Loops: for, while, do-while

Why this matters

Printing tables, summing marks, scanning arrays — almost every useful program repeats something.

Syntax

for (initialisation; condition; update) { ... }
while (condition) { ... }
do { ... } while (condition);

Example

public class Loops {
    public static void main(String[] args) {
        for (int i = 1; i <= 5; i++) {
            System.out.println("5 x " + i + " = " + (5 * i));
        }

        int count = 3;
        while (count > 0) {
            System.out.println("Countdown: " + count);
            count--;
        }

        int n = 0;
        do {
            System.out.println("Runs at least once, n = " + n);
        } while (n > 0);
    }
}

Step by step

1. 1`int i = 1` runs once at the start.
2. 2`i <= 5` is checked before every repetition; when false the loop ends.
3. 3`i++` runs after each repetition.
4. 4`while` checks the condition first, so it may run zero times.
5. 5`do-while` runs the body first and checks afterwards, so it always runs at least once.

Key points

• for is best when you know how many repetitions you need
• while is best when you repeat until something happens
• do-while always executes at least once
• break exits a loop, continue skips to the next repetition

Common mistakes

• Forgetting i++ and creating an infinite loop
• Using <= where < was needed (off-by-one errors)
• Declaring i inside the loop body and losing it every round

Try it yourself

• Print the multiplication table of a number entered by the user.
• Find the sum of the first 10 natural numbers with a while loop.