標準ライブラリでコレクションを反復・変換する

コレクションをパイプラインとして扱う

map：1対1の変換

val prices = listOf(10, 20, 30)
val withTax = prices.map { it * 1.1 }
println(withTax)  // [11.0, 22.0, 33.0]

flatMap：1対多の変換

val sentences = listOf("hello world", "foo bar")
val words = sentences.flatMap { it.split(" ") }
println(words)  // [hello, world, foo, bar]

filterとfilterNot

val nums = (1..10).toList()
val evens = nums.filter { it % 2 == 0 }
val odds = nums.filterNot { it % 2 == 0 }
println(evens)  // [2, 4, 6, 8, 10]

groupBy：キーでグループ化

val words = listOf("one", "two", "three", "four", "five")
val byLength = words.groupBy { it.length }
println(byLength)
// {3=[one, two], 5=[three], 4=[four, five]}

associateとassociateBy

val items = listOf("apple", "banana", "cherry")
val byFirst = items.associateBy { it.first() }
println(byFirst)  // {a=apple, b=banana, c=cherry}

partition：2つに分割

val (evens, odds) = (1..8).partition { it % 2 == 0 }
println(evens)  // [2, 4, 6, 8]
println(odds)   // [1, 3, 5, 7]

distinctとdistinctBy

val nums = listOf(1, 2, 2, 3, 1, 4)
println(nums.distinct())  // [1, 2, 3, 4]
val words = listOf("Hi", "hello", "HI")
println(words.distinctBy { it.lowercase() })

take、drop、chunked

val list = (1..10).toList()
println(list.take(3))    // [1, 2, 3]
println(list.drop(7))    // [8, 9, 10]
println(list.chunked(3)) // [[1,2,3],[4,5,6],[7,8,9],[10]]

zip：2つのコレクションをペアにする

val names = listOf("Alice", "Bob", "Charlie")
val scores = listOf(90, 85, 92)
val combined = names.zip(scores)
println(combined)  // [(Alice, 90), (Bob, 85), (Charlie, 92)]

sumOf、minOf、maxOf、averageOf

val items = listOf("apple", "banana", "cherry")
println(items.sumOf { it.length })    // 18
println(items.maxOf { it.length })    // 6
println(items.minOf { it.length })    // 5

確認問題

まとめ