Nested loops

Nested loops — the inner loop runs completely for every single pass of the outer loop:

for (int row = 1; row <= 3; row++) {
    for (int col = 1; col <= 4; col++) {
        System.out.print(col);
    }
    System.out.println();
}
// 1234
// 1234
// 1234

The inner body ran 3 × 4 = 12 times. When you see nested loops, compute the total iteration count first: it is the product.

Triangle patterns show the inner bound depending on the outer variable:

for (int row = 1; row <= 4; row++) {
    for (int star = 1; star <= row; star++) {
        System.out.print("*");
    }
    System.out.println();
}

prints a staircase: 1, 2, 3, then 4 stars. Total work: 1+2+3+4 = 10 iterations — the shape of many O(n²) discussions later.

do-while (rare on the exam, know its shape): the body runs first, the test runs after — so the body always executes at least once. Use while/for in your own code; trace do-when it appears.