swift4-dictionary-subscript-with-default-2.swift

let array = [2, 3, 3, 4, 7, 2, 1, 4, 0, 9, 1, 2]

// array の要素の値ごとに集計
// 2 が 3 個、 3 が 2 個 などを知りたい
let nToCount: [Int: Int] = array.reduce(into: [:]) { nToCount, n in
    nToCount[n, default: 0] += 1
}

// 結果を表示
for (n, count) in (nToCount.sorted { $0.key < $1.key }) {
    print("\(n): \(count)")
}