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'.