While and do-while loops in Kotlin - Time & Space Complexity

Time Complexity: While and do-while loops

O(n)

Understanding Time Complexity

We want to understand how the time it takes to run while and do-while loops changes as the input grows.

Specifically, how many times the loop runs affects the total work done.

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

var count = 0
while (count < n) {
    println("Count is: $count")
    count++
}

var index = 0
do {
    println("Index is: $index")
    index++
} while (index < n)
    

This code runs two loops: a while loop and a do-while loop, each running until a counter reaches n.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: The body of each loop runs once per count from 0 up to n - 1.
• How many times: Each loop runs approximately n times.

How Execution Grows With Input

As n grows, the number of times the loops run grows in the same way.

Input Size (n)	Approx. Operations
10	About 10 times each loop runs
100	About 100 times each loop runs
1000	About 1000 times each loop runs

Pattern observation: The work grows directly in proportion to n.

Final Time Complexity

Time Complexity: O(n)

This means the time to run these loops grows linearly as the input size n grows.

Common Mistake

[X] Wrong: "A do-while loop always runs faster than a while loop because it runs at least once."

[OK] Correct: Both loops run about n times here, so their time depends on n, not on the loop type.

Interview Connect

Understanding how loops grow with input size helps you explain your code's efficiency clearly and confidently.

Self-Check

"What if we added a nested loop inside the while loop that also runs n times? How would the time complexity change?"

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