Java Loops Notes

1️⃣ What Is a Loop?

A loop repeats code multiple times.

Instead of writing:

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

You can write:

for (int i = 0; i < 3; i++) {
    System.out.println("Hello");
}

2️⃣ Why Do We Use Loops?

• Repeating actions
• Processing arrays and lists
• Validating user input
• Counting / totals
• Patterns (shapes, grids)
• Game logic

3️⃣ The Three Main Java Loops

4️⃣ The for Loop

Structure

for (initialization; condition; update) {
    // code repeats
}

Example

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

What each part means

Counting backwards

for (int i = 5; i > 0; i--) {
    System.out.println(i);
}

Skipping numbers

for (int i = 0; i <= 10; i += 2) {
    System.out.println(i);
}

5️⃣ The while Loop

Structure

while (condition) {
    // code repeats
}

Example

int i = 0;

while (i < 5) {
    System.out.println(i);
    i++;
}

6️⃣ The do-while Loop

Structure

do {
    // code runs
} while (condition);

Example

int i = 0;

do {
    System.out.println(i);
    i++;
} while (i < 5);

7️⃣ Infinite Loops ⚠️

An infinite loop never stops.

Example 1: Always true condition

while (true) {
    System.out.println("Forever...");
}

Example 2: Forgetting to update

int i = 0;

while (i < 5) {
    System.out.println(i);
    // i++ is missing!
}

8️⃣ Loop Control: break and continue

break (stop the loop)

for (int i = 0; i < 10; i++) {
    if (i == 5) break;
    System.out.println(i);
}

continue (skip one iteration)

for (int i = 0; i < 5; i++) {
    if (i == 2) continue;
    System.out.println(i);
}

9️⃣ Nested Loops

A nested loop is a loop inside another loop. Great for grids, tables, and 2D arrays.

for (int row = 1; row <= 3; row++) {
    for (int col = 1; col <= 4; col++) {
        System.out.print("* ");
    }
    System.out.println();
}

Output:

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

1️⃣0️⃣ Loops With Arrays

Standard for loop

int[] numbers = {10, 20, 30, 40};

for (int i = 0; i < numbers.length; i++) {
    System.out.println(numbers[i]);
}

Enhanced for (for-each)

for (int num : numbers) {
    System.out.println(num);
}

1️⃣1️⃣ Common Loop Mistakes

❌ Off-by-one error

// WRONG (crashes with ArrayIndexOutOfBoundsException)
for (int i = 0; i <= numbers.length; i++) {
    System.out.println(numbers[i]);
}

// RIGHT
for (int i = 0; i < numbers.length; i++) {
    System.out.println(numbers[i]);
}

❌ Forgetting to update

int x = 0;
while (x < 10) {
    System.out.println(x);
    // x++ is missing
}

❌ Condition that never runs

// This loop runs 0 times because 10 is not less than 0
for (int i = 10; i < 0; i++) {
    System.out.println(i);
}

1️⃣2️⃣ Real-World Example: Guessing Game

import java.util.Scanner;

public class GuessingGameDemo {
    public static void main(String[] args) {
        Scanner input = new Scanner(System.in);
        int secret = 7;
        int guess = 0;

        while (guess != secret) {
            System.out.print("Enter guess: ");
            guess = input.nextInt();
        }

        System.out.println("Correct!");
        input.close();
    }
}

1️⃣3️⃣ When To Choose Each Loop

1️⃣4️⃣ AP CSA Exam Tips

• Practice loop tracing (what prints? how many times?)
• Watch < vs <=
• Know nested loop outputs
• Understand variable scope inside loops
• Recognize infinite loops quickly
• Use array.length correctly

1️⃣5️⃣ Practice Problems

1. Print numbers 1–50
2. Print even numbers 1–100
3. Print a multiplication table (1–12)
4. Count how many vowels in a string
5. Reverse a number
6. Sum numbers from 1–100
7. Print a triangle pattern using *