While and do-while loops in Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a while loop with decrement

What is the output of this Kotlin code snippet?

Kotlin

var x = 3
while (x > 0) {
    print(x)
    x--
}

A 123

B 321

C 3

D 0

💡 Hint

Remember that the loop runs while x is greater than zero and x decreases each time.

The loop prints x starting from 3 down to 1, so the output is '321'.

❓ Predict Output

intermediate

Output of a do-while loop with increment

What will this Kotlin code print?

Kotlin

var count = 0
do {
    print(count)
    count += 2
} while (count < 5)

A 024

B 0246

C 02

D 0

💡 Hint

The loop runs at least once and increments count by 2 each time.

The loop prints 0, then increments count to 2, prints 2, increments to 4, prints 4, then increments to 6 and stops because 6 is not less than 5.

🔧 Debug

advanced

Identify the error in this while loop

What error does this Kotlin code produce?

Kotlin

var i = 0
while (i < 3) {
    println(i)
}

A Infinite loop (no increment of i)

B Syntax error: missing parentheses

C Type error: i is not an integer

D No error, prints 0 1 2

💡 Hint

Check if the loop variable changes inside the loop.

The variable i is never incremented, so the condition i < 3 is always true, causing an infinite loop.

❓ Predict Output

advanced

Output of nested while and do-while loops

What is the output of this Kotlin code?

Kotlin

var a = 1
while (a <= 2) {
    var b = 1
    do {
        print(a * b)
        b++
    } while (b <= 2)
    a++
}

A 124

B 1122

C 1224

D 2244

💡 Hint

The outer loop runs twice, and the inner loop runs twice each time.

For a=1, b=1 and 2 print 1 and 2; for a=2, b=1 and 2 print 2 and 4; combined output is '1224'.

🧠 Conceptual

expert

Difference between while and do-while loops

Which statement correctly describes the difference between while and do-while loops in Kotlin?

A While loops can only count up; do-while loops can only count down.

B A while loop runs the body once before checking the condition; a do-while loop checks the condition first.

C Both loops check the condition before running the body, but do-while loops run faster.

D A while loop checks the condition before running the loop body; a do-while loop runs the body once before checking the condition.

💡 Hint

Think about when the condition is checked in each loop type.

While loops test the condition before running the loop body, so they may not run at all. Do-while loops run the body first, then test the condition, so they always run at least once.

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