Infix functions for readable calls in Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of infix function call

What is the output of this Kotlin code using an infix function?

Kotlin

infix fun Int.times(str: String) = str.repeat(this)

fun main() {
    val result = 3 times "Hi"
    println(result)
}

A HiHiHi

B Hi 3 times

C 3Hi

D Error: infix function cannot be called like this

💡 Hint

Remember infix functions allow calling like 'a times b' instead of 'a.times(b)'.

The infix function 'times' repeats the string the number of times specified by the integer. So 3 times "Hi" produces "HiHiHi".

❓ Predict Output

intermediate

Result of chained infix calls

What is the output of this Kotlin code with chained infix calls?

Kotlin

infix fun String.append(s: String) = this + s
infix fun String.exclaim(times: Int) = this + "!".repeat(times)

fun main() {
    val result = "Hello" append " World" exclaim 3
    println(result)
}

A Hello World!!!

B Error: cannot chain infix functions without parentheses

C Hello World exclaim 3

D Hello World!3

💡 Hint

Infix functions can be chained if the return type matches the next function's receiver.

The first infix call appends " World" to "Hello", then the second adds three exclamation marks, resulting in "Hello World!!!".

🔧 Debug

advanced

Identify the error in infix function usage

What error does this Kotlin code produce?

Kotlin

infix fun Int.add(other: Int) = this + other

fun main() {
    val sum = 5 add
    println(sum)
}

A TypeError: Cannot add Int and Unit

B SyntaxError: Missing argument for infix function

C Compilation error: Infix function call must have exactly one argument

D No error, prints 5

💡 Hint

Infix functions require exactly one argument when called.

The call '5 add' is missing the argument for the infix function 'add', causing a compilation error.

🧠 Conceptual

advanced

Understanding infix function constraints

Which of the following is NOT a requirement for defining an infix function in Kotlin?

A It must have exactly one parameter

B It must return Unit (void)

C It must be marked with the 'infix' keyword

D It must be a member function or an extension function

💡 Hint

Think about what return types infix functions can have.

Infix functions can return any type, not just Unit. The other options are required.

❓ Predict Output

expert

Output of infix function with nullable receiver

What is the output of this Kotlin code?

Kotlin

infix fun String?.orDefault(default: String) = this ?: default

fun main() {
    val a: String? = null
    val b: String? = "Kotlin"
    println(a orDefault "Default")
    println(b orDefault "Default")
}

A Compilation error: infix function cannot have nullable receiver

B

null
Kotlin

C

Default
Default

D

Default
Kotlin

💡 Hint

The Elvis operator ?: returns the left side if not null, otherwise the right side.

For 'a' which is null, the default is returned. For 'b' which is "Kotlin", it returns 'b'.