分组数组值

如果我们有这样的结构

struct Box {
    let name: String
    let thingsInside: Int
}

和一系列的 Box(es)

let boxes = [
    Box(name: "Box 0", thingsInside: 1),
    Box(name: "Box 1", thingsInside: 2),
    Box(name: "Box 2", thingsInside: 3),
    Box(name: "Box 3", thingsInside: 1),
    Box(name: "Box 4", thingsInside: 2),
    Box(name: "Box 5", thingsInside: 3),
    Box(name: "Box 6", thingsInside: 1)
]

我们可以通过 thingsInside 属性对方框进行分组，以获得一个 Dictionary，其中 key 是事物的数量，值是一个盒子的数组。

let grouped = boxes.reduce([Int:[Box]]()) { (res, box) -> [Int:[Box]] in
    var res = res
    res[box.thingsInside] = (res[box.thingsInside] ?? []) + [box]
    return res
}

现在分组是 [Int:[Box]] 并具有以下内容

[
    2: [Box(name: "Box 1", thingsInside: 2), Box(name: "Box 4", thingsInside: 2)], 
    3: [Box(name: "Box 2", thingsInside: 3), Box(name: "Box 5", thingsInside: 3)],
    1: [Box(name: "Box 0", thingsInside: 1), Box(name: "Box 3", thingsInside: 1), Box(name: "Box 6", thingsInside: 1)]
]