Kotlin - built-in higher-order functions - Power Up Your Code - Map(keys, values), fold, groupBy, sumBy, List(size)

Head First Kotlin
A Brain-Friendly Guide
楽天ブックス(Kobo) 紀伊國屋書店(Kinoppy)

開発環境

• macOS Mojave - Apple (OS)
• Windows 10 Pro (OS)
• IntelliJ IDEA CE(Community Edition) (IDE(統合開発環境))
• Kotlin (プログラミング言語)

Head First Kotlin: A Brain-Friendly Guide (Dawn Griffiths(著)、David Griffiths(著)、O'Reilly Media)のChapter 12(built-in higher-order functions - Power Up Your Code)、Mixed Messages(391)の解答を求めてみる。

コード

fun main() {
    val myMap = mapOf("A" to 4, "B" to 3, "C" to 2, "D" to 1, "E" to 2)
    var x1 = ""
    var x2 = 0
    println("1.")
    x1 = myMap.keys.fold("") { x, y -> x + y }
    x2 = myMap.entries.fold(0) { x, y -> x * y.value }
    println("$x1$x2" == "ABCDE0")

    x1 = "ABCDE"
    x2 = 0
    println("2.")
    x2 = myMap.values.groupBy { it }.keys.sumBy { it }
    println("$x1$x2" == "ABCDE10")

    x1 = "ABCDE"
    println("3.")
    x2 = myMap.values.fold(12) { x, y -> x - y }
    println("$x1$x2" == "ABCDE0")

    x1 = "ABCDE"
    x2 = 0
    println("4.")
    x2 = myMap.entries.fold(1) { x, y -> x * y.value }
    println("$x1$x2" == "ABCDE48")

    x1 = "ABCDE"
    x2 = 0
    println("4.")
    x1 = myMap.values.fold("") { x, y -> x + y }
    println("$x1$x2" == "432120")

    x1 = "ABCDE"
    x2 = 0
    println("5.")
    x1 = myMap.values
        .fold(0) { x, y -> x + y }
        .toString()
    x2 = myMap.keys.groupBy { it }.size
    println("$x1$x2" == "125")

}

入出力結果

1.
true
2.
true
3.
true
4.
true
4.
true
5.
true

Process finished with exit code 0