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.

Practice

1. What is a key feature of a single-expression function in Kotlin?

easy

A. It returns the value of the expression after the equals sign automatically.

B. It requires a block body with explicit return statements.

C. It cannot have parameters.

D. It must be declared inside a class.

Solution

1. Step 1: Understand single-expression function syntax

   Single-expression functions use an equals sign followed by an expression, without braces.

2. Step 2: Identify the return behavior

   They automatically return the value of that expression without needing an explicit return keyword.

3. Final Answer:

   It returns the value of the expression after the equals sign automatically. -> Option A

4. Quick Check:

   Single-expression function = auto return [OK]

Hint: Look for '=' and no braces for single-expression functions [OK]

Common Mistakes:

• Thinking explicit return is needed
• Assuming parameters are not allowed
• Believing it must be inside a class

2. Which of the following is the correct syntax for a single-expression function in Kotlin that adds two integers?

easy

A. fun add(a: Int, b: Int) { return a + b }

B. fun add(a: Int, b: Int) = a + b

C. fun add(a: Int, b: Int): Int { a + b }

D. fun add(a: Int, b: Int): Int => a + b

Solution

1. Step 1: Identify single-expression function syntax

   It uses '=' followed by the expression without braces or explicit return.

2. Step 2: Check each option

   fun add(a: Int, b: Int) = a + b matches this syntax exactly: fun add(a: Int, b: Int) = a + b.

3. Final Answer:

   fun add(a: Int, b: Int) = a + b -> Option B

4. Quick Check:

   Equals sign and expression = correct syntax [OK]

Hint: Single-expression functions use '=' not braces [OK]

Common Mistakes:

• Using braces with return in single-expression functions
• Using '=>' instead of '='
• Omitting '=' sign

3. What will be the output of this Kotlin code?

fun square(x: Int) = x * x
println(square(4))

medium

A. 16

B. 4

C. x * x

D. Compilation error

Solution

1. Step 1: Understand the function behavior

   The function square returns the product of x times x.

2. Step 2: Calculate the output for input 4

   4 * 4 = 16, so square(4) returns 16, which is printed.

3. Final Answer:

   16 -> Option A

4. Quick Check:

   4 squared = 16 [OK]

Hint: Multiply input by itself for square function [OK]

Common Mistakes:

• Expecting the expression as a string output
• Confusing input with output
• Thinking it causes a compile error

4. Identify the error in this Kotlin single-expression function:

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

medium

A. Missing return type declaration.

B. String interpolation is invalid.

C. Using braces {} after equals sign is incorrect.

D. Function must have no parameters.

Solution

1. Step 1: Check single-expression function syntax

   Single-expression functions use '=' followed by an expression without braces.

2. Step 2: Identify the error

   The braces after '=' create a lambda block, which is not allowed here; it should be just the expression.

3. Final Answer:

   Using braces {} after equals sign is incorrect. -> Option C

4. Quick Check:

   No braces allowed after '=' in single-expression functions [OK]

Hint: No braces after '=' in single-expression functions [OK]

Common Mistakes:

• Adding braces after '='
• Thinking return type is mandatory
• Misusing string interpolation syntax

5. You want to write a Kotlin single-expression function that returns the length of a string only if it is not empty; otherwise, it returns zero. Which of these is correct?

hard

A. fun lengthOrZero(s: String) = s.length ?: 0

B. fun lengthOrZero(s: String) { if (s.isNotEmpty()) return s.length else return 0 }

C. fun lengthOrZero(s: String): Int = { if (s.isNotEmpty()) s.length else 0 }

D. fun lengthOrZero(s: String) = if (s.isNotEmpty()) s.length else 0

Solution

1. Step 1: Understand the requirement

   The function must return length if string is not empty, else zero, in a single expression.

2. Step 2: Check each option

   fun lengthOrZero(s: String) = if (s.isNotEmpty()) s.length else 0 uses a single-expression function with an if-else expression correctly. fun lengthOrZero(s: String) { if (s.isNotEmpty()) return s.length else return 0 } uses braces and explicit returns, not single-expression. fun lengthOrZero(s: String): Int = { if (s.isNotEmpty()) s.length else 0 } wrongly uses braces after '='. fun lengthOrZero(s: String) = s.length ?: 0 misuses the Elvis operator on Int.

3. Final Answer:

   fun lengthOrZero(s: String) = if (s.isNotEmpty()) s.length else 0 -> Option D

4. Quick Check:

   If-else expression after '=' = correct single-expression function [OK]

Hint: Use if-else expression directly after '=' for conditions [OK]

Common Mistakes:

• Using braces after '='
• Trying to use return inside single-expression function
• Misusing Elvis operator on non-nullable Int