lists, range()
Readability	Flexible	Concise

Choosing which to use:

Use WHILE when:
- You don’t know how many iterations are needed
- The condition depends on what happens inside the loop
- You need an infinite loop with a break

Use FOR when:
- You’re iterating over a list / string / range
- You know the exact number of iterations
- You need to visit every element

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💡 Useful techniques

1. enumerate() — index + element:

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

for index, fruit in enumerate(fruits):
    print(f"{index+1}. {fruit}")

# 1. apple
# 2. banana
# 3. orange

2. Iterating over a dictionary:

student = {"name": "Anna", "age": 20, "year": 2}

for key, value in student.items():
    print(f"{key}: {value}")

# name: Anna
# age: 20
# year: 2

3. else after a loop:

for i in range(5):
    if i == 10:  # Never True
        break
else:
    print("Loop completed without break")

else runs only if the loop was not interrupted by break.

4. Range with a step:

# Even numbers:
for i in range(0, 10, 2):
    print(i)  # 0, 2, 4, 6, 8

# Reverse order:
for i in range(10, 0, -1):
    print(i)  # 10, 9, 8...1

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🎮 Practical examples

1. Calculator with a loop:

while True:
    print("\n🧮 Calculator")
    print("1. Addition")
    print("2. Subtraction")
    print("3. Multiplication")
    print("4. Division")
    print("5. Exit")

    choice = input("Choice: ")

    if choice == "5":
        print("Goodbye!")
        break

    if choice in ["1", "2", "3", "4"]:
        a = float(input("First number: "))
        b = float(input("Second number: "))

        if choice == "1":
            print(f"Result: {a + b}")
        elif choice == "2":
            print(f"Result: {a - b}")
        elif choice == "3":
            print(f"Result: {a * b}")
        elif choice == "4":
            if b != 0:
                print(f"Result: {a / b}")
            else:
                print("❌ Division by zero!")
    else:
        print("❌ Invalid choice")

2. FizzBuzz — the classic exercise:

for i in range(1, 101):
    if i % 15 == 0:
        print("FizzBuzz")
    elif i % 3 == 0:
        print("Fizz")
    elif i % 5 == 0:
        print("Buzz")
    else:
        print(i)

3. Password generator:

import random
import string

length = int(input("Password length: "))
password = ""

for i in range(length):
    char = random.choice(string.ascii_letters + string.digits)
    password += char

print(f"Your password: {password}")

4. Find prime numbers:

n = int(input("Find primes up to: "))

for num in range(2, n+1):
    is_prime = True

    for i in range(2, int(num ** 0.5) + 1):
        if num % i == 0:
            is_prime = False
            break

    if is_prime:
        print(num, end=" ")

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⚠️ Common mistakes

1. Missing indentation:

# ❌ Wrong:
for i in range(5):
print(i)  # SyntaxError!

# ✅ Correct:
for i in range(5):
    print(i)

2. Modifying the loop counter inside a for loop:

# ❌ Don't do this:
for i in range(5):
    print(i)
    i = 10  # Has no effect on the loop!

# ✅ Use while instead:
i = 0
while i < 5:
    print(i)
    i += 1  # Works correctly

3. Forgetting to update the variable in while:

# ❌ Infinite loop:
i = 0
while i < 5:
    print(i)
    # Forgot i += 1

# ✅ Correct:
i = 0
while i < 5:
    print(i)
    i += 1

4. Wrong range:

# ❌ range(5, 1) — prints nothing:
for i in range(5, 1):
    print(i)

# ✅ Need a step of -1:
for i in range(5, 1, -1):
    print(i)  # 5, 4, 3, 2

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🚀 Summary

Loops are a fundamental building block of programming!

You’ve learned to:

✅ Use while for condition-based loops
✅ Use for to iterate over sequences
✅ Work with range() for numbers
✅ Apply break to exit early
✅ Apply continue to skip iterations
✅ Create nested loops
✅ Avoid infinite loops

With loops you can:
- Process large amounts of data
- Build games with repeating actions
- Automate repetitive tasks
- Implement complex algorithms

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Practice in CodeHS! Write several programs using loops. Experiment with while, for, break, continue. Mix different techniques! 💪

Next step: Learn about functions to organize your code even better! 🎯