Control Structures — Looping

A loop (iteration) repeats a block of statements as long as a condition remains true. Loops save us from writing the same code many times — printing 1 to 100 needs one loop, not a hundred statements.

Every loop needs three things: an initialisation (a starting value), a condition (when to keep going) and an update (how the value changes each time), or it will never stop — an infinite loop.

The while loop

Tests the condition before each repetition, so the body may run zero times:

int i = 1;
while (i <= 5) {
    System.out.println(i);
    i++;                 // update — without this the loop never ends
}

The do...while loop

Tests the condition after the body, so the body always runs at least once:

int n = 1;
do {
    System.out.println(n);
    n++;
} while (n <= 5);

The for loop

Puts initialisation, condition and update on one line — ideal when the number of repetitions is known:

for (int i = 1; i <= 5; i++) {
    System.out.println(i);
}

Nested loops

A loop inside another loop is used for tables and patterns; the inner loop completes fully for each single pass of the outer loop:

for (int r = 1; r <= 3; r++) {
    for (int c = 1; c <= 3; c++) {
        System.out.print(r * c + "\t");
    }
    System.out.println();     // new line after each row
}

break and continue

• break exits the loop immediately. …