Loops

Topic: Control Flow (Extended)
Concepts Covered: for, while, break, continue

Why Use Loops?

Loops allow us to repeat actions efficiently instead of writing repetitive code:

• Log numbers from 1 to 200
• Keep asking for user input until valid
• Generate multiplication tables
• Process repetitive calculations

While Loops

The while loop executes code as long as a condition is true:

Basic While Loop

// Syntax
while (condition) {
  // Code to execute
  // Don't forget to update the condition!
}

Examples

Simple Counter

let count = 0;

while (count < 5) {
  console.log(`Number: ${count}`);
  count = count + 1; // or count++
}
// Output: Number: 0, Number: 1, Number: 2, Number: 3, Number: 4

Multiplication Table

let multiplier = 1;

while (multiplier <= 12) {
  console.log(`4 x ${multiplier} = ${4 * multiplier}`);
  multiplier += 1;
}

Password Checker with Attempts

const correct_password = "test123";
let input = prompt("Input your password");
let user_attempt = 0;

while (input !== correct_password) {
  user_attempt += 1;

  if (user_attempt === 3) {
    alert("Try again after 3 minutes");
    break; // Exit the loop
  }

  input = prompt("Wrong password, Input correct password");
}

if (input === correct_password) {
  alert("Correct password!!");
}

For Loops

The for loop is used when you know how many times you want to repeat:

Basic For Loop

// Syntax
for (initial value; condition; increment/decrement) {
  // Code to execute
}

Examples

Multiplication Table

for (let i = 1; i <= 12; i++) {
  console.log(`6 x ${i} = ${6 * i}`);
}

Sum Calculator

let input = +prompt("Input a number");
let sum = 0;

for (let i = 1; i <= input; i++) {
  sum += i;
}

console.log(`Sum from 1 to ${input} = ${sum}`);

Reverse Numbers

let input = +prompt("Input a number");

for (let i = input; i > 0; i--) {
  console.log(i);
}

Loop Control Statements

Break

Exits the loop completely:

for (let i = 1; i <= 10; i++) {
  if (i === 5) {
    break; // Exit when i equals 5
  }
  console.log(i);
}
// Output: 1, 2, 3, 4

Continue

Skips the current iteration and continues with the next:

for (let i = 1; i <= 10; i++) {
  if (i % 2 === 0) {
    continue; // Skip even numbers
  }
  console.log(i);
}
// Output: 1, 3, 5, 7, 9

Common Loop Patterns

Range Even Numbers

let start = +prompt("Enter start number");
let end = +prompt("Enter end number");

let current = start;
while (current <= end) {
  if (current % 2 === 0) {
    console.log(current);
  }
  current++;
}

FizzBuzz

for (let i = 1; i <= 100; i++) {
  if (i % 3 === 0 && i % 5 === 0) {
    console.log("FizzBuzz");
  } else if (i % 3 === 0) {
    console.log("Fizz");
  } else if (i % 5 === 0) {
    console.log("Buzz");
  } else {
    console.log(i);
  }
}

When to Use Which Loop

Use for when:

• You know the exact number of iterations
• Working with counters or ranges
• Need the index value

Use while when:

• The number of iterations is unknown
• Looping until a condition is met
• Processing user input until valid

Common Pitfalls

Infinite Loops

Always ensure your loop condition will eventually become false:

// ❌ Infinite loop - counter never changes
let counter = 20;
while (counter > 0) {
  console.log(`Number: ${counter}`);
  // Missing: counter--;
}

// ✅ Fixed version
let counter = 20;
while (counter > 0) {
  console.log(`Number: ${counter}`);
  counter--; // or counter = counter - 1;
}

Off-by-One Errors

Be careful with your loop conditions:

// Common mistake - stopping too early
for (let i = 1; i < 10; i++) {
  console.log(i); // This only goes to 9, not 10!
}

// Correct version - if you want 1 through 10
for (let i = 1; i <= 10; i++) {
  console.log(i); // This goes from 1 to 10
}

Best Practices

1. Use meaningful variable names: i, j, k for simple counters, descriptive names for complex loops
2. Avoid modifying loop variables inside the loop body (for for loops)
3. Always ensure the loop will terminate to avoid infinite loops
4. Use break and continue sparingly and only when they make code clearer
5. Choose the appropriate loop type for your specific use case

Key Takeaways

• while loops are condition-based, for loops are count-based
• Always ensure your loops will terminate to avoid infinite loops
• Use break to exit early, continue to skip iterations
• Choose the right loop type for your specific use case
• Be careful with loop conditions to avoid off-by-one errors

Practice Exercises

1. Write a program that finds all prime numbers between 1 and 100
2. Create a guessing game where the user has limited attempts
3. Build a multiplication table generator for any number
4. Write a program that reverses a string using loops
5. Create a program that counts vowels in a sentence