Repeat Statements (Loops)

Learning Objectives

• Master while loops for repetition
• Learn for loops for controlled iteration
• Understand loop control with break and continue
• Recognize common loop mistakes
• Apply loops to solve real problems

Section 5.1: The While Loop

What is a Loop?

A loop repeats a block of code multiple times until a condition becomes false.

While Loop Syntax

while (condition) {
    // Code to repeat
}

Simple Example

let count = 1;

while (count <= 5) {
    console.log(count);
    count = count + 1;
}

// Output:
// 1
// 2
// 3
// 4
// 5

Real-World Example: Password Validation

let password = "";

while (password !== "secret123") {
    password = prompt("Enter password:");
}

console.log("Access granted!");

Infinite Loops (Avoid!)

while (true) {
    console.log("This never stops!");
    // ❌ DANGEROUS: Freezes the browser
}

Loop with Break

let attempt = 0;

while (true) {
    attempt++;
    let password = prompt("Enter password:");
    
    if (password === "secret123") {
        console.log("Correct!");
        break;  // Exit the loop
    }
    
    if (attempt === 3) {
        console.log("Too many attempts");
        break;
    }
}

Section 5.2: The For Loop

For Loop Syntax

for (initialization; condition; increment) {
    // Code to repeat
}

Simple For Loop

for (let i = 1; i <= 5; i++) {
    console.log(i);
}

// Output: 1, 2, 3, 4, 5

Breaking It Down

for (let i = 1;    // Initialize counter
     i <= 5;       // Repeat while true
     i++           // Increment each iteration
) {
    console.log(i);
}

Counting Backwards

for (let i = 5; i >= 1; i--) {
    console.log(i);
}

// Output: 5, 4, 3, 2, 1

Skipping Values

for (let i = 0; i <= 10; i += 2) {
    console.log(i);  // 0, 2, 4, 6, 8, 10
}

Using continue

Skip to the next iteration:

for (let i = 1; i <= 5; i++) {
    if (i === 3) {
        continue;  // Skip 3
    }
    console.log(i);  // 1, 2, 4, 5
}

Section 5.3: Common Mistakes

Mistake 1: Infinite Loops

// ❌ Wrong: condition never becomes false
let i = 0;
while (i < 10) {
    console.log(i);
    // forgot to increment i
}

// ✅ Correct
let i = 0;
while (i < 10) {
    console.log(i);
    i++;  // Increment
}

Mistake 2: Off-by-One Error

// ❌ Wrong: prints 1-4 instead of 1-5
for (let i = 1; i < 5; i++) {
    console.log(i);
}

// ✅ Correct
for (let i = 1; i <= 5; i++) {
    console.log(i);
}

Mistake 3: Wrong Condition Check

// ❌ Wrong: loop never executes
for (let i = 10; i < 5; i++) {
    console.log(i);
}

// ✅ Correct
for (let i = 1; i < 10; i++) {
    console.log(i);
}

Mistake 4: Modifying Loop Variable Incorrectly

// ❌ Wrong: changes step size unexpredictably
for (let i = 0; i < 10; i++) {
    i++;  // Skips every other number
}

// ✅ Correct
for (let i = 0; i < 10; i = i + 2) {
    console.log(i);  // 0, 2, 4, 6, 8
}

Section 5.4: Which Loop to Use?

Use while When:

• You don't know how many iterations you need
• Condition is complex
• Loop might not execute at all

while (userInput !== "quit") {
    userInput = prompt("Enter command:");
}

Use for When:

• You know the exact number of iterations
• You need a counter
• Iterating through a range

for (let i = 1; i <= 100; i++) {
    console.log(i);
}

Coding Challenges

Challenge 5.1: Carousel

Task: Create a rotating carousel that displays numbers 1-5 repeatedly, 3 times total

Expected Output:

1 2 3 4 5
1 2 3 4 5
1 2 3 4 5

Challenge 5.2: Use of Modulo Operator

Task: Print numbers 1-20, but:

• Say "Fizz" for multiples of 3
• Say "Buzz" for multiples of 5
• Say "FizzBuzz" for multiples of both

Challenge 5.3: Input Validation

Task: Ask user to enter a number between 1 and 10

• Keep asking until valid input is provided
• Show error message for invalid input

Challenge 5.4: Multiplication Table

Task: Create a multiplication table for a given number

Example for 5:

5 × 1 = 5
5 × 2 = 10
5 × 3 = 15
...
5 × 10 = 50

Challenge 5.5: Neither Yes nor No

Task: Ask user yes/no questions until they give the correct answer

• Keep asking until "yes" is entered
• Count the number of attempts

Challenge 5.6: FizzBuzz

Task: Print numbers 1-100 with special rules:

• "Fizz" for multiples of 3
• "Buzz" for multiples of 5
• "FizzBuzz" for multiples of both
• Otherwise print the number

Key Takeaways

✅ while loops repeat while a condition is true
✅ for loops are best for counting/iteration
✅ Use break to exit a loop early
✅ Use continue to skip to next iteration
✅ Avoid infinite loops by always update loop variables
✅ Be careful with off-by-one errors

Quiz Questions

• What's the difference between while and for loops?
• How do you exit a loop early?
• What does i++ do?
• Explain an off-by-one error
• What would for (let i = 0; i < 5; i++) print?

Next Module: Module 6 - Write Functions