Sequence operators (map, filter) in Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Kotlin code using map and filter?

Consider this Kotlin code that uses sequence operators map and filter. What will it print?

Kotlin

val numbers = sequenceOf(1, 2, 3, 4, 5)
val result = numbers.filter { it % 2 == 1 }.map { it * 3 }.toList()
println(result)

A [6, 12, 18]

B [2, 4]

C [3, 9, 15]

D [1, 3, 5]

💡 Hint

First filter odd numbers, then multiply each by 3.

The code filters odd numbers (1,3,5) and then multiplies each by 3, resulting in [3,9,15].

❓ Predict Output

intermediate

What does this Kotlin code print after filtering and mapping?

Look at this Kotlin code using sequence operators. What is the output?

Kotlin

val words = sequenceOf("apple", "banana", "cherry", "date")
val result = words.filter { it.length > 5 }.map { it.uppercase() }.toList()
println(result)

A ["BANANA", "CHERRY"]

B ["apple", "banana", "cherry", "date"]

C ["APPLE", "BANANA", "CHERRY", "DATE"]

D ["date"]

💡 Hint

Filter words longer than 5 letters, then convert to uppercase.

Only "banana" and "cherry" have length > 5, so after uppercase, the list is ["BANANA", "CHERRY"].

🔧 Debug

advanced

Why does this Kotlin code throw an exception?

This Kotlin code throws an exception. What is the cause?

Kotlin

val nums = sequenceOf(1, 2, 3, 4)
val result = nums.map { if (it == 3) null else it * 2 }.filter { it!! > 2 }.toList()
println(result)

A Throws ClassCastException due to null in sequence

B Throws IllegalArgumentException because map returns null

C No exception, prints [4, 8]

D Throws NullPointerException because filter uses 'it!!' on null values

💡 Hint

Check how null values are handled in filter with 'it!!'.

The map returns null for element 3. The filter uses 'it!!' which throws NullPointerException when 'it' is null.

📝 Syntax

advanced

Which option is the correct Kotlin syntax to filter even numbers and double them using sequences?

Choose the correct Kotlin code that filters even numbers from a sequence and doubles them.

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

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

C val result = sequenceOf(1,2,3,4).filter(it % 2 == 0).map(it * 2).toList()

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

💡 Hint

Remember to use lambda syntax with curly braces and 'it'.

Option A uses correct lambda syntax with braces and 'it'. Options A and D miss braces, causing syntax errors. Option A changes order, doubling before filtering, which is logically different.

🚀 Application

expert

How many items are in the resulting list after this Kotlin sequence operation?

Given this Kotlin code, how many items does the resulting list contain?

Kotlin

val data = sequenceOf(10, 15, 20, 25, 30)
val result = data.filter { it > 15 }.map { it / 5 }.filter { it % 2 == 0 }.toList()
println(result.size)

A 1

B 2

C 3

D 4

💡 Hint

Step through filtering and mapping carefully.

Filter >15 gives [20,25,30]. Map divides by 5: [4,5,6]. Filter even numbers: [4,6]. So size is 2.