While and do-while loops in Kotlin

Introduction

Loops help you repeat actions without writing the same code many times. While and do-while loops let you repeat code as long as a condition is true.

When you want to keep asking a user for input until they give a valid answer.

When you want to process items in a list until you reach the end.

When you want to repeat a task until a certain condition changes.

When you want to keep checking if a game is over before continuing.

When you want to run a block of code at least once, then repeat it if needed.

Syntax

Kotlin

while (condition) {
    // code to repeat
}

do {
    // code to repeat
} while (condition)

The while loop checks the condition before running the code.

The do-while loop runs the code once before checking the condition.

Examples

This while loop prints numbers 1 to 3.

Kotlin

var count = 1
while (count <= 3) {
    println("Count is $count")
    count++
}

This do-while loop also prints numbers 1 to 3, but runs the code block at least once.

Kotlin

var count = 1
do {
    println("Count is $count")
    count++
} while (count <= 3)

Sample Program

This program shows how while and do-while loops work. The first loop counts down from 5 to 1. The second loop counts up from 1 to 3.

Kotlin

fun main() {
    var number = 5
    println("Using while loop:")
    while (number > 0) {
        println(number)
        number--
    }

    var count = 1
    println("Using do-while loop:")
    do {
        println(count)
        count++
    } while (count <= 3)
}

Important Notes

Be careful to change the condition inside the loop, or it may run forever.

do-while loops are useful when you want the code to run at least once.

Summary

While loops check the condition before running the code.

Do-while loops run the code once before checking the condition.

Use loops to repeat tasks easily and avoid writing repeated code.

Practice

1. What is the main difference between a while loop and a do-while loop in Kotlin?

easy

A. while loops are faster than do-while loops.

B. while runs code once before checking; do-while checks the condition before running code.

C. while checks the condition before running the code; do-while runs code once before checking.

D. while loops only run once; do-while loops run multiple times.

Solution

1. Step 1: Understand while loop behavior

   A while loop checks its condition before running the code inside it, so if the condition is false initially, the code inside won't run at all.

2. Step 2: Understand do-while loop behavior

   A do-while loop runs the code inside once first, then checks the condition to decide if it should repeat.

3. Final Answer:

   while checks the condition before running the code; do-while runs code once before checking. -> Option C

4. Quick Check:

   Loop condition check timing = while checks the condition before running the code; do-while runs code once before checking. [OK]

Hint: Remember: do-while runs code once before condition check [OK]

Common Mistakes:

• Thinking both loops check condition before running code
• Confusing which loop runs code first
• Assuming do-while runs only once

2. Which of the following is the correct syntax for a do-while loop in Kotlin?

easy

A. do { /* code */ } while (condition)

B. while (condition) { /* code */ } do

C. do (condition) { /* code */ } while

D. while { /* code */ } do (condition)

Solution

1. Step 1: Recall correct do-while syntax

   The do-while loop syntax starts with do, then the code block in braces, followed by while with the condition in parentheses.

2. Step 2: Compare options to syntax

   do { /* code */ } while (condition) matches the correct syntax exactly: do { /* code */ } while (condition). Others have wrong order or misplaced keywords.

3. Final Answer:

   do { /* code */ } while (condition) -> Option A

4. Quick Check:

   Syntax order for do-while = do { /* code */ } while (condition) [OK]

Hint: Remember: do { } while(condition) is the correct order [OK]

Common Mistakes:

• Placing while before do
• Using parentheses incorrectly
• Mixing up braces and parentheses

3. What will be the output of this Kotlin code?

var count = 3
while (count > 0) {
    println(count)
    count--
}

medium

A. 3 2 1

B. 1 2 3

C. 3 2 1 0

D. 0 1 2 3

Solution

1. Step 1: Analyze initial value and condition

   The variable count starts at 3. The loop runs while count > 0, so it runs for 3, 2, and 1.

2. Step 2: Trace each iteration and output

   Each loop prints the current count then decreases it by 1. So it prints 3, then 2, then 1, then stops before printing 0.

3. Final Answer:

   3 2 1 -> Option A

4. Quick Check:

   Loop prints count down from 3 to 1 = 3 2 1 [OK]

Hint: Count down loops print from start to 1, not zero [OK]

Common Mistakes:

• Including 0 in output
• Printing in ascending order
• Not decrementing count

4. Identify the error in this Kotlin code snippet:

var i = 1
do {
    println(i)
    i++
} while i <= 3

medium

A. Missing braces around the do block

B. Incorrect use of do keyword

C. Variable i should be declared inside the loop

D. Missing parentheses around the while condition

Solution

1. Step 1: Check do-while syntax

   The while condition must be inside parentheses after the while keyword.

2. Step 2: Identify the error in the code

   The code has while i <= 3 without parentheses. It should be while (i <= 3).

3. Final Answer:

   Missing parentheses around the while condition -> Option D

4. Quick Check:

   Parentheses required for while condition = Missing parentheses around the while condition [OK]

Hint: Always put condition in parentheses after while [OK]

Common Mistakes:

• Omitting parentheses in while condition
• Placing variable declaration inside loop unnecessarily
• Confusing braces and parentheses

5. You want to write a Kotlin program that asks the user to enter a positive number and keeps asking until the user enters a number greater than zero. Which loop should you use and why?

hard

A. Use a while loop because it checks the condition before asking the user.

B. Use a do-while loop because it asks the user at least once before checking the condition.

C. Use a while loop because it runs the code at least once.

D. Use a do-while loop because it never checks the condition.

Solution

1. Step 1: Understand the problem requirement

   The program must ask the user at least once to enter a number, then repeat if the number is not positive.

2. Step 2: Choose the correct loop type

   A do-while loop runs the code block once before checking the condition, so it ensures the user is asked at least once.

3. Final Answer:

   Use a do-while loop because it asks the user at least once before checking the condition. -> Option B

4. Quick Check:

   Ask user once then check = do-while loop = Use a do-while loop because it asks the user at least once before checking the condition. [OK]

Hint: Use do-while to run code once before condition check [OK]

Common Mistakes:

• Choosing while loop which may skip asking once
• Thinking do-while never checks condition
• Confusing loop behavior with input requirements