While and do-while loops in Kotlin - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a while loop in Kotlin?

A while loop repeats a block of code as long as a given condition is true. It checks the condition before running the code each time.

beginner

How does a do-while loop differ from a while loop?

A do-while loop runs the code block first, then checks the condition. This means it always runs at least once, even if the condition is false at the start.

beginner

Write a simple Kotlin while loop that prints numbers 1 to 3.

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

beginner

Write a Kotlin do-while loop that prints numbers 1 to 3.

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

intermediate

When should you use a do-while loop instead of a while loop?

Use a do-while loop when you want the code to run at least once, no matter what the condition is. For example, when asking a user for input and you want to check it after the first input.

What happens first in a while loop?

A The condition is checked

B The code block runs

C The loop ends

D The variable is reset

In a while loop, the condition is checked before running the code block.

How many times does a do-while loop run if the condition is false at the start?

A Zero times

B At least once

C Twice

D Infinite times

A do-while loop always runs the code block at least once before checking the condition.

Which loop is better when you don't know how many times you need to repeat but want to check before running?

A for loop

B do-while loop

C while loop

D if statement

A while loop checks the condition first, so it is good when you want to run only if the condition is true.

What is the output of this Kotlin code? var i = 3 while (i < 3) { println(i); i++ }

A No output

B 1 2 3

C 3

D Error

The condition i < 3 is false at the start, so the while loop does not run and prints nothing.

What keyword is used to repeat a block of code while a condition is true in Kotlin?

A repeat

B cycle

C loop

D while

The keyword 'while' is used to create a while loop in Kotlin.

Explain the difference between a while loop and a do-while loop in Kotlin.

Think about when the condition is checked in each loop.

Write a Kotlin program using a while loop to print numbers from 1 to 5.

Start with var i = 1 and increase i inside the loop.

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