Let function with safe calls in Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of let with safe call on nullable String

What is the output of this Kotlin code snippet?

Kotlin

val name: String? = "Anna"
val result = name?.let { it.uppercase() } ?: "No name"
println(result)

A ANNA

B No name

C anna

D null

💡 Hint

Remember that let runs only if the value is not null and uppercase converts to capital letters.

The safe call ?. lets the let block run only if name is not null. Since name is "Anna", let converts it to uppercase, so output is "ANNA".

❓ Predict Output

intermediate

Result of let with null value and safe call

What will be printed by this Kotlin code?

Kotlin

val number: Int? = null
val output = number?.let { it * 2 } ?: -1
println(output)

A Exception

B -1

C null

D 0

💡 Hint

If the variable is null, let block does not run and the Elvis operator ?: provides the default.

Since number is null, the safe call ?. skips let and the Elvis operator returns -1, so output is -1.

🔧 Debug

advanced

Identify the error in let with safe call

This Kotlin code is intended to print the length of a nullable string if it is not null. What error will it produce?

Kotlin

val text: String? = null
val length = text.let { it.length }
println(length)

A Prints 0

B NullPointerException

C Compilation error: Only safe (?.) or non-null asserted (!!.) calls are allowed on a nullable receiver

D Prints null

💡 Hint

Check how let is called on a nullable variable without safe call operator.

Calling let directly on a nullable variable without safe call causes a compilation error because the receiver might be null.

❓ Predict Output

advanced

Output of nested let with safe calls

What is the output of this Kotlin code?

Kotlin

val a: String? = "Hi"
val b: String? = null
val result = a?.let { x -> b?.let { y -> x + y } } ?: "Empty"
println(result)

A Empty

B Hi

C Hinull

D null

💡 Hint

The inner let runs only if b is not null. If b is null, the whole expression after ?: is used.

Since b is null, b?.let does not run and the outer let returns null, so Elvis operator returns "Empty".

❓ Predict Output

expert

Value of variable after let with safe call and side effect

Consider this Kotlin code. What is the final value of the variable 'counter' after execution?

Kotlin

var counter = 0
var text: String? = "abc"
text?.let {
    counter += it.length
}
text = null
text?.let {
    counter += it.length
}
println(counter)

A Compilation error

B 0

C 6

D 3

💡 Hint

The let block runs only when the variable is not null. The second let does not run because text is null.

The first let runs and adds 3 to counter. The second let does not run because text is null. So counter is 3.