2.11 Nested Iteration

1. A loop inside a loop

A nested iteration statement is a loop placed inside the body of another loop. The order is strict: once the inner loop starts, it finishes every one of its iterations, and only then can the outer loop go on to its next iteration 2.11.A.1.

for (int round = 1; round <= 2; round++) {
    for (int player = 1; player <= 3; player++) {
        System.out.println("Round " + round + ", player " + player);
    }
    System.out.println("End of round " + round);
}

Output:

Round 1, player 1
Round 1, player 2
Round 1, player 3
End of round 1
Round 2, player 1
Round 2, player 2
Round 2, player 3
End of round 2

The inner loop ran all 3 players before the outer loop moved from round 1 to round 2, and it started again at player 1 in round 2. The inner body ran 2 times 3 = 6 times.

2. Rows and columns

Think of the outer loop as rows and the inner loop as columns. print stays on the same line; println after the inner loop ends the row.

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

Output:

1 2 3 4 
2 4 6 8 
3 6 9 12 

Watch out: a per-row result must be reset inside the outer loop. This code totals each row of the table above:

for (int row = 1; row <= 3; row++) {
    int rowTotal = 0;
    for (int col = 1; col <= 4; col++) {
        rowTotal += row * col;
    }
    System.out.println("Row " + row + " total: " + rowTotal);
}

Output:

Row 1 total: 10
Row 2 total: 20
Row 3 total: 30

If int rowTotal = 0; were moved above the outer loop, the totals would pile up: 10, 30, 60.

3. Inner bounds that depend on the outer loop

When the inner loop’s Boolean expression uses the outer loop control variable, each row has a different length. A triangle:

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

Output:

*
**
***
****

i	inner loop values of j	stars printed
1	1	1
2	1, 2	2
3	1, 2, 3	3
4	1, 2, 3, 4	4

The inner body ran 1 + 2 + 3 + 4 = 10 times. Changing the inner loop to start at j = i and run while j <= 4 would print the rows in the other order: 4, 3, 2, 1 stars.

A number pattern built the same way:

for (int i = 1; i <= 3; i++) {
    for (int j = i; j <= 3; j++) {
        System.out.print(j);
    }
    System.out.println();
}

Output:

123
23
3

4. Checking pairs in a string

Nested loops can compare every character with every later character. Does a code contain any repeated letter? The inner loop starts at j = i + 1, so each pair is checked once and no letter is compared with itself.

String code = "PLANET";
boolean repeat = false;
for (int i = 0; i < code.length(); i++) {
    for (int j = i + 1; j < code.length(); j++) {
        if (code.substring(i, i + 1).equals(code.substring(j, j + 1))) {
            repeat = true;
        }
    }
}
System.out.println("Repeat: " + repeat);

Output:

Repeat: false

With "LETTER" instead, the code prints Repeat: true (E and T both repeat). For a 6-letter string the inner body runs 5 + 4 + 3 + 2 + 1 = 15 times.

A while loop can be nested in a for loop too. Count the digits of each number from 8 to 12:

for (int n = 8; n <= 12; n++) {
    int temp = n;
    int digits = 0;
    while (temp > 0) {
        digits++;
        temp /= 10;
    }
    System.out.print(digits + " ");
}
System.out.println();

Output:

1 1 2 2 2 

The copy temp matters: dividing n itself would change the outer loop control variable and break the outer loop.

5. Common nested loop errors

Using the same name for both loop control variables does not compile when both are declared in their headers:

for (int i = 0; i < 3; i++) {
    for (int i = 0; i < 2; i++) {
        System.out.print(i);
    }
}

error: variable i is already defined in method main(String[])

Using the outer variable in the inner header by mistake (j < i when you meant j < 3) changes how many times the inner body runs. The first row can even run zero times:

int count = 0;
for (int i = 0; i < 3; i++) {
    for (int j = 0; j < i; j++) {
        count++;
    }
}
System.out.println(count);

Output:

3

With j < 3 the count would be 9. Here the inner loop runs 0, 1 and 2 times. To check nested loops, write a small table with one row per outer iteration and list what the inner loop does in that row.