Sequence vs collection performance in Kotlin - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a sequence from a list.

Kotlin

val numbers = listOf(1, 2, 3, 4, 5)
val seq = numbers.[1]()

Drag options to blanks, or click blank then click option'

Afilter

BasSequence

Cmap

DtoList

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'toList()' which returns a list, not a sequence.

Using 'map()' or 'filter()' directly without converting to sequence first.

The asSequence() function converts a collection to a sequence for lazy operations.

2fill in blank

medium

Complete the code to get the first element greater than 3 from a sequence.

Kotlin

val seq = sequenceOf(1, 2, 3, 4, 5)
val first = seq.filter { it > 3 }.[1]()

Drag options to blanks, or click blank then click option'

Alast

Bmap

Cfind

Dfirst

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'last()' which returns the last element.

Using 'find()' which returns nullable and requires a predicate.

The first() function returns the first element matching the condition.

3fill in blank

hard

Fix the error in the code to sum numbers lazily using a sequence.

Kotlin

val numbers = listOf(1, 2, 3, 4, 5)
val sum = numbers.[1]().map { it * 2 }.sum()

Drag options to blanks, or click blank then click option'

AtoList

Bfilter

CasSequence

Dmap

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'toList()' which does not enable lazy evaluation.

Using 'map()' directly without converting to sequence.

Using asSequence() allows lazy evaluation before summing.

4fill in blank

hard

Fill both blanks to create a sequence and filter even numbers.

Kotlin

val numbers = listOf(1, 2, 3, 4, 5)
val evens = numbers.[1]().[2] { it % 2 == 0 }.toList()

Drag options to blanks, or click blank then click option'

AasSequence

BtoList

Cfilter

Dmap

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'toList()' before filtering which is eager evaluation.

Using 'map' instead of 'filter' to select elements.

First convert to sequence with asSequence(), then filter even numbers with filter.

5fill in blank

hard

Fill all three blanks to create a sequence, map values, and find the max.

Kotlin

val numbers = listOf(1, 2, 3, 4, 5)
val maxVal = numbers.[1]().[2] { it * 3 }.[3]()

Drag options to blanks, or click blank then click option'

AasSequence

Bmap

Cmax

Dfilter

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'filter' instead of 'map' for transformation.

Using 'maxBy' which requires a selector function.

Not converting to sequence first.

Convert to sequence with asSequence(), map each value by multiplying by 3, then find the max with max().

Practice

1. What is the main advantage of using a Sequence over a Collection in Kotlin when working with large datasets?

easy

A. Sequences process data lazily, saving memory and improving performance.

B. Sequences store all data in memory for faster access.

C. Sequences automatically sort data for you.

D. Sequences are easier to write than collections.

Solution

1. Step 1: Understand how sequences process data

   Sequences process elements one by one only when needed, which is called lazy processing.

2. Step 2: Compare with collections

   Collections hold all elements in memory and process them eagerly, which can use more memory for large data.

3. Final Answer:

   Sequences process data lazily, saving memory and improving performance. -> Option A

4. Quick Check:

   Lazy processing = better for large data [OK]

Hint: Remember: sequences delay work, collections do it all at once [OK]

Common Mistakes:

• Thinking sequences store all data in memory
• Confusing lazy processing with sorting
• Assuming sequences are always simpler to write

2. Which of the following Kotlin code snippets correctly creates a sequence from a list?

easy

A. val seq = listOf(1, 2, 3).toSequence()

B. val seq = sequenceOf(listOf(1, 2, 3))

C. val seq = listOf(1, 2, 3).asSequence()

D. val seq = Sequence(listOf(1, 2, 3))

Solution

1. Step 1: Identify correct method to convert list to sequence

   The method asSequence() converts a collection to a sequence lazily.

2. Step 2: Check other options for correctness

   sequenceOf() creates a sequence from given elements, not a list; toSequence() and Sequence() constructor do not exist.

3. Final Answer:

   val seq = listOf(1, 2, 3).asSequence() -> Option C

4. Quick Check:

   Use asSequence() to convert collections [OK]

Hint: Use asSequence() to convert collections to sequences [OK]

Common Mistakes:

• Using non-existent toSequence() method
• Confusing sequenceOf() with asSequence()
• Trying to call Sequence() constructor directly

3. Consider the following Kotlin code:

val numbers = (1..5).asSequence()
    .map { it * 2 }
    .filter { it > 5 }
    .toList()
println(numbers)

What is the output?

medium

A. [12, 14, 16]

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

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

D. [6, 8, 10]

Solution

1. Step 1: Apply map transformation

   Numbers 1 to 5 are doubled: [2, 4, 6, 8, 10].

2. Step 2: Apply filter condition

   Keep only numbers greater than 5: [6, 8, 10].

3. Final Answer:

   [6, 8, 10] -> Option D

4. Quick Check:

   Map then filter = [6, 8, 10] [OK]

Hint: Map first, then filter to get final list [OK]

Common Mistakes:

• Filtering before mapping
• Forgetting to convert sequence to list
• Assuming original range is unchanged

4. What is wrong with this Kotlin code snippet?

val list = listOf(1, 2, 3, 4, 5)
val seq = list.asSequence()
val result = seq.map { it * 2 }
println(result)

medium

A. The map function does not exist for sequences.

B. The map operation is not executed because sequences are lazy.

C. The println statement causes a syntax error.

D. You cannot convert a list to a sequence using asSequence().

Solution

1. Step 1: Understand sequence laziness

   Sequences process operations lazily, so map returns a sequence but does not execute immediately.

2. Step 2: Check what is printed

   Printing result prints the sequence object, not the transformed data, because no terminal operation like toList() was called.

3. Final Answer:

   The map operation is not executed because sequences are lazy. -> Option B

4. Quick Check:

   Sequences need terminal operation to execute [OK]

Hint: Call toList() or similar to execute sequence operations [OK]

Common Mistakes:

• Expecting immediate execution of map on sequences
• Thinking asSequence() is invalid
• Assuming println shows transformed data

5. You have a large database query result as a Kotlin list of 1 million records. You want to filter records where age > 30, then map to names, and finally collect the first 10 names. Which approach is best for performance?

hard

A. Use a sequence: convert list to sequence, filter, map, then take(10) and toList().

B. Use a collection: filter, map on the list, then take first 10 elements.

C. Convert list to array, then filter and map using loops.

D. Sort the list first, then filter and map.

Solution

1. Step 1: Consider data size and operations

   With 1 million records, processing eagerly wastes memory and time.

2. Step 2: Use sequence for lazy processing

   Sequence processes elements one by one, stops after taking 10, saving resources.

3. Step 3: Compare other options

   Collections process all data eagerly; arrays and sorting add unnecessary overhead.

4. Final Answer:

   Use a sequence: convert list to sequence, filter, map, then take(10) and toList(). -> Option A

5. Quick Check:

   Lazy sequence + take(10) = best for large data [OK]

Hint: Use sequences with take() to limit processing early [OK]

Common Mistakes:

• Processing entire list eagerly wasting memory
• Sorting unnecessarily before filtering
• Using arrays without lazy evaluation