Kotlin Loops

Kotlin provides multiple looping constructs to repeat code execution: for, while, and do-while. Understanding when and how to use each loop is essential for writing clean, efficient code.

1. For Loop

Why use?

The for loop is ideal for iterating over a known range, array, list, or collection. It is concise and readable for fixed iterations.

How to use? Examples:

// Iterating over a range
for (i in 1..5) {
    println(i)
}

// Iterating over a collection with index
val colors = listOf("Red","Green","Blue")
for ((index, color) in colors.withIndex()) {
    println("$index -> $color")
}

// Using step and downTo
for (i in 10 downTo 1 step 2) println(i)

When to use and when not:

• Use for known iteration counts or collections.
• Do not use when the number of iterations depends on runtime conditions — prefer while loops.
• Great for concise traversal of arrays, lists, and ranges.

Complex Example:

// Nested loop with continue
for (i in 1..3) {
    for (j in 1..3) {
        if (i == j) continue // Skip diagonal elements
        println("i=$i, j=$j")
    }
}

Interview Questions:

• What is a for loop and when is it used?
• How do you iterate over a collection with index and value?
• Explain the difference between a for loop and a while loop.
• What are step, downTo, and until in ranges?

2. While Loop

Why use?

The while loop is used when the number of iterations is not known in advance and depends on a runtime condition.

How to use? Examples:

var i = 1
while (i <= 5) {
    println(i)
    i++
}

// Loop based on user input
var input: String
while (true) {
    print("Enter text (type exit to quit): ")
    input = readLine() ?: ""
    if (input == "exit") break
}

When to use and when not:

• Use when iterations depend on a runtime condition.
• Avoid for known counts — use for loop.
• Ensure the condition eventually becomes false to avoid infinite loops.

Complex Example:

// Accumulate sum until a limit
var sum = 0
var number = 1
while (sum < 20) {
    sum += number
    println("Added $number, sum=$sum")
    number++
}

Interview Questions:

• What is the difference between while and do-while loops?
• When would you prefer a while loop over a for loop?
• How can infinite loops occur in while loops?
• How do you exit a while loop prematurely?

3. Do-While Loop

Why use?

The do-while loop guarantees the loop executes at least once before checking the condition.

How to use? Examples:

var i = 1
do {
    println(i)
    i++
} while (i <= 5)

// User input example
var input: String
do {
    print("Enter text (type exit to quit): ")
    input = readLine() ?: ""
} while (input != "exit")

When to use and when not:

• Use when you need at least one iteration.
• Do not use if the loop may need to skip execution entirely — prefer while.
• Useful for menus or prompts that must appear at least once.

Complex Example:

// Input validation example
var age: Int
do {
    print("Enter your age (must be > 0): ")
    age = readLine()?.toIntOrNull() ?: 0
} while (age <= 0)

Interview Questions:

• What is the difference between do-while and while loops?
• When is a do-while loop preferred?
• Can a do-while loop execute zero times? Why or why not?
• How can break and continue be used inside do-while loops?

Summary Table