Functions Workbook

Practice problems for In Kotlin, Functions Don’t Need a Class. Each takes a minute or two. Write your own answer first, then click Show answer to check — nothing here is a trick question, just direct practice of the syntax from the lesson.

Declaring and single-expression functions

1. Square a number

Write a single-expression function square that takes an Int and returns it multiplied by itself.

fun square(x: Int) = x * x

2. The larger of two

Write max(a: Int, b: Int) that returns the larger of the two, using if as an expression.

fun max(a: Int, b: Int) = if (a > b) a else b

3. Tighten a block body

Rewrite this as a single-expression function:

fun double(n: Int): Int {
    return n * 2
}

fun double(n: Int) = n * 2

4. A function that returns nothing

Write greet(name: String) that prints Hi, <name> and returns no meaningful value. What is its return type?

fun greet(name: String) {
    println("Hi, $name")
}

5. Build a full name

Write fullName(first: String, last: String) that returns the two joined by a space, using a string template.

fun fullName(first: String, last: String) = "$first $last"

Default and named arguments

6. A default port

Write connect(host: String, port: Int = 8080) that returns "host:port". Calling connect("localhost") should give "localhost:8080".

fun connect(host: String, port: Int = 8080) = "$host:$port"

7. Skip to the argument you want

Given fun connect(host: String, port: Int = 443, timeoutMs: Int = 5000), write a call that keeps the default port but sets timeoutMs to 10000.

connect("example.com", timeoutMs = 10000)

8. Replace two overloads with one

In Java you’d write two methods: String label(String text) and String label(String text, boolean bold). Write a single Kotlin function label where bold defaults to false; return the text wrapped in <b>…</b> when bold is true, otherwise the text unchanged.

fun label(text: String, bold: Boolean = false) =
    if (bold) "<b>$text</b>" else text

9. Arguments out of order

Given fun rect(width: Int, height: Int), call it with the arguments written in the opposite order, using named arguments.

rect(height = 20, width = 10)

vararg

10. Sum any number of values

Write sum that accepts any number of Int arguments and returns their total.

fun sum(vararg n: Int) = n.sum()

11. Pass an array into a vararg

Given fun sum(vararg n: Int): Int and val xs = intArrayOf(1, 2, 3), write the call that passes the array’s elements as the individual arguments.

sum(*xs)

12. Join words

Write joinWords(vararg words: String) that returns all the words joined by single spaces.

fun joinWords(vararg words: String) = words.joinToString(" ")

Top-level and local functions

13. A utility with no class

Write gcd(a: Int, b: Int) as a top-level, single-expression recursive function (greatest common divisor). No surrounding class.

fun gcd(a: Int, b: Int): Int = if (b == 0) a else gcd(b, a % b)

14. A helper that stays local

Inside a function banner(text: String), define a local function line() that prints 20 dashes, then use it above and below the text.

fun banner(text: String) {
    fun line() = println("-".repeat(20))

    line()
    println(text)
    line()
}

15. Make a call self-documenting

The call setEnabled(true) tells a reader nothing. Given fun setEnabled(enabled: Boolean), rewrite the call so it reads itself.

setEnabled(enabled = true)

Done? Head back to the lesson, In Kotlin, Functions Don’t Need a Class, or move on to the next one: control flow and ranges.