Nested loops

Written Explanation

The shape

for (int row = 1; row <= 3; row++) {
    for (int col = 1; col <= 4; col++) {
        // inner body
    }
}

The inner loop is part of the outer loop's body. Every time the outer loop starts an iteration, the inner loop starts again from its own start: col is created fresh, runs 1 to 4, and disappears.

Counting the runs

If the outer loop runs R times and the inner loop runs C times each time, the inner body runs R times C in total. For 3 rows and 4 columns that is 12. The outer body (everything outside the inner loop) runs only R times.

This is the key habit of the lesson: before running a nested loop, say the number out loud. For 5 rows by 6 columns the answer is 30. For 1,000 by 1,000 it is a million. Counting like this is the beginning of estimating cost, which BEG-039 develops.

Rows and columns

Use the outer loop for rows and the inner loop for columns. To print a row of stars on one line you use System.out.print inside the inner loop and a System.out.println() after it, still inside the outer loop:

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

The println() is after the inner loop but inside the outer one. Move it and the output changes: that is the most common nested-loop bug.

Use meaningful, different names

Name the counters row and col (or day and slot), not i and i. If you reuse the same name in the inner loop, Java rejects it, because the outer variable is still in scope. Different names also make your trace tables easier.

The inner loop depends on the outer

The inner limit can use the outer counter. That produces triangles:

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

Row 1 prints one star, row 2 prints two, and so on. The inner body now runs 1 + 2 + 3 + 4 = 10 times, not 4 times 4, so the "multiply" rule is only for loops whose inner count does not change. When it does, add up the inner counts row by row.

Tracing a nested loop

Trace the triangle above with a table that has one row per inner iteration:

Read it from the top: row stays fixed while col runs through its whole list, then row moves on and col starts over. If you lose your place in a nested loop, this "outer fixed, inner sweeping" picture gets you back. A useful sanity check is to print row and col together inside the inner body, as in System.out.println(row + "," + col);, and compare the pairs with your table.

Why the inner counter restarts

The inner for header runs its start part every time the inner loop begins, so int col = 1 runs again for each new row. That is why the same inner code can print many identical rows. It also means the inner counter cannot carry information from one row to the next: if you need a total across all rows, declare it before both loops, as you did for a running total in L01.

break and continue in nested loops

As L04 said, they affect only the innermost loop. A break in the inner loop ends that row's inner loop; the outer loop then moves to the next row.