Sequence operators (map, filter) in Kotlin - Interactive Code 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

Bmap

CasSequence

DtoList

Attempts:

3 left

💡 Hint

Common Mistakes

Using map or filter directly on the list without converting to sequence.

The asSequence() function converts a collection into a sequence, enabling lazy operations.

2fill in blank

medium

Complete the code to filter even numbers from the sequence.

Kotlin

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

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

Afilter

BflatMap

Cmap

Dreduce

Attempts:

3 left

💡 Hint

Common Mistakes

Using map instead of filter, which transforms elements but does not select.

The filter function selects elements that satisfy the given condition, here even numbers.

3fill in blank

hard

Fix the error in the code to map numbers to their squares.

Kotlin

val numbers = sequenceOf(1, 2, 3)
val squares = numbers.[1] { it * it }

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

Afilter

Bfold

Creduce

Dmap

Attempts:

3 left

💡 Hint

Common Mistakes

Using filter which selects elements, not transforms them.

The map function transforms each element, here squaring each number.

4fill in blank

hard

Fill both blanks to create a sequence of squares of even numbers.

Kotlin

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

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

Afilter

Bmap

Creduce

Dfold

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping filter and map order, which changes the result.

First, filter selects even numbers, then map squares them.

5fill in blank

hard

Fill all three blanks to create a sequence of uppercase strings longer than 3 characters.

Kotlin

val words = listOf("cat", "dog", "elephant", "fox")
val result = words.asSequence().[1] { it.length > 3 }.[2] { it.[3]() }.toList()

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

Afilter

Bmap

Cuppercase

Dlowercase

Attempts:

3 left

💡 Hint

Common Mistakes

Using lowercase instead of uppercase.

Swapping filter and map order.

First, filter selects words longer than 3 letters, then map converts them to uppercase.

Practice

1. What does the map operator do in a Kotlin sequence?

easy

A. It removes items that do not match a condition.

B. It transforms each item in the sequence to a new value.

C. It sorts the items in the sequence.

D. It counts the number of items in the sequence.

Solution

1. Step 1: Understand the purpose of map

   The map operator applies a function to each item, changing it to a new value.

2. Step 2: Compare with other operators

   filter removes items, sort orders items, and count counts items, so these are different.

3. Final Answer:

   It transforms each item in the sequence to a new value. -> Option B

4. Quick Check:

   map = transform each item [OK]

Hint: Map changes items; filter removes items [OK]

Common Mistakes:

• Confusing map with filter
• Thinking map sorts or counts items
• Assuming map removes items

2. Which of the following is the correct syntax to filter even numbers from a sequence seq in Kotlin?

easy

A. seq.filter { it % 2 == 0 }

B. seq.map { it % 2 == 0 }

C. seq.filter(it % 2 == 0)

D. seq.map { it / 2 == 0 }

Solution

1. Step 1: Identify correct filter syntax

   In Kotlin, filter takes a lambda with condition inside curly braces: { it % 2 == 0 }.

2. Step 2: Check other options

   map changes values, not filters; seq.filter(it % 2 == 0) misses braces; seq.map { it / 2 == 0 } uses wrong condition.

3. Final Answer:

   seq.filter { it % 2 == 0 } -> Option A

4. Quick Check:

   filter uses braces and condition [OK]

Hint: filter uses braces with condition inside [OK]

Common Mistakes:

• Using map instead of filter
• Missing curly braces in lambda
• Wrong condition inside lambda

3. What is the output of this Kotlin code?

val numbers = sequenceOf(1, 2, 3, 4)
val result = numbers.filter { it % 2 == 0 }.map { it * 10 }.toList()
println(result)

medium

A. [10, 20]

B. [2, 4]

C. [20, 40]

D. [1, 3]

Solution

1. Step 1: Apply filter to keep even numbers

   Filter keeps numbers where it % 2 == 0, so 2 and 4 remain.

2. Step 2: Apply map to multiply each by 10

   Mapping multiplies each by 10: 2 * 10 = 20, 4 * 10 = 40.

3. Final Answer:

   [20, 40] -> Option C

4. Quick Check:

   filter evens then map *10 = [20, 40] [OK]

Hint: Filter first, then map transforms filtered items [OK]

Common Mistakes:

• Mapping before filtering
• Confusing filter condition
• Forgetting to convert sequence to list

4. Find the error in this Kotlin code snippet:

val seq = sequenceOf(1, 2, 3, 4)
val filtered = seq.filter(it % 2 == 0)
println(filtered.toList())

medium

A. Missing curly braces around lambda in filter

B. Using map instead of filter

C. Sequence cannot be converted to list

D. Filter condition should be it % 2 != 0

Solution

1. Step 1: Check filter lambda syntax

   Filter requires a lambda with curly braces: { it % 2 == 0 }. The code misses braces.

2. Step 2: Validate other options

   Map is not used here; sequence can convert to list; condition can be any boolean expression.

3. Final Answer:

   Missing curly braces around lambda in filter -> Option A

4. Quick Check:

   filter needs braces for lambda [OK]

Hint: Always use braces {} for lambdas in filter [OK]

Common Mistakes:

• Omitting braces in lambda
• Confusing filter and map
• Wrong filter condition assumed as error

5. Given a sequence of strings seq = sequenceOf("apple", "banana", "cherry", "date"), which Kotlin code correctly filters strings containing letter 'a' and converts them to uppercase?

hard

A. seq.filter { it.contains('a') }.map { it.lowercase() }.toList()

B. seq.map { it.uppercase() }.filter { it.contains('a') }.toList()

C. seq.map { it.lowercase() }.filter { it.contains('a') }.toList()

D. seq.filter { it.contains('a') }.map { it.uppercase() }.toList()

Solution

1. Step 1: Filter strings containing 'a'

   Use filter { it.contains('a') } to keep only strings with 'a'. This keeps "apple", "banana", "date".

2. Step 2: Convert filtered strings to uppercase

   Use map { it.uppercase() } to change these strings to uppercase.

3. Final Answer:

   seq.filter { it.contains('a') }.map { it.uppercase() }.toList() -> Option D

4. Quick Check:

   Filter then map uppercase = seq.filter { it.contains('a') }.map { it.uppercase() }.toList() [OK]

Hint: Filter first, then map to uppercase [OK]

Common Mistakes:

• Mapping before filtering changes all strings
• Using lowercase instead of uppercase
• Filtering after mapping changes string case before filter