Map transformation in Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Kotlin map transformation?

Consider the following Kotlin code that transforms a map. What will be printed?

Kotlin

val original = mapOf("a" to 1, "b" to 2, "c" to 3)
val transformed = original.mapValues { it.value * 2 }
println(transformed)

A {a=2, b=4, c=6}

B Compilation error

C {a=3, b=4, c=5}

D {a=1, b=2, c=3}

💡 Hint

mapValues applies a transformation only to the values, keeping keys the same.

The mapValues function creates a new map with the same keys but transforms each value by multiplying it by 2. So 1 becomes 2, 2 becomes 4, and 3 becomes 6.

❓ Predict Output

intermediate

What does this Kotlin code print after filtering and transforming a map?

Given this Kotlin code, what is the output?

Kotlin

val numbers = mapOf(1 to "one", 2 to "two", 3 to "three", 4 to "four")
val filtered = numbers.filterKeys { it % 2 == 0 }.mapValues { it.value.uppercase() }
println(filtered)

A {2=two, 4=four}

B {2=TWO, 4=FOUR}

C {1=ONE, 3=THREE}

D Runtime exception

💡 Hint

filterKeys keeps only keys that satisfy the condition, then mapValues transforms the values.

The filterKeys keeps only keys 2 and 4 (even numbers). Then mapValues converts their values to uppercase strings.

🔧 Debug

advanced

Why does this Kotlin map transformation code cause a compilation error?

Look at this Kotlin code snippet. It tries to create a new map with values doubled only if the value is greater than 2. Why does it fail to compile?

Kotlin

val data = mapOf(1 to 1, 2 to 3, 3 to 5)
val result = data.mapValues { if (it.value > 2) it.value * 2 }
println(result)

A The lambda does not return a value for all cases, causing a type mismatch.

B mapValues cannot be used with if statements inside the lambda.

C The mapOf keys must be strings, not integers.

D The println statement is missing parentheses.

💡 Hint

Consider what happens when the if condition is false inside the lambda.

The lambda returns a value only when it.value > 2. When false, it returns Unit implicitly, causing a type mismatch because mapValues expects a consistent return type.

📝 Syntax

advanced

Which option correctly creates a map with keys doubled and values squared in Kotlin?

Choose the Kotlin code snippet that correctly creates a new map where each key is doubled and each value is squared from the original map mapOf(1 to 2, 3 to 4).

Kotlin

val original = mapOf(1 to 2, 3 to 4)

A original.map { it.key * 2 to it.value * it.value }

B original.mapValues { (k, v) -> k * 2 to v * v }

C original.mapKeys { it.key * 2 }.mapValues { it.value * it.value }

D original.map { (k, v) -> k * 2 to v * v }.toMap()

💡 Hint

map returns a list of pairs, which can be converted back to a map with toMap().

Option D correctly maps each entry to a new pair with doubled key and squared value, then converts the list of pairs back to a map. Option D is invalid because mapValues lambda receives only the value, not key-value pair. Option D doubles keys but does not correctly pair them with squared values. Option D returns a list, not a map.

🚀 Application

expert

How many entries are in the resulting map after this Kotlin transformation?

Given the code below, how many entries does the result map contain?

Kotlin

val input = mapOf("x" to 10, "y" to 20, "z" to 30)
val result = input.mapKeys { it.key + it.value.toString() }.filterKeys { it.length > 2 }

A 0

B 1

C 3

D 2

💡 Hint

Check the length of each new key after concatenation.

The new keys are "x10", "y20", and "z30". Each has length 3, which is greater than 2, so all three entries pass the filterKeys condition. Thus, the map contains 3 entries.