Chaining scope functions in Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of chained scope functions with let and also

What is the output of this Kotlin code snippet?

Kotlin

val result = "hello".let {
    println(it.uppercase())
    it.length
}.also {
    println(it * 2)
}
println(result)

A

hello
10
5

B

HELLO
5
10

C

HELLO
10
5

D

HELLO
5
5

💡 Hint

Remember that let returns the last expression inside its block, and also returns the receiver.

The let block prints the uppercase string "HELLO" and returns the length 5.
The also block receives 5, prints 5 * 2 = 10, and returns the original receiver which is 5.
Finally, println(result) prints 5.

❓ Predict Output

intermediate

Result of chaining apply and run

What will be printed by this Kotlin code?

Kotlin

val builder = StringBuilder().apply {
    append("Hi")
}.run {
    append(" there")
    toString()
}
println(builder)

A there

B Hi thereHi

C Hi

D Hi there

💡 Hint

apply returns the receiver, run returns the last expression.

apply appends "Hi" and returns the StringBuilder.
run appends " there" and returns the string representation.
So builder is "Hi there".

🔧 Debug

advanced

Identify the error in chained let and with

What error does this Kotlin code produce?

Kotlin

val text = "abc".let {
    it.uppercase()
}.with {
    println(this)
    length
}
println(text)

A Unresolved reference: length

B Type mismatch: inferred type is Unit but Int was expected

C No error, prints ABC and 3

D NullPointerException

💡 Hint

The let block returns the result of the last expression: it.uppercase(), which is a String.

This code produces an error. The let block returns a String "ABC".
However, with is a top-level function that takes the receiver as its first argument, but here it is called as an extension function, which is invalid.
Therefore, the compiler reports an error: Unresolved reference: with.

❓ Predict Output

advanced

Output of nested run and also with mutable list

What is the output of this Kotlin code?

Kotlin

val list = mutableListOf(1, 2, 3).run {
    add(4)
    also {
        it.removeAt(0)
    }
    size
}
println(list)

A 3

B 4

C 5

D Compilation error

💡 Hint

Remember what run returns and what also returns.

run returns the last expression, which is size (an Int).
also returns the receiver list but is not the last expression.
After adding 4 and removing the first element, the list has 3 elements.
So list is 3.

🧠 Conceptual

expert

Understanding return values in chained scope functions

Given the Kotlin code below, what is the final value of result?

Kotlin

val result = "Kotlin".also {
    println(it.lowercase())
}.let {
    it.length
}.apply {
    println(this)
}

A Unit

B 6

C Compilation error

D Kotlin

💡 Hint

Check what apply returns and what let returns.

also prints "kotlin" and returns the original string "Kotlin".
let returns the length 6.
apply is called on the integer 6 (scope functions are extensions on Any, so work on Int), prints 6, and returns the receiver 6.
Thus, result is 6.