Single-expression functions in Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a single-expression function with string interpolation

What is the output of this Kotlin code?

Kotlin

fun greet(name: String) = "Hello, $name!"

fun main() {
    println(greet("Anna"))
}

A Hello, Anna!

B Hello, name!

C greet(Anna)

D Error: Missing return type

💡 Hint

Look at how the function returns a string using $name inside quotes.

The function greet is a single-expression function that returns the string "Hello, " concatenated with the value of name and an exclamation mark. When called with "Anna", it prints "Hello, Anna!".

❓ Predict Output

intermediate

Return type inference in single-expression functions

What will this Kotlin program print?

Kotlin

fun square(x: Int) = x * x

fun main() {
    println(square(5))
}

A 0

B Error: Return type required

C 25

D 5

💡 Hint

The function returns the product of x times x.

The function square returns the square of the input number. For input 5, it returns 25.

🔧 Debug

advanced

Identify the error in this single-expression function

What error does this Kotlin code produce?

Kotlin

fun add(a: Int, b: Int) = return a + b

fun main() {
    println(add(3, 4))
}

A Output: 7

B SyntaxError: Unexpected 'return' in single-expression function

C TypeError: Missing return type

D RuntimeException: Null pointer

💡 Hint

Single-expression functions do not use the 'return' keyword explicitly.

In Kotlin, single-expression functions automatically return the expression value. Using 'return' explicitly causes a syntax error.

❓ Predict Output

advanced

Output of a single-expression function with conditional expression

What is the output of this Kotlin code?

Kotlin

fun max(a: Int, b: Int) = if (a > b) a else b

fun main() {
    println(max(10, 20))
}

A 10

B Error: Missing braces in if expression

C true

D 20

💡 Hint

The function returns the greater of two numbers using if-else.

The function max returns the larger of a and b. For inputs 10 and 20, it returns 20.

🧠 Conceptual

expert

Understanding single-expression function behavior with side effects

Consider this Kotlin code. What is the output when main() runs?

Kotlin

var counter = 0
fun increment() = counter++

fun main() {
    println(increment())
    println(increment())
}

A

0
1

B

1
2

C

0
0

D Error: Variable counter must be immutable

💡 Hint

The post-increment operator returns the value before incrementing.

The function increment returns the current value of counter, then increases it by 1. So the first call prints 0, the second prints 1.