🐍 Day 4: Loops — Making Programs Repeat

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1. Learning Objectives

By the end of Day 4, you will be able to:

• Use for loops to iterate over sequences
• Use while loops to repeat code while a condition holds
• Control loop flow with break and continue
• Use the else clause on loops for clean completion checks
• Avoid the dreaded infinite loop

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2. Concept Explanation

2.1 Why Loops? — Doing Things More Than Once

Programs often need to repeat actions. Without loops, you'd have to write the same code over and over:

# Without loops — pain!
print("Student 1")
print("Student 2")
print("Student 3")
# ... imagine 1000 students

Loops let you write the action once and repeat it as many times as needed.

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2.2 The for Loop — Iterating Over Sequences

The for loop takes each item in a sequence, one at a time, and runs its block with that item.

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

Output:

apple
banana
cherry

How it works: The variable fruit is assigned "apple", then the block runs; then "banana", block runs; then "cherry", block runs; then the loop ends.

range() — the loop counter's best friend:

range(n) generates numbers from 0 to n-1. Very useful for repeating a block a specific number of times.

for i in range(5):
    print(i)

Output:

0
1
2
3
4

range(start, stop, step) gives finer control:

for i in range(2, 10, 3):  # 2, 5, 8
    print(i)

Looping over strings:

for char in "Hello":
    print(char)

Output: H, e, l, l, o (each on new line)

Looping over other sequences we'll learn later: lists, tuples, dictionaries, sets, files — all work with for.

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2.3 The while Loop — Repeat While Condition is True

A while loop checks a condition before each iteration. If it's True, the block runs; if False, the loop ends.

count = 1
while count <= 3:
    print(count)
    count += 1

Output:

1
2
3

⚠️ Danger: If the condition never becomes False, you get an infinite loop. Your program will run forever (press Ctrl+C to stop).

# Infinite loop — don't run unless you want to see it!
x = 1
while x > 0:
    print(x)
    x += 1   # never becomes <=0

When to use while vs for:

• Use for when you know how many times to iterate (or iterate over an existing collection).
• Use while when you need to loop as long as a condition is true, and you don't know the number of iterations in advance (e.g., waiting for user input).

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2.4 Loop Control: break and continue

Sometimes you need to exit a loop early or skip an iteration.

break — immediately exits the entire loop (no more iterations).

for i in range(10):
    if i == 5:
        break
    print(i)
# Output: 0 1 2 3 4   (stops before 5)

continue — skips the rest of the current iteration and jumps to the next one.

for i in range(5):
    if i == 2:
        continue
    print(i)
# Output: 0 1 3 4   (2 is skipped)

Think of break as "stop the loop entirely" and continue as "skip this one and go to next".

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2.5 The else Clause on Loops — Did It Finish Naturally?

Python allows an else block after a for or while loop. It executes only if the loop completed normally (i.e., not interrupted by break).

for i in range(5):
    print(i)
else:
    print("Loop finished without break.")

Output: 0 1 2 3 4, then the message.

If a break occurs, the else block is skipped:

for i in range(5):
    if i == 3:
        break
    print(i)
else:
    print("This won't print")
# Output: 0 1 2

This is particularly useful for search loops: if you find the item, break; if not, the else runs to say "not found".

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2.6 Nested Loops

You can put loops inside loops:

for i in range(3):
    for j in range(2):
        print(f"i={i}, j={j}")

This prints 3×2=6 combinations.

Be careful: nesting increases total iterations dramatically. A loop inside a loop that both run 1000 times gives 1,000,000 iterations.

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3. Code Examples

Example 1: Multiplication Table

# Print 5's multiplication table
n = 5
for i in range(1, 11):
    print(f"{n} x {i} = {n * i}")

Example 2: Password Attempts with while

correct = "python123"
attempts = 3

while attempts > 0:
    guess = input("Enter password: ")
    if guess == correct:
        print("Access granted.")
        break
    attempts -= 1
    print(f"Wrong. {attempts} attempts left.")
else:
    print("Account locked.")

Example 3: Filtering with continue

# Print only odd numbers from 1 to 10
for num in range(1, 11):
    if num % 2 == 0:
        continue
    print(num)

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4. Hands-On Exercises

Exercise 1: Countdown

Ask the user for a number, then print a countdown from that number to 0 using a while loop.

Exercise 2: Sum of Numbers

Using a for loop, calculate the sum of all numbers from 1 to 100 (inclusive). Print the sum.

Exercise 3: FizzBuzz

Print numbers from 1 to 30. For multiples of 3, print "Fizz" instead of the number. For multiples of 5, print "Buzz". For numbers that are multiples of both 3 and 5, print "FizzBuzz".

Exercise 4: Prime Checker

Ask for an integer >1. Use a loop to test if it's prime (divisible only by 1 and itself). Print "Prime" or "Not prime". (Hint: use % and break)

Exercise 5: Menu Repeater

Write a program that repeatedly displays a menu:

1. Say Hello
2. Show Date (just print a placeholder like "2026-05-06")
3. Quit

Keep showing the menu until the user chooses 3. Use while and break.

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5. Applied Challenge Task 🏗️

Number Guessing Game

Build a game where the computer thinks of a random number between 1 and 100, and the player has to guess it.

1. Generate a secret number using random.randint(1, 100). (You'll need import random at the top.)
2. Give the player 7 attempts.
3. After each guess, tell the player if the guess is too high, too low, or correct.
4. If the player guesses correctly, congratulate them and show how many attempts they used.
5. If they run out of attempts, reveal the secret number.
6. Use break to end the loop when they win.

Example output:

I'm thinking of a number between 1 and 100. Can you guess it? You have 7 tries.
Guess 1: 50
Too high!
Guess 2: 25
Too low!
Guess 3: 37
Correct! You got it in 3 tries. 🎉

Stretch goals:

• After the game, ask if they want to play again (loop the whole game).
• Validate that the guess is a number between 1–100.
• Track and print the player's guess history after they win/lose.

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6. Brief Review Summary

Concept	Key Points
for loop	Iterates over a sequence (list, string, range, etc.)
while loop	Repeats while condition is True
break	Exits the loop immediately
continue	Skips to the next iteration
else on loops	Runs only if loop finished without break
Infinite loop	Condition never becomes False — use Ctrl+C to interrupt
Nested loops	Loop inside loop — beware of performance

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7. Preview of Next Topic — Day 5

Tomorrow we'll dive into one of the most important concepts in programming:

• Functions — defining reusable blocks of code
• Parameters & arguments — passing data into functions
• Return values — sending data back
• Scope — where variables live and how they're accessed

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🎯 Your Action Items for Day 4:

1. ✅ Complete all 5 exercises
2. ✅ Complete the Number Guessing Game challenge
3. ✅ Experiment with an infinite loop and kill it with Ctrl+C
4. ✅ Try the else clause on loops with both success and break cases