while and do-while Loops
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while (condition) {
// body
}
do {
// body runs at least once
} while (condition)The while Loop
while repeatedly runs its body as long as the condition remains true, checking the condition before every iteration -- including the very first one.
Example: The while Loop
// Entry point: execution of the program starts here
fun main() {
// Declare a mutable variable `count`, initialized to `0`
var count = 0
// Keep looping while `count < 5` holds
while (count < 5) {
// Print "Count is $count" to the console, with a trailing newline
println("Count is $count")
count++
}
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The do-while Loop
do-while runs its body first and checks the condition afterward, guaranteeing the body executes at least once even if the condition is false from the start.
Example: The do-while Loop
// Entry point: execution of the program starts here
fun main() {
// Declare a mutable variable `attempts`, initialized to `0`
var attempts = 0
do {
// Print "Attempt number ${attempts + 1}" to the console, with a trailing newline
println("Attempt number ${attempts + 1}")
attempts++
// Keep looping while `attempts < 3` holds
} while (attempts < 3)
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Infinite Loops with break
while (true) combined with a break statement inside the body creates a loop whose exit condition is easier to express in the middle of the logic rather than up front.
while (true) loop has a reachable break, or it will run forever.Example: Infinite Loops with break
// Entry point: execution of the program starts here
fun main() {
// Declare a mutable variable `n`, initialized to `1`
var n = 1
// Keep looping while `true` holds
while (true) {
// Print "n is $n" to the console, with a trailing newline
println("n is $n")
n *= 2
if (n > 20) break
}
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Choosing Between while and do-while
Use while when the body might legitimately need to run zero times, and do-while when the body must always run at least once before checking whether to continue.
Example: Choosing Between while and do-while
// Entry point: execution of the program starts here
fun main() {
// Declare a read-only variable `condition`, set to `false`
val condition = false
// Keep looping while `condition` holds
while (condition) {
// Print "This never prints" to the console, with a trailing newline
println("This never prints")
}
do {
// Print "This always prints once" to the console, with a trailing newline
println("This always prints once")
// Keep looping while `condition` holds
} while (condition)
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- Forgetting to update the loop's condition variable inside the body, causing an infinite loop that never terminates.
- Using
whilewhen ado-whileis actually needed, resulting in a block that should run at least once being skipped entirely when the condition starts false. - Confusing the check order:
whilechecks before running the body,do-whilechecks after, changing how many times the body might run.
whilechecks its condition before each iteration, so the body may run zero times.do-whilechecks its condition after each iteration, guaranteeing the body runs at least once.- Both loops require the condition to eventually become false, usually by updating a variable inside the body.
while (true) { ... break ... }is a common pattern for loops whose exit condition is easier to express inside the body.
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