Unit 4 Notes

Unit 4: Iteration – AP Computer Science A

🔑 Key Concepts

• for loops
• while loops
• do-while loops
• Infinite loops and off-by-one errors
• Nested loops
• Loop control: break and continue

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🔁 for Loop

Best for when the number of iterations is known.

for (int i = 0; i < 5; i++) {
    System.out.println(i);
}
// prints 0 to 4

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🔁 while Loop

Best for when the number of iterations is unknown.

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

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🔁 do-while Loop

Runs the loop at least once before checking the condition.

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

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⚠️ Infinite Loops

Loops that never terminate due to a condition that never becomes false.

while (true) {
    // endless loop
}

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🧠 Common Pitfall: Off-by-One Errors

Be careful with loop conditions using <, <=, etc.

for (int i = 0; i <= 5; i++) { // runs 6 times: 0 to 5

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🔂 Nested Loops

A loop inside another loop, often used with 2D arrays.

for (int row = 0; row < 3; row++) {
    for (int col = 0; col < 3; col++) {
        System.out.println("[" + row + "," + col + "]");
    }
}

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⛔ Loop Control Statements

• break — exits the loop
• continue — skips the current iteration

for (int i = 0; i < 10; i++) {
    if (i % 2 == 0) continue;
    System.out.print(i + " "); // prints odd numbers
}

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🧪 Practice Multiple Choice (5 Questions)

1. What will the following code print?

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

(A) 0 1 2
(B) 1 2 3
(C) 0 1 2 3
(D) 1 2
✅ Answer: (A)

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2. How many times will this loop run?

for (int i = 0; i <= 4; i++) {
    System.out.println("Loop");
}

(A) 4
(B) 5
(C) 6
(D) Infinite
✅ Answer: (B)

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3. What is true about a do-while loop?
(A) It may never run
(B) It always runs at least once
(C) It always runs exactly once
(D) It runs forever
✅ Answer: (B)

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4. Which loop will print all even numbers from 0 to 10?

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

(A) Correct
(B) Skips 0
(C) Prints odd numbers
(D) Infinite loop
✅ Answer: (A)

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5. What does continue do in a loop?
(A) Ends the loop
(B) Restarts the loop
(C) Skips the current iteration
(D) Pauses the loop
✅ Answer: (C)

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