Function references (::functionName) in Kotlin - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a function reference to the 'println' function.

Kotlin

val printer = [1]println
printer("Hello, Kotlin!")

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

A->

B.

C::

D=>

Attempts:

3 left

💡 Hint

Common Mistakes

Using '.' instead of '::' which calls the function instead of referencing it.

Using '->' or '=>' which are not valid for function references in Kotlin.

In Kotlin, the '::' operator is used to create a function reference. Here, '::println' creates a reference to the println function.

2fill in blank

medium

Complete the code to pass a function reference of 'isEven' to the 'filter' function.

Kotlin

fun isEven(n: Int) = n % 2 == 0
val numbers = listOf(1, 2, 3, 4)
val evens = numbers.filter([1]isEven)
println(evens)

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

A::

B.

C->

D=>

Attempts:

3 left

💡 Hint

Common Mistakes

Using '.' which tries to call the function instead of referencing it.

Using lambda syntax '->' or '=>' which is incorrect here.

The '::' operator creates a reference to the 'isEven' function, which can be passed to 'filter' to select even numbers.

3fill in blank

hard

Fix the error by completing the code to create a function reference to the 'toUpperCase' method of String.

Kotlin

val toUpper: (String) -> String = [1]String::toUpperCase
println(toUpper("kotlin"))

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

AString::

B::

CString.

D.

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'String::' before 'String::toUpperCase' which is redundant.

Using '.' which tries to call the method instead of referencing it.

The correct syntax to reference a member function is '::' followed by the class and method name, like 'String::toUpperCase'.

4fill in blank

hard

Fill both blanks to create a map of words to their lengths using a function reference and a property reference.

Kotlin

val words = listOf("apple", "banana", "cherry")
val lengths = words.associateWith([1]) { it.[2] }
println(lengths)

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

AString::length

Blength

DString.length

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'String.length' which is invalid syntax.

Using 'length' without context or as a string.

The 'associateWith' function takes a function reference to get the value for each key. 'String::length' is the correct function reference, and 'length' is the property accessed on 'it'.

5fill in blank

hard

Fill all three blanks to filter a list of strings by length using a function reference and a lambda.

Kotlin

val words = listOf("cat", "dog", "elephant", "fox")
val filtered = words.filter { word -> word.length [1] [2] }
val lengthCheck: (String) -> Boolean = [3] { it.length > 3 }
println(filtered)
println(filtered.filter(lengthCheck))

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

A>

B3

C::

D<

Attempts:

3 left

💡 Hint

Common Mistakes

Using '<' instead of '>' which filters the wrong words.

Not using '::' when assigning the lambda to the function variable.

The filter uses '>' and '3' to select words longer than 3 characters. The function reference operator '::' is used to assign the lambda to 'lengthCheck'.

Practice

1. What does the Kotlin syntax ::functionName represent?

easy

A. A call to the function named functionName

B. A comment about the function named functionName

C. A variable declaration named functionName

D. A reference to the function named functionName

Solution

1. Step 1: Understand the :: operator in Kotlin

   The :: operator is used to get a reference to a function, not to call it.

2. Step 2: Differentiate between calling and referencing

   Calling a function uses parentheses like functionName(), while referencing uses ::functionName.

3. Final Answer:

   A reference to the function named functionName -> Option D

4. Quick Check:

   Function reference = ::functionName [OK]

Hint: Remember :: means 'point to function', not call it [OK]

Common Mistakes:

• Confusing function reference (::) with function call (())
• Thinking :: declares a variable
• Assuming :: is a comment or annotation

2. Which of the following is the correct syntax to pass a function reference to map in Kotlin?

easy

A. list.map(::functionName)

B. list.map(:functionName)

C. list.map(functionName())

D. list.map(functionName)

Solution

1. Step 1: Identify how to pass function references in Kotlin

   Function references use the :: operator before the function name when passed as arguments.

2. Step 2: Check each option for correct syntax

   list.map(::functionName) uses ::functionName, which is the correct way to pass a function reference to map.

3. Final Answer:

   list.map(::functionName) -> Option A

4. Quick Check:

   Pass function reference with :: [OK]

Hint: Use :: before function name when passing as argument [OK]

Common Mistakes:

• Passing function call instead of reference (using parentheses)
• Omitting :: operator
• Using incorrect single colon : instead of double ::

3. What is the output of this Kotlin code?

fun square(x: Int) = x * x
val numbers = listOf(1, 2, 3)
val squares = numbers.map(::square)
println(squares)

medium

A. [1, 4, 9]

B. [square, square, square]

C. [1, 2, 3]

D. Compilation error

Solution

1. Step 1: Understand the function reference usage

   The ::square passes the square function to map, which applies it to each element.

2. Step 2: Calculate the result of applying square to each number

   Applying square to 1, 2, 3 gives 1, 4, 9 respectively.

3. Final Answer:

   [1, 4, 9] -> Option A

4. Quick Check:

   map(::square) squares each element [OK]

Hint: map(::function) applies function to each list item [OK]

Common Mistakes:

• Expecting original list instead of transformed list
• Confusing function reference with function call
• Thinking output is function names instead of results

4. Identify the error in this Kotlin code snippet:

fun greet(name: String) = "Hello, $name"
val greeter = greet
println(greeter("Alice"))

medium

A. Missing parentheses when assigning function reference

B. Cannot assign function directly without :: operator

C. Function greet must return Unit

D. No error, code runs fine

Solution

1. Step 1: Check how function references are assigned

   Assigning a function to a variable requires the :: operator to get a reference.

2. Step 2: Identify the missing :: operator

   The code assigns greet instead of ::greet, causing a type mismatch error.

3. Final Answer:

   Cannot assign function directly without :: operator -> Option B

4. Quick Check:

   Use :: to assign function reference [OK]

Hint: Use :: when assigning function to variable [OK]

Common Mistakes:

• Assigning function name without ::
• Adding parentheses which calls function instead of referencing
• Assuming function must return Unit

5. Given this Kotlin code, what will be the output?

fun isEven(n: Int) = n % 2 == 0
val numbers = listOf(1, 2, 3, 4, 5, 6)
val evenNumbers = numbers.filter(::isEven)
println(evenNumbers)

Choose the correct output and explain why function references are useful here.

hard

A. [true, false, true, false, true, false]

B. [1, 3, 5]

C. [2, 4, 6]

D. Compilation error due to filter usage

Solution

1. Step 1: Understand the function reference with filter

   The ::isEven passes the isEven function to filter, which keeps elements where the function returns true.

2. Step 2: Apply isEven to each number and filter

   Numbers 2, 4, and 6 are even, so they remain in the filtered list.

3. Step 3: Explain usefulness of function references

   Function references let us pass existing functions easily without writing extra lambdas.

4. Final Answer:

   [2, 4, 6] -> Option C

5. Quick Check:

   filter(::isEven) keeps even numbers [OK]

Hint: filter(::function) keeps items where function returns true [OK]

Common Mistakes:

• Confusing filter with map output
• Expecting boolean list instead of filtered list
• Not using :: for function reference