Grouping Array values

If we have a struct like this

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

and an array of 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)
]

we can group the boxes by the thingsInside property in order to get a Dictionary where the key is the number of things and the value is an array of boxes.

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

Now grouped is a [Int:[Box]] and has the following content

[
    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)]
]

Found a mistake? Have a question or improvement idea? Let me know.

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