Loops

Learn how to repeat actions in your code efficiently using loops.

What are Loops?

Loops allow you to execute the same block of code multiple times without writing it repeatedly. They're essential for processing collections of data and automating repetitive tasks.

Instead of writing:

System.out.println("Hello");
System.out.println("Hello");
System.out.println("Hello");
System.out.println("Hello");
System.out.println("Hello");

You can write:

for (int i = 0; i < 5; i++) {
    System.out.println("Hello");
}

The for Loop

Best used when you know how many times you want to repeat something.

Syntax

for (initialisation; condition; update) {
    // Code to repeat
}

Example

for (int i = 0; i < 5; i++) {
    System.out.println("Iteration: " + i);
}

// Output:
// Iteration: 0
// Iteration: 1
// Iteration: 2
// Iteration: 3
// Iteration: 4

How it Works

1. Initialisation: int i = 0 - Create and initialise the counter
2. Condition: i < 5 - Check if loop should continue
3. Code Block: Execute the code inside {}
4. Update: i++ - Increment the counter
5. Repeat steps 2-4 until condition is false

More Examples

// Count from 1 to 10
for (int i = 1; i <= 10; i++) {
    System.out.println(i);
}

// Count backwards
for (int i = 10; i > 0; i--) {
    System.out.println(i);
}

// Count by 2s
for (int i = 0; i <= 20; i += 2) {
    System.out.println(i);  // 0, 2, 4, 6, ..., 20
}

// Multiply by 2 each time
for (int i = 1; i <= 100; i *= 2) {
    System.out.println(i);  // 1, 2, 4, 8, 16, 32, 64
}

Practical Examples

Calculate Sum

int sum = 0;
for (int i = 1; i <= 100; i++) {
    sum += i;
}
System.out.println("Sum of 1 to 100: " + sum);  // 5050

Times Table

int number = 7;
for (int i = 1; i <= 10; i++) {
    System.out.println(number + " x " + i + " = " + (number * i));
}

The while Loop

Best used when you don't know how many iterations you'll need.

Syntax

while (condition) {
    // Code to repeat
}

Example

int count = 0;

while (count < 5) {
    System.out.println("Count: " + count);
    count++;
}

// Output:
// Count: 0
// Count: 1
// Count: 2
// Count: 3
// Count: 4

How it Works

1. Check Condition: Is it true?
2. Execute Code: If true, run the code block
3. Repeat: Go back to step 1
4. Exit: When condition becomes false

More Examples

// Countdown
int countdown = 10;
while (countdown > 0) {
    System.out.println(countdown);
    countdown--;
}
System.out.println("Blast off!");

// Find first power of 2 greater than 1000
int power = 1;
while (power <= 1000) {
    power *= 2;
}
System.out.println("First power of 2 > 1000: " + power);

Practical Example: User Input

Scanner scanner = new Scanner(System.in);
String input = "";

while (!input.equals("exit")) {
    System.out.print("Enter command (or 'exit' to quit): ");
    input = scanner.nextLine();
    System.out.println("You entered: " + input);
}

The do-while Loop

Similar to while, but always executes at least once.

Syntax

do {
    // Code to repeat
} while (condition);

Example

int count = 0;

do {
    System.out.println("Count: " + count);
    count++;
} while (count < 5);

while vs do-while

// while - may not execute at all
int x = 10;
while (x < 5) {
    System.out.println("This won't print");
}

// do-while - always executes at least once
int y = 10;
do {
    System.out.println("This prints once");  // Executes!
} while (y < 5);

Practical Example: Input Validation

Scanner scanner = new Scanner(System.in);
int age;

do {
    System.out.print("Enter your age (1-120): ");
    age = scanner.nextInt();
} while (age < 1 || age > 120);

System.out.println("Valid age entered: " + age);

Enhanced for Loop (For-Each)

Best for iterating through arrays and collections.

Syntax

for (type variable : collection) {
    // Use variable
}

Example

String[] names = {"Alice", "Bob", "Charlie"};

for (String name : names) {
    System.out.println("Hello, " + name);
}

// Output:
// Hello, Alice
// Hello, Bob
// Hello, Charlie

More Examples

// Array of numbers
int[] numbers = {1, 2, 3, 4, 5};
int sum = 0;

for (int num : numbers) {
    sum += num;
}
System.out.println("Sum: " + sum);  // 15

// Finding maximum
int[] scores = {85, 92, 78, 95, 88};
int max = scores[0];

for (int score : scores) {
    if (score > max) {
        max = score;
    }
}
System.out.println("Highest score: " + max);  // 95

Nested Loops

Loops inside other loops:

// Multiplication table
for (int i = 1; i <= 5; i++) {
    for (int j = 1; j <= 5; j++) {
        System.out.print(i * j + "\t");
    }
    System.out.println();
}

// Output:
// 1  2  3  4  5
// 2  4  6  8  10
// 3  6  9  12 15
// 4  8  12 16 20
// 5  10 15 20 25

Pattern Printing

// Print a triangle
for (int i = 1; i <= 5; i++) {
    for (int j = 1; j <= i; j++) {
        System.out.print("* ");
    }
    System.out.println();
}

// Output:
// *
// * *
// * * *
// * * * *
// * * * * *

Loop Control Statements

break - Exit the Loop

// Find first number divisible by 7 and 13
for (int i = 1; i <= 1000; i++) {
    if (i % 7 == 0 && i % 13 == 0) {
        System.out.println("Found: " + i);
        break;  // Exit the loop
    }
}
// Output: Found: 91

continue - Skip to Next Iteration

// Print only odd numbers
for (int i = 1; i <= 10; i++) {
    if (i % 2 == 0) {
        continue;  // Skip even numbers
    }
    System.out.println(i);
}
// Output: 1, 3, 5, 7, 9

Examples

// Search for a name
String[] names = {"Alice", "Bob", "Charlie", "David"};
String searchName = "Charlie";
boolean found = false;

for (String name : names) {
    if (name.equals(searchName)) {
        System.out.println("Found: " + name);
        found = true;
        break;
    }
}

if (!found) {
    System.out.println("Name not found");
}

Infinite Loops

Be careful - loops that never end!

// Infinite loop - don't do this!
while (true) {
    System.out.println("This runs forever");
}

// Another infinite loop
for (int i = 0; i >= 0; i++) {
    System.out.println("This never stops");
}

warning

Always ensure your loop has a way to exit, or use break intentionally.

Intentional Infinite Loop

// Game loop - exit when player chooses to quit
while (true) {
    String choice = getPlayerChoice();

    if (choice.equals("quit")) {
        break;  // Exit the loop
    }

    playGame();
}

Choosing the Right Loop

Loop Type	When to Use
for	You know how many iterations needed
while	You don't know iteration count, may not execute at all
do-while	Same as while, but must execute at least once
for-each	Iterating through arrays or collections

Common Mistakes

1. Off-By-One Errors

// Wrong - runs 11 times (0-10)
for (int i = 0; i <= 10; i++) {
    System.out.println(i);
}

// Correct - runs 10 times (0-9)
for (int i = 0; i < 10; i++) {
    System.out.println(i);
}

2. Forgetting to Update Counter

// Infinite loop - i never changes!
int i = 0;
while (i < 5) {
    System.out.println(i);
    // Missing: i++
}

// Correct
int i = 0;
while (i < 5) {
    System.out.println(i);
    i++;
}

3. Modifying Loop Variable

// Confusing - avoid modifying i inside the loop
for (int i = 0; i < 10; i++) {
    System.out.println(i);
    i += 2;  // Don't do this!
}

Practice Exercises

1. Sum Calculator

Calculate the sum of all numbers from 1 to 100.

2. Factorial

Calculate the factorial of a number (e.g., 5! = 5 × 4 × 3 × 2 × 1 = 120).

3. Fizz Buzz

Print numbers 1 to 100, but:

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

4. Pattern Printing

Print this pattern:

*****
****
***
**
*

5. Prime Number Checker

Check if a number is prime (only divisible by 1 and itself).

Solution Template

public class LoopsPractice {
    public static void main(String[] args) {
        // 1. Sum Calculator
        int sum = 0;
        // Your code here

        // 2. Factorial
        int n = 5;
        int factorial = 1;
        // Your code here

        // 3. FizzBuzz
        // Your code here

        // 4. Pattern
        // Your code here

        // 5. Prime Checker
        int number = 17;
        boolean isPrime = true;
        // Your code here
    }
}

Key Takeaways

• Use for loops when you know the iteration count
• Use while loops when the iteration count depends on a condition
• Use do-while when code must execute at least once
• Use for-each loops for arrays and collections
• break exits the loop completely
• continue skips to the next iteration
• Beware of infinite loops and off-by-one errors

Next Steps

Now that you can repeat actions with loops, let's learn about functions - how to organise your code into reusable blocks.