Lambdas Workbook (Part 2): The Collection API

Practice problems for Lambdas at Work: Transforming Collections. Each takes a minute or two. Write your own answer first, then click Show answer — nothing here is a trick question, just direct practice of the syntax from the lesson.

passing a lambda to a function

1. forEach

Given val names = listOf("Ada", "Linus"), print each name with forEach.

names.forEach { println(it) }

2. The trailing-lambda convention

Rewrite names.forEach({ n -> println(n) }) in idiomatic form (lambda outside the parentheses, using it).

names.forEach { println(it) }

map and filter

3. map to squares

Turn listOf(1, 2, 3) into [1, 4, 9].

listOf(1, 2, 3).map { it * it }

4. map to lengths

Given words: List<String>, produce a list of their lengths.

words.map { it.length }

5. filter

Keep only the even numbers from nums.

nums.filter { it % 2 == 0 }

6. Chain them

From nums, keep the evens and then square them.

nums.filter { it % 2 == 0 }.map { it * it }

the rest of the toolkit

7. any

Is any element of nums negative?

nums.any { it < 0 }

8. all

Are all elements of nums positive?

nums.all { it > 0 }

9. count

How many names in names are longer than three characters?

names.count { it.length > 3 }

10. find

Get the first element of nums greater than 10, or null if there’s none.

nums.find { it > 10 }

11. sumOf

Add up the lengths of all words.

words.sumOf { it.length }

your own higher-order function

12. Take a function as a parameter

Write repeatTimes(n: Int, action: (Int) -> Unit) that calls action with 0, 1, … n-1.

fun repeatTimes(n: Int, action: (Int) -> Unit) {
    for (i in 0 until n) action(i)
}

13. Call it with a trailing lambda

Call your repeatTimes to print tick 0, tick 1, tick 2.

repeatTimes(3) { println("tick $it") }

Back to the lesson, Lambdas at Work, or on to part three: closures.