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.

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