Single-expression functions in Kotlin - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define a single-expression function that returns the square of a number.

Kotlin

fun square(x: Int): Int = [1]

Drag options to blanks, or click blank then click option'

Ax * x

Breturn x * x

Cx + x

Dprintln(x)

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'return' keyword inside a single-expression function.

Using curly braces instead of '=' for single-expression functions.

The single-expression function uses the '=' sign followed by the expression. Here, x * x correctly returns the square.

2fill in blank

medium

Complete the code to define a single-expression function that returns the length of a string.

Kotlin

fun lengthOf(str: String): Int = [1]

Drag options to blanks, or click blank then click option'

Astr.length()

Bstr.length

Cstr.size

Dstr.count()

Attempts:

3 left

💡 Hint

Common Mistakes

Using length() with parentheses which is invalid in Kotlin.

Using size which is for collections, not strings.

In Kotlin, the length of a string is accessed using the length property without parentheses.

3fill in blank

hard

Fix the error in the single-expression function that returns true if a number is even.

Kotlin

fun isEven(num: Int): Boolean = num [1] 2 == 0

Drag options to blanks, or click blank then click option'

A%

B/

C*

D+

Attempts:

3 left

💡 Hint

Common Mistakes

Using division / instead of modulo %.

Using multiplication or addition operators which don't check evenness.

The modulo operator % gives the remainder. If num % 2 == 0, the number is even.

4fill in blank

hard

Complete the code to create a single-expression function that returns the last character of a string.

Kotlin

fun lastChar(str: String): Char = str[1]

Drag options to blanks, or click blank then click option'

A[

B]

Cstr.length - 1

DlastIndex

Attempts:

3 left

💡 Hint

Common Mistakes

Using parentheses instead of square brackets for indexing.

Omitting the str. prefix before length.

To get the last character, use the indexing operator [] with str.length - 1 which gives the last valid index.

5fill in blank

hard

Fill all three blanks to define a single-expression function that returns a greeting message with the user's name.

Kotlin

fun greet(name: String): String = "Hello, [1]!".[2]() + [3]

Drag options to blanks, or click blank then click option'

A$name

Buppercase

C" Have a nice day."

DtoUpperCase

Attempts:

3 left

💡 Hint

Common Mistakes

Using uppercase() which is Kotlin 1.5+ but not always available.

Forgetting to add parentheses after the uppercase function.

Not adding the extra greeting string at the end.

The function inserts the name inside the greeting string, converts it to uppercase with toUpperCase(), and adds a friendly message.

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