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The while Loop

The while loop is a pre-test loop that checks the condition before each iteration. If the condition is false initially, the loop body never executes.

Syntax

while (condition) {
    // loop body
    // update statement
}

✓ When to Use: Use while when you don't know the number of iterations in advance, or when the termination depends on a dynamic condition.

while vs for

for: Best when iteration count is known
while: Best for unknown/dynamic iteration counts
Both can be converted to each other

⚠️ Warning: Always ensure the condition will eventually become false. Forgetting the update statement causes infinite loops!

Code Examples

Basic while loop patterns

java

1// Basic while loop
2int count = 1;
3while (count <= 5) {
4    System.out.println("Count: " + count);
5    count++;
6}
7// Output: Count: 1, 2, 3, 4, 5
8
9// Sum numbers until condition
10int sum = 0;
11int num = 1;
12while (num <= 100) {
13    sum += num;
14    num++;
15}
16System.out.println("Sum 1-100: " + sum);  // 5050

Input validation with while loop

java

1// User input validation
2Scanner scanner = new Scanner(System.in);
3int input = -1;
4
5while (input < 0 || input > 100) {
6    System.out.print("Enter a number (0-100): ");
7    input = scanner.nextInt();
8    
9    if (input < 0 || input > 100) {
10        System.out.println("Invalid! Try again.");
11    }
12}
13
14System.out.println("You entered: " + input);

Sentinel-controlled loop and string processing

java

1// Read until sentinel value
2Scanner scanner = new Scanner(System.in);
3int total = 0;
4int value;
5
6System.out.println("Enter numbers (-1 to stop):");
7value = scanner.nextInt();
8
9while (value != -1) {
10    total += value;
11    value = scanner.nextInt();
12}
13
14System.out.println("Total: " + total);
15
16// Process string character by character
17String text = "Hello";
18int index = 0;
19while (index < text.length()) {
20    System.out.println(text.charAt(index));
21    index++;
22}

Mathematical operations with while

java

1// Find digits in a number
2int number = 12345;
3int digitCount = 0;
4int temp = number;
5
6while (temp > 0) {
7    temp = temp / 10;
8    digitCount++;
9}
10System.out.println(number + " has " + digitCount + " digits");
11
12// Reverse a number
13int original = 12345;
14int reversed = 0;
15temp = original;
16
17while (temp > 0) {
18    int digit = temp % 10;
19    reversed = reversed * 10 + digit;
20    temp = temp / 10;
21}
22System.out.println("Reversed: " + reversed);  // 54321

Use Cases

Reading input until sentinel value
Polling for conditions
Game loops (until game over)
Processing linked data structures
Waiting for events
Mathematical algorithms (digit extraction, GCD)

Common Mistakes to Avoid

Infinite loop - forgetting update statement
Off-by-one errors in condition
Not initializing loop variable before while
Condition never becoming false
Using = instead of == in condition
Modifying wrong variable in update