Java Programming Language (Loops: Nested loop)

Nested loop

Same as conditions, we can use a loop inside another loop. For example,

public class Loops {
    public static void main(String[] args) {
        for (int person = 1; person <= 2; person ++) {
            for (int task = 1; task <= 3; task ++) {
                System.out.println("Person " + person + " completed task " + task);
            }
        }
    }
}

Output

Person 1 completed task 1
Person 1 completed task 2
Person 1 completed task 3
Person 2 completed task 1
Person 2 completed task 2
Person 2 completed task 3

Code always executes from top to bottom. If we think about this manner, The compiler first goes inside the outer for loop. Then it will execute the code inside the block until the loop condition is false. However, inside the loop block compiler found another for loop. Now the compiler will execute the code inside this inner for loop block until it terminates. Then the outer loop will go to the next iteration.

In this case, the outer loop will iterate two times and the inner loop will iterate three times.

• First, while the outer loop is in it's first iteration the inner loop will iterate three times.
• Once the inner loop terminates the outer loop will move on to it's second iteration.
• In the second iteration the inner loop will reinitialise itself. As a result, while the outer loop is in it's second iteration the inner loop will again iterate over three times.
• After the inner loop terminates itself, the outer loop will also terminate as it does not meet the condition of outer loop.
• Then the compiler will execute the code from the next line.

There can be multiple nested loops and it can be performed with while loops as well.

break statement

break statement is generally used to terminate a block of code. Earlier we used break inside the switch statement in the Conditions section to terminate the subsequent cases. In loops we can use break as well to terminate the iterations.

public class Loops {
    public static void main(String[] args) {
        int count = 1;
        System.out.println("Counting to 3");
        while (true) {
            System.out.println("Count: " + count);
            if (count == 3) {
                break;
            }
            count++;
        }
    }
}

Here, on every iteration I am incrementing the count variable by 1. When the count becomes 3, the condition becomes true inside the if statement. As the compiler executes the break statement, it terminates the loop and goes to the next line after the loop.

Output

Counting to 3
Count: 1
Count: 2
Count: 3

continue statement

continue statement is used to skip a particular iteration. Suppose, I want to print number 1 to 5 except number 3.

public class Loops {
    public static void main(String[] args) {
        for (int i = 1; i <= 5; i++) {
            if (i == 3) {
                continue;
            }
            System.out.println(i);
        }
    }
}

When the code executes continue statement, it skips the current iteration and move on to the next iteration.
Output

1
2
4
5

Appendix

while using nested loop, break or continue statement becomes ambiguous as it cannot figure out which loop it wants to terminate or skip. In these cases, we can use label to identify the loops.

public class Loops {
    public static void main(String[] args) {
        // label
        outer:
        for (int i = 1; i <= 2; i++) {
            // label
            inner:
            for (int j = 1; j <= 3; j++) {
                if (j == 2) {
                    continue outer;
                }
                System.out.println("i = " + i + ", j = " + j);
            }
        }
    }
}

Here, we are labelling the loops before initialising so that we can specify the particular loop later. In the continue statement I am specifying which loop I want to skip when the condition meets.
Output

i = 1, j = 1
i = 2, j = 1

Find out what would happed if we skipped the inner loop instead of the outer loop here.

label can be used in break statements as well to terminate a certain loop.

public class Loops {
    public static void main(String[] args) {
        outer:
        for (int i = 1; i <= 2; i++) {
            inner:
            for (int j = 1; j <= 3; j++) {
                if (j == 2) {
                    break inner;
                }
                System.out.println("i = " + i + ", j = " + j);
            }
        }
    }
}

Output

i = 1, j = 1
i = 2, j = 1

Did you get the idea? Let me know in the comment section.

Happy coding