Nested Loops and Loop Patterns

A nested loop is simply a loop placed inside another loop. The inner loop completes all of its iterations for every single iteration of the outer loop — this is the foundation for working with grids, matrices, and pattern printing.

Basic Nested Loop Structure

for (int i = 1; i <= 2; i++) {
    System.out.println("Outer: " + i);       // runs 2 times
    for (int j = 1; j <= 3; j++) {
        System.out.println("  Inner: " + j); // runs 2 × 3 = 6 times total
    }
}

For every single pass of the outer loop, the inner loop runs completely from start to finish before control returns to the outer loop.

Classic Use Case: Multiplication Table

for (int i = 1; i <= 3; i++) {
    for (int j = 1; j <= 3; j++) {
        System.out.print(i * j + " ");
    }
    System.out.println(); // moves to next line after each row
}
// Output:
// 1 2 3
// 2 4 6
// 3 6 9

Pattern Printing — A Common Interview Warm-Up

// Right-angled triangle of stars
for (int i = 1; i <= 5; i++) {
    for (int j = 1; j <= i; j++) {
        System.out.print("* ");
    }
    System.out.println();
}
// Output:
// *
// * *
// * * *
// * * * *
// * * * * *

Time Complexity of Nested Loops

If the outer loop runs n times and the inner loop runs m times for each outer pass, the total number of inner-loop executions is n × m. When both loops run over the same input size n, this gives O(n²) time complexity — a key concept that resurfaces constantly in DSA when analyzing brute-force algorithms.

Interview tip: Pattern-printing questions (triangles, pyramids, diamonds) are common in early technical screening rounds specifically to test comfort with nested loop logic — practice a few different shapes, not just triangles.