Lesson 10 of 20

Loops

Repeat program actions with for, while, and do-while loops.

Learning objectives

• Choose an appropriate loop for a task.
• Control repetition with counters and conditions.
• Use break and continue carefully.
• Recognize and avoid infinite loops.
• Build a repeated-input mini project.

1. for loops

A for loop is ideal when the number of repetitions is known or controlled by a counter.

Example: print 1 to 5

for (int i = 1; i <= 5; i++) {
    System.out.println(i);
}

2. while loops

A while loop checks its condition before each repetition. It may execute zero times.

int balance = 100;

while (balance >= 20) {
    balance -= 20;
    System.out.println("Remaining balance: " + balance);
}

3. do-while loops

A do-while loop checks the condition after the body, so the body runs at least once.

int choice = 1;

do {
    System.out.println("Menu displayed");
    choice++;
} while (choice <= 3);

4. break and continue

for (int i = 1; i <= 10; i++) {
    if (i == 3) {
        continue; // skip 3
    }

    if (i == 8) {
        break;    // stop the loop
    }

    System.out.println(i);
}

Mini project: marks average

import java.util.Scanner;

public class MarksAverage {
    public static void main(String[] args) {
        Scanner input = new Scanner(System.in);

        System.out.print("How many marks? ");
        int count = Integer.parseInt(input.nextLine());

        int total = 0;

        for (int i = 1; i <= count; i++) {
            System.out.print("Enter mark " + i + ": ");
            int mark = Integer.parseInt(input.nextLine());
            total += mark;
        }

        double average = (double) total / count;
        System.out.printf("Average: %.2f%n", average);

        input.close();
    }
}

Common mistakes

Summary

Loops remove repetitive code and are essential for processing groups of data. Lesson 11 combines loops with arrays so you can store and process multiple related values.