Kotlin labelled break

While working with loops say you want to stop the execution of loop immediately if a certain condition is satisfied. In this case you can use either break or return expression to exit from the loop.

In this article, we are going to learn how to use break expression to exit a loop. When break expression encounters in a program it terminates to nearest enclosing loop.

There are two types of break expressions in Kotlin: 
As we all know, Unlabelled break is used to terminate the closest enclosing loop when a certain condition is satisfied. 
But labelled break is used to terminate a loop when a certain condition is satisfied. It can be done with the help of labels. An identifier followed by the @ sign is called a label, e.g., inner@, outer@, first@, second@, etc. You can use a label with any expression, and it should be written in front of it.

We are going to learn how to use labelled break expressions in while, do-while, and for loops.

Use of labelled break in while loop

Labelled break is used to exit to the desired block when it satisfy a specific condition without checking the condition in while loop. Then, transfers the control to following statement of while block. If you mark the outer loop using the label outer@ then you can easily break the outer loop using break@outer in the break condition block.

Syntax of labelled break in while loop

outer@ while(condition) {
      // code
      inner@ while(condition) {
            // code
            if(break condition) {
               break @outer
            } 
      }
}

Kotlin program using labelled break in a while loop 
 

fun main(args: Array<String>) {
    var num1 = 4
    outer@ while (num1 > 0) {
        var num2 = 4
        inner@ while (num2 > 0) {
            if (num1==2)
                break@outer
            println("num1 = $num1, num2 = $num2")
            num2--
        }
        num1--
    }
}

Output:

num1 = 4, num2 = 4
num1 = 4, num2 = 3
num1 = 4, num2 = 2
num1 = 4, num2 = 1
num1 = 3, num2 = 4
num1 = 3, num2 = 3
num1 = 3, num2 = 2
num1 = 3, num2 = 1

When (num1 == 2) expression is evaluated to be true, the break@outer is executed, which terminates the desired loop marked with outer@.
 

Use of labelled break in do-while loop

In do-while loop also the labelled break is executed to terminate the desired loop. Here we have used outer@ for the outer do-while and inner@ for the inner do-while loop.

Syntax of labelled break in do-while loop

outer@ do {
       // code
       inner@ do {
           // code
           if(break condition) {
              break@outer
           }
       } while(condition)
} while(condition)

Kotlin program using labelled break in a do-while loop

fun main(args: Array<String>) {
    var num1 = 4
    outer@ do {
        var num2 = 4

        inner@ do {
            if (num1 == 2)
                break@outer
            println("num1 = $num1; num2 = $num2")
            num2--
        } while (num2 > 0)
        num1--
    } while (num1 > 0)
}

Output:

num1 = 4; num2 = 4
num1 = 4; num2 = 3
num1 = 4; num2 = 2
num1 = 4; num2 = 1
num1 = 3; num2 = 4
num1 = 3; num2 = 3
num1 = 3; num2 = 2
num1 = 3; num2 = 1

Here, we print the same output as while loop. When (num1 == 2) expression is evaluated to be true, the break@outer is executed which terminates the desired loop marked with outer@.
 

Use of labelled break in a for loop

In for loop also we can use the labelled break to terminate the desired loop for certain condition. We have labelled the outer for loop as outer@ and inner for loop as inner@. In for loop, iteration is to be done through iterator. 

Syntax of labelled break in for loop 

outer@ for(iteration through iterator) {
    // code
      inner@ for(iteration through iterator)
           // code
          if(break condition) {
          break@outer
          }
      }
}

Kotlin program using labelled break in a for-loop

fun main(args: Array<String>) {
    outer@ for (num1 in 4 downTo 1) {

        inner@ for (num2 in 4 downTo 1) {
            if (num1 == 2)
                break@outer
            println("num1 = $num1; num2 = $num2")
        }
    }
}

Output:

num1 = 4; num2 = 4
num1 = 4; num2 = 3
num1 = 4; num2 = 2
num1 = 4; num2 = 1
num1 = 3; num2 = 4
num1 = 3; num2 = 3
num1 = 3; num2 = 2
num1 = 3; num2 = 1