Do-While Loop

A do-while loop is a post-test control flow structure that guarantees the execution of a code block at least once before evaluating a boolean loop-continuation condition. If the condition evaluates to true, the loop iterates; if false, execution proceeds to the next statement following the loop.

Syntax

do {
    // Statements to execute
} while (condition)

Execution Mechanics

1. Unconditional Execution: The instruction pointer enters the do block and executes all statements sequentially. This occurs before any condition check.
2. Condition Evaluation: After the block executes, the boolean condition within the while clause is evaluated.
3. Branching:
   • If the condition evaluates to true, the instruction pointer jumps back to the start of the do block for the next iteration.
   • If the condition evaluates to false, the loop terminates, and control flow passes to the subsequent program statement.

Variable Scoping

A distinct feature of Kotlin’s do-while loop, contrasting with several other C-family languages, is its scoping rule. Variables declared locally within the do block remain in lexical scope and are fully accessible within the while condition evaluation.

fun main() {
    var countdown = 3
    
    do {
        val currentStatus = countdown--
        println("Status: $currentStatus")
    } while (currentStatus > 0) // 'currentStatus' is legally accessed here
}

Structural Jump Expressions

The do-while loop supports Kotlin’s structural jump expressions to alter control flow during an iteration. These jumps can be unlabelled (affecting the innermost enclosing loop) or labelled (affecting a specifically designated loop).

• break: Immediately terminates the innermost loop entirely, bypassing the while condition check, and transfers control to the statement following the loop.
• continue: Immediately halts the current iteration within the do block and jumps directly to the while condition evaluation of the innermost loop to determine if the next iteration should commence.
• break@label: Immediately terminates the specific loop marked with the matching label.
• continue@label: Immediately halts the current iteration and jumps to the while condition evaluation of the specific loop marked with the matching label.

fun main() {
    var x = 0
    
    outerLoop@ do {
        x++
        var y = 0
        
        do {
            y++
            
            // Halts current inner iteration, jumps to while (y < 3)
            if (y == 2) continue 
            
            // Terminates the outer loop entirely, bypassing all remaining iterations
            if (x == 3) break@outerLoop 
            
            println("x = $x, y = $y")
        } while (y < 3)
        
    } while (x < 5)
}

Master Kotlin with Deep Grasping Methodology!

Learn More