Generating infinite sequences in Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a simple infinite sequence with take

What is the output of this Kotlin code snippet?

Kotlin

val seq = generateSequence(1) { it + 2 }
println(seq.take(5).toList())

A [1, 4, 7, 10, 13]

B [2, 4, 6, 8, 10]

C [1, 2, 3, 4, 5]

D [1, 3, 5, 7, 9]

💡 Hint

Look at how the sequence starts and how the lambda adds 2 each time.

The sequence starts at 1 and each next element adds 2, so the first five elements are 1, 3, 5, 7, 9.

🧠 Conceptual

intermediate

Understanding termination of infinite sequences

Which statement about Kotlin infinite sequences is true?

A Infinite sequences in Kotlin are stored fully in memory.

B An infinite sequence always runs forever and cannot be limited.

C You can limit an infinite sequence by using terminal operations like take or first.

D You must convert an infinite sequence to a list before using it.

💡 Hint

Think about how you can get a finite result from an infinite source.

Infinite sequences generate values lazily. Using functions like take(n) or first() stops the sequence after a finite number of elements.

❓ Predict Output

advanced

Output of a Fibonacci infinite sequence

What is the output of this Kotlin code?

Kotlin

val fibs = generateSequence(Pair(0, 1)) { Pair(it.second, it.first + it.second) }
println(fibs.take(7).map { it.first }.toList())

A [0, 1, 1, 2, 3, 5, 8]

B [1, 1, 2, 3, 5, 8, 13]

C [0, 1, 2, 3, 5, 8, 13]

D [1, 2, 3, 5, 8, 13, 21]

💡 Hint

The sequence generates pairs where the first is the current Fibonacci number.

The sequence starts with (0,1). Each next pair is (previous second, sum of previous first and second). Taking first elements gives Fibonacci numbers starting at 0.

❓ Predict Output

advanced

Terminating a sequence with null

What is the output of this Kotlin code?

Kotlin

val seq = generateSequence(1) { if (it < 5) it + 1 else null }
println(seq.toList())

A [1, 2, 3, 4, 5]

B Throws IllegalStateException due to null in sequence

C [1, 2, 3, 4]

D Throws NullPointerException

💡 Hint

Check what happens when the lambda returns null.

The lambda returns null when it >= 5, which signals the end of the sequence. toList() collects [1, 2, 3, 4, 5] and stops. No error occurs.

📝 Syntax

expert

Which option produces a valid infinite sequence of powers of 2?

Select the Kotlin code snippet that correctly generates an infinite sequence of powers of 2 starting at 1.

A val seq = generateSequence(1) { it + it }

B val seq = generateSequence(1) { it * 2 }

C val seq = generateSequence(1) { it xor 2 }

D val seq = generateSequence(1) { it ** 2 }

💡 Hint

Check Kotlin operators for multiplication and exponentiation.

Option B uses it * 2 which doubles the value each time, producing powers of 2. Option B also doubles but is equivalent to C. Option B uses ^ which is bitwise XOR in Kotlin, not exponentiation. Option B uses ** which is invalid syntax in Kotlin.

Practice

1. What does an infinite sequence in Kotlin do?
sequence { yield(1); yieldAll(generateSequence(2) { it + 1 }) }

easy

A. Generates only the first two numbers and stops

B. Creates a fixed list of numbers from 1 to 10

C. Generates values endlessly, one at a time, starting from 1

D. Throws an error because sequences can't be infinite

Solution

1. Step 1: Understand sequence behavior

   The sequence starts by yielding 1, then continues with an infinite sequence starting at 2, increasing by 1 each time.

2. Step 2: Recognize infinite generation

   Since generateSequence with a lambda that adds 1 never returns null, it produces values endlessly.

3. Final Answer:

   Generates values endlessly, one at a time, starting from 1 -> Option C

4. Quick Check:

   Infinite sequence = endless values [OK]

Hint: Infinite sequences produce values one by one without stopping [OK]

Common Mistakes:

• Thinking infinite sequences create fixed lists
• Assuming infinite sequences cause errors immediately
• Believing sequences stop after a few values

2. Which of the following is the correct syntax to create an infinite sequence of even numbers starting from 0 in Kotlin?

easy

A. sequence { yield(0); yield(2); yield(4) }

B. sequenceOf(0, 2, 4, 6)

C. generateSequence(0, 2)

D. generateSequence(0) { it + 2 }

Solution

1. Step 1: Identify infinite sequence syntax

   generateSequence(seed) { nextValue } creates an infinite sequence by applying the lambda repeatedly.

2. Step 2: Check each option

   generateSequence(0) { it + 2 } correctly uses generateSequence(0) { it + 2 } to produce even numbers endlessly. Options B and D create finite sequences. generateSequence(0, 2) has incorrect syntax.

3. Final Answer:

   generateSequence(0) { it + 2 } -> Option D

4. Quick Check:

   Correct syntax = generateSequence(0) { it + 2 } [OK]

Hint: Use generateSequence(seed) { next } for infinite sequences [OK]

Common Mistakes:

• Using sequenceOf for infinite sequences
• Passing two arguments to generateSequence incorrectly
• Not providing a lambda for next values

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

val seq = generateSequence(1) { it + 3 }
println(seq.take(4).toList())

medium

A. [1, 2, 3, 4]

B. [1, 4, 7, 10]

C. [1, 3, 6, 9]

D. [4, 7, 10, 13]

Solution

1. Step 1: Understand sequence generation

   The sequence starts at 1, then each next value adds 3: 1, 4, 7, 10, ...

2. Step 2: Apply take and toList

   take(4) takes first 4 values: 1, 4, 7, 10. toList() converts them to a list.

3. Final Answer:

   [1, 4, 7, 10] -> Option B

4. Quick Check:

   Sequence values = [1,4,7,10] [OK]

Hint: Add the step value repeatedly starting from seed [OK]

Common Mistakes:

• Adding 1 instead of 3 each step
• Confusing take with drop
• Expecting a finite sequence without take

4. Identify the error in this Kotlin code that tries to generate an infinite sequence of odd numbers:

val odds = generateSequence(1) { it + 2 }
println(odds.take(5).toList())

medium

A. No error; code runs correctly and prints odd numbers

B. Missing yield keyword inside generateSequence

C. The lambda should return null to stop sequence

D. Seed value should be 0 instead of 1

Solution

1. Step 1: Check generateSequence usage

   The code uses generateSequence(1) { it + 2 }, which correctly generates odd numbers starting at 1.

2. Step 2: Verify output and syntax

   take(5).toList() safely takes first 5 odd numbers. No syntax or runtime error exists.

3. Final Answer:

   No error; code runs correctly and prints odd numbers -> Option A

4. Quick Check:

   Correct infinite sequence = No error; code runs correctly and prints odd numbers [OK]

Hint: generateSequence needs only seed and next lambda, no yield [OK]

Common Mistakes:

• Thinking yield is needed inside generateSequence
• Expecting lambda to return null to continue
• Confusing seed value for even numbers

5. You want to generate an infinite sequence of Fibonacci numbers in Kotlin. Which code correctly creates this infinite sequence?

hard

A. Both A and B

B. sequence { var a=0; var b=1; while(true) { yield(a); val c=a+b; a=b; b=c } }

C. generateSequence(Pair(0, 1)) { Pair(it.second, it.first + it.second) }.map { it.first }

D. generateSequence(0) { it + it }

Solution

1. Step 1: Analyze generateSequence(Pair(0, 1)) { Pair(it.second, it.first + it.second) }.map { it.first }

   generateSequence(Pair(0, 1)) { Pair(it.second, it.first + it.second) }.map { it.first } uses generateSequence with pairs to generate Fibonacci pairs, then maps to first element, correctly producing Fibonacci numbers infinitely.

2. Step 2: Analyze sequence { var a=0; var b=1; while(true) { yield(a); val c=a+b; a=b; b=c } }

   sequence { var a=0; var b=1; while(true) { yield(a); val c=a+b; a=b; b=c } } uses a sequence builder with a while(true) loop yielding Fibonacci numbers step-by-step, also correct and infinite.

3. Step 3: Analyze generateSequence(0) { it + it }

   generateSequence(0) { it + it } doubles the number each time, not Fibonacci, so it's incorrect.

4. Final Answer:

   Both A and B -> Option A

5. Quick Check:

   Both methods generate Fibonacci infinite sequences [OK]

Hint: Use pairs or sequence builder with yield for Fibonacci [OK]

Common Mistakes:

• Using generateSequence without pairs for Fibonacci
• Trying to double numbers instead of summing previous two
• Not using yield in sequence builder for infinite values