while Loops

Master indefinite iteration in Java: learn how while loops repeat code as long as a Boolean condition remains true.

AP Computer Science A

What Is a Loop?

A loop is a control structure that repeats a block of code multiple times. Instead of writing the same statement over and over, a loop automates the repetition for you.

Without Loops

You write 100 print statements to print numbers 1 to 100.

With Loops

You write 3 lines: a condition, a body, and an update. Done.

Anatomy of a while Loop

while (condition) {
    // body: code to repeat
    // update: change something so
    //   condition eventually becomes false
}

Condition

A Boolean expression checked before each iteration.

Body

Statements that execute each time the condition is true.

Update

Part of the body that changes the loop variable to avoid infinite loops.

Execution Flow

Formal Syntax in Java

int count = 1;

while (count <= 5) {
    System.out.println(count);
    count++;  // update step
}

while vs for

	while loop	for loop
Init location	Before the loop	Inside the header
Condition	In the parentheses	In the header (middle)
Update	Inside the body	In the header (end)
Best for	Unknown iterations	Known iterations
Risk	Easier to forget update	Update always visible

Step-by-Step Approach

1

Identify the loop variable. What value controls when the loop stops?

2

Initialize it before the loop. Set the starting value (e.g., int i = 0).

3

Write the condition. Express when the loop should continue, not when it should stop.

4

Write the body. Include the useful work and the update step that changes the loop variable.

5

Trace it. Mentally run through the first 2-3 iterations and the final iteration to verify correctness.

Example 1: Sum of Integers

Calculate the sum \(1 + 2 + 3 + \dots + n\) using a while loop.

int n = 10;
int sum = 0;
int i = 1;

while (i <= n) {
    sum += i;
    i++;
}
System.out.println(sum);  // 55

Example 2: Sentinel Value Loop

Read user input until a sentinel value of -1 is entered. This is a classic use case where the number of iterations is unknown.

Scanner sc = new Scanner(System.in);
int num = sc.nextInt();

while (num != -1) {
    System.out.println("Got: " + num);
    num = sc.nextInt();  // read next
}
System.out.println("Done.");

Example 3: String Processing

Count the number of vowels in a String using a while loop with .charAt().

String word = "algorithm";
int idx = 0;
int vowels = 0;

while (idx < word.length()) {
    char ch = word.charAt(idx);
    if ("aeiou".indexOf(ch) != -1) {
        vowels++;
    }
    idx++;
}
// vowels == 3  (a, o, i)

Tracing a Loop

int x = 1;
while (x < 16) {
    x *= 2;
}
System.out.println(x);

Iteration	x (before)	x (after x *= 2)	\(x < 16\)?
1	1	2	true
2	2	4	true
3	4	8	true
4	8	16	false

The Dreaded Infinite Loop

An infinite loop occurs when the condition never becomes false. The program freezes or runs forever.

int x = 5;
while (x > 0) {
    System.out.println(x);
    // Oops! Forgot x--;
}

Off-by-One Errors

An off-by-one error happens when your loop executes one too many or one too few times. This is the most common loop bug.

One Too Many

Using \(\le\) when you meant \(<\). E.g., printing indices 0 through 5 for a size-5 array causes an ArrayIndexOutOfBoundsException.

One Too Few

Using \(<\) when you meant \(\le\). E.g., summing 1 to n but stopping at n-1 because you wrote \(i < n\) instead of \(i \le n\).

Common Mistakes

✕ Missing Update

Forgetting to change the loop variable inside the body, causing an infinite loop.

✕ Wrong Operator

Using \(<\) instead of \(\le\) (or vice versa), causing an off-by-one error.

✕ Semicolon After while

Writing while(\(x < 5\)); puts an empty body. The real body never repeats.

Key Concepts Summary

Concept	Description
Condition	Boolean expression checked before each iteration
Body	Code that executes when condition is true
Update	Statement that changes the loop variable
Infinite loop	Condition never becomes false: program hangs
Off-by-one	Loop runs one too many or too few times
Sentinel	Special value that signals the loop to stop
Zero iterations	If condition is initially false, body never runs

Question 1 of 5

Consider the following code segment:

int x = 1; while (x < 10) { x += 3; } System.out.println(x);

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What is printed as a result of executing the code segment?

Trace: x=1 → 4 → 7 → 10. When x=10, the condition \(10 < 10\) is false, so the loop ends and prints 10.

Question 2 of 5

Consider the following code segment:

int count = 0; int num = 128; while (num > 1) { num /= 2; count++; } System.out.println(count);

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What value is printed?

128 → 64 → 32 → 16 → 8 → 4 → 2 → 1. That's 7 divisions, so count = 7. The loop essentially computes \(\log_2(128) = 7\).

Question 3 of 5

Which of the following will cause an infinite loop?

In option D, k starts at 5 and increases each iteration. Since k is always greater than 0 and keeps growing, the condition \(k > 0\) is never false.

Question 4 of 5

Consider the following code segment:

int a = 0; int b = 1; while (b < 20) { int temp = b; b = a + b; a = temp; } System.out.println(b);

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What value of b is printed?

This computes Fibonacci numbers. Trace: b = 1, 1, 2, 3, 5, 8, 13, 21. When b=13, \(13 < 20\) is true, so one more iteration gives b=21. Then \(21 < 20\) is false, and 21 is printed.

Question 5 of 5

Consider the following code segment:

String s = "banana"; int i = 0; int count = 0; while (i < s.length()) { if (s.charAt(i) == 'a') { count++; } i++; } System.out.println(count);

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What value is printed?

The string "banana" has characters: b, a, n, a, n, a. The letter 'a' appears at indices 1, 3, and 5: that is 3 times.