Single-expression functions in Kotlin - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a single-expression function in Kotlin?

A single-expression function is a function that returns the result of a single expression directly without using curly braces or the return keyword.

beginner

How do you write a single-expression function that adds two numbers in Kotlin?

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

intermediate

Can single-expression functions have explicit return types in Kotlin?

Yes, you can specify the return type explicitly, but Kotlin can often infer it automatically.

beginner

What is the benefit of using single-expression functions?

They make the code shorter, cleaner, and easier to read by removing unnecessary syntax like braces and return statements.

beginner

Is this a valid single-expression function in Kotlin?

fun greet() = println("Hello")

Yes, it is valid. The function calls println as a single expression and returns Unit.

Which of the following is a correct single-expression function in Kotlin?

A fun square(x: Int): Int { x * x }

B fun square(x: Int) { return x * x }

C fun square(x: Int) { x * x }

D fun square(x: Int) = x * x

Option D uses the single-expression syntax with '=' and no braces, which is correct.

What is the return type of this function?

fun isEven(n: Int) = n % 2 == 0

A Boolean

B Int

C Unit

D String

The expression 'n % 2 == 0' returns a Boolean value.

Can a single-expression function have multiple statements inside it?

A No, it can only have one expression.

B Only if wrapped in curly braces.

C Yes, but only if separated by semicolons.

D Yes, it can have any number of statements.

Single-expression functions can only have one expression, no multiple statements.

Which keyword is NOT needed in a single-expression function?

A fun

B return

C =

D parameter types

The 'return' keyword is not needed because the expression result is returned automatically.

What does this function return?

fun greet() = println("Hi")

A String "Hi"

B Nothing

C Unit

D Boolean

println returns Unit, so the function returns Unit.

Explain what a single-expression function is and how it differs from a regular function in Kotlin.

Think about how the function body is written and how the result is returned.

Write a single-expression function in Kotlin that checks if a number is positive.

Use a comparison expression like 'n > 0'.

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