Destructuring in collection iteration in Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Output of destructuring a list of pairs

What is the output of this Kotlin code that uses destructuring in a for loop to iterate over a list of pairs?

Kotlin

val pairs = listOf(Pair("a", 1), Pair("b", 2), Pair("c", 3))
for ((letter, number) in pairs) {
    println("$letter$number")
}

A a1\nb2\nc3

B (a,1)\n(b,2)\n(c,3)

C a\nb\nc

D 1\n2\n3

💡 Hint

Look at how the pair is split into two variables inside the loop.

The for loop destructures each Pair into two variables: letter and number. Then it prints them concatenated, so the output is a1, b2, c3 each on a new line.

📝 Syntax

intermediate

Identify the syntax error in destructuring declaration

Which option contains a syntax error when destructuring a map entry in a for loop?

Kotlin

val map = mapOf("x" to 10, "y" to 20)
for ((key, value) in map) {
    println("$key -> $value")
}

A for ((key, value) in map) { println("$key -> $value") }

B for ((k, v) in map) { println("$k -> $v") }

C for (key, value in map) { println("$key -> $value") }

D for ((key, value) in map.entries) { println("$key -> $value") }

💡 Hint

Check the parentheses around the destructured variables.

Option C misses parentheses around key and value, which is required for destructuring declarations in Kotlin for loops.

❓ optimization

advanced

Optimizing iteration with destructuring on large map

Given a large map, which option is the most efficient way to iterate with destructuring to print keys and values?

Kotlin

val largeMap = (1..1000000).associateWith { it * 2 }

A largeMap.keys.forEach { println("$it -> ${largeMap[it]}") }

B largeMap.forEach { (k, v) -> println("$k -> $v") }

C largeMap.entries.forEach { println("${it.key} -> ${it.value}") }

D for ((k, v) in largeMap) { println("$k -> $v") }

💡 Hint

Consider the overhead of accessing values inside the loop.

Using forEach with destructuring (option B) is efficient and concise. Option B accesses the map value by key each iteration, which is slower. Options B and C are valid but forEach with destructuring is preferred for readability and performance.

🔧 Debug

advanced

Debug the error in destructuring a list of triples

What error occurs when running this code and why?

Kotlin

val triples = listOf(Triple(1, 2, 3), Triple(4, 5, 6))
for ((a, b) in triples) {
    println("$a and $b")
}

A Type mismatch error: expected 2 variables but Triple has 3

B No error, prints '1 and 2' and '4 and 5'

C Runtime exception: IndexOutOfBounds

D Syntax error: missing third variable in destructuring

💡 Hint

Check how many variables Triple expects in destructuring.

Triple requires three variables in destructuring. Using only two variables causes a type mismatch error at compile time.

🧠 Conceptual

expert

Understanding destructuring declarations with data classes

Given this data class and list, what is the output of the code below?

Kotlin

data class Person(val name: String, val age: Int)
val people = listOf(Person("Alice", 30), Person("Bob", 25))
for ((name, age) in people) {
    println("$name is $age years old")
}

A name age\nname age

B Person(name=Alice, age=30)\nPerson(name=Bob, age=25)

C Compilation error: cannot destructure Person

D Alice is 30 years old\nBob is 25 years old

💡 Hint

Data classes automatically support destructuring declarations.

Data classes in Kotlin automatically generate componentN functions, enabling destructuring. The loop extracts name and age from each Person and prints them.