Testing scope functions and lambdas in Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of nested scope functions with lambdas

What is the output of this Kotlin code snippet?

Kotlin

val result = "Hello".run {
    val upper = this.uppercase()
    also { println("Inside also: $it") }
    lowercase()
}
println(result)

A

Inside also: hello
hello

B

Inside also: HELLO
hello

C

Inside also: Hello
hello

D

Inside also: Hello
HELLO

💡 Hint

Remember what 'run' and 'also' return and what 'this' and 'it' refer to inside lambdas.

The run function uses the string as this. Inside, upper is the uppercase version but not returned. The also lambda prints the original string it which is "Hello". Finally, lowercase() is the last expression and returned by run. So the printed lines are:
Inside also: Hello
hello

❓ Predict Output

intermediate

Return value of 'apply' with lambda

What does this Kotlin code print?

Kotlin

val list = mutableListOf(1, 2, 3).apply {
    add(4)
    remove(2)
}
println(list)

A [1, 3]

B [1, 2, 3, 4]

C [4]

D [1, 3, 4]

💡 Hint

Recall that 'apply' returns the object it was called on after running the lambda.

The apply function runs the lambda on the list and returns the list itself. Inside the lambda, add(4) adds 4, and remove(2) removes the element 2. So the final list is [1, 3, 4].

❓ Predict Output

advanced

Effect of 'let' on nullable types

What is the output of this Kotlin code?

Kotlin

val name: String? = null
val length = name?.let {
    println("Name is not null: $it")
    it.length
} ?: -1
println(length)

A -1

B

Name is not null: null
0

C

Name is not null: null
-1

D NullPointerException

💡 Hint

Check how safe call operator and 'let' work with null values.

The variable name is null. The safe call name?.let { ... } skips the lambda because name is null. So the Elvis operator ?: -1 returns -1. Nothing is printed inside the lambda.

❓ Predict Output

advanced

Output of 'also' modifying external variable

What will be printed by this Kotlin code?

Kotlin

var count = 0
val result = listOf(1, 2, 3).also {
    count = it.sum()
}
println("count = $count, result = $result")

A count = 6, result = [1, 2, 3]

B count = 0, result = 6

C count = 6, result = 6

D count = 0, result = [1, 2, 3]

💡 Hint

'also' returns the original object but allows side effects inside the lambda.

The also function runs the lambda with the list as it. Inside, count is set to the sum of the list elements (6). The also returns the original list. So the print shows count = 6 and result = [1, 2, 3].

❓ Predict Output

expert

Complex chaining of scope functions and lambdas

What is the output of this Kotlin code?

Kotlin

val output = mutableListOf<String>().run {
    add("Start")
    apply {
        add("Middle")
        also {
            add("Almost End")
        }
    }
    add("End")
    this
}
println(output)

A [Start, Almost End, End]

B [Start, Middle, Almost End, End]

C [Start, Middle, End]

D [Start, Middle, Almost End]

💡 Hint

Trace each scope function carefully and remember what each returns.

The run block starts with an empty mutable list. add("Start") adds "Start". Then apply adds "Middle" and inside also adds "Almost End". apply returns the list, so the chain continues. Then add("End") adds "End". Finally, run returns the list. The final list contains all four strings in order.