How to transform array to 2d array using reduce(into:_:)?

Question

I have an array of custom model:

struct Event {
    var day: Int // let's assume its Int for now
    var title: String
}

as:

let array = [Event(day: 1, title: "Pizza Party!"),
             Event(day: 1, title: "Another Pizza Party"),
             Event(day: 2, title: "Cinema - Moive 01"),
             Event(day: 2, title: "Cinema - Moive 02")]

I want to transform array to be a 2 dimensional array, each array should contains events that have the same day; According to array, the result should be:

[
    [Event(day: 1, title: "Pizza Party!"), Event(day: 1, title: "Another Pizza Party")]
    [Event(day: 2, title: "Cinema - Moive 01"), Event(day: 2, title: "Cinema - Moive 02")]
]

The first array -in the outer 2d array- contains events for day 1 and the second one contains events for day 2.

Is there a way to get the above result using the reduce(into:_:) method?

Despite of wanting to do it using reduce(into:_:), I was able to achieve by implementing:

func transfrom(_ models: [Event]) -> [[Event]] {
    let uniqueDates = Set(array.map { $0.day }).sorted()
    var twoDArray = [[Event]]()

    for date in uniqueDates {
        var array = [Event]()
        for model in models {
            if date == model.day {
                array.append(model)
            }
        }

        twoDArray.append(array)
    }

    return twoDArray
}

let transfomredArray = transfrom(array) // wanted result

Why reduce(into:_:) ? Something like Array(Dictionary(grouping: array, by: { $0.day }).values) should do the trick. — Martin R
– Martin R, Commented Nov 5, 2018 at 14:27
What makes you think that reduce(into:-:) is suitable for the task? I mean, is it really a requirement? — Alladinian
– Alladinian, Commented Nov 5, 2018 at 14:33
You can't do what you want using reduce(into:_:) Reduce reduces all the elements of an array into a single value. D V's answer of using dictionary(grouping:) should work. — Duncan C
– Duncan C, Commented Nov 5, 2018 at 14:38
Related (duplicate?): stackoverflow.com/questions/31220002/… — Martin R
– Martin R, Commented Nov 5, 2018 at 14:49

D V · Accepted Answer · 2018-11-05 14:28:53Z

4

You can use the grouping functionality of Dictionary to achieve your requirement:

// This dictionary will have all your events grouped based on the day it belongs to.
let dict = Dictionary(grouping: array) { (element) -> Int in
    return element.day
}

// If you need a sorted list of events based on the day.
let groupedItems = dict.sorted {
    return $0.key < $1.key
}

// groupedItems would now have all the data, grouped and ready to be used.
for day in groupedItems {
    print("\(day.key): \(day.value.count)")
}

answered Nov 5, 2018 at 14:28

D V

3482 silver badges10 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

ielyamani · Accepted Answer · 2018-11-05 14:51:47Z

0

If you still want reduce(into:):

let result: [[Event]] = array.sorted { $0.day < $1.day }
    .reduce(into: [[Event]]())
    { tempo, event in
        if let lastArray = tempo.last {
            if lastArray.first?.day == event.day {
                let head: [[Event]] = Array(tempo.dropLast())
                let tail: [[Event]] = [lastArray + [event]]
                tempo = head + tail
            }
            else {
                tempo.append([event])
            }
        } else {
            tempo = [[event]]
        }
}

answered Nov 5, 2018 at 14:51

ielyamani

18.6k11 gold badges59 silver badges94 bronze badges

Comments

Ankit Jayaswal · Accepted Answer · 2018-11-05 14:55:27Z

To do it in simpler way, you can use @MartinR's comment:

let resultArray = Dictionary(grouping: array, by: { $0.day }).values

But there might be some extra operations you want to perform over it, lets say sorting. Then you can use following approach:

Group the data, based on day parameter. Result would be [Int: [Event]]
```
let grouped = Dictionary(grouping: array, by: { $0.day })
```
Sort the grouped data on basis of key. result would be [Int: [Event]]
```
let sorted = grouped.sorted(by: { $0.key < $1.key })
```
You can use reduce to get output in desired format, note that simply applying reduce will flatten the array so use { $0 + [$1.value] } instead.
```
let resultArray: [[Event]] = sorted.reduce([]) { $0 + [$1.value] }
```

Collectives™ on Stack Overflow

How to transform array to 2d array using reduce(into:_:)?

3 Answers 3

Comments

Comments

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

Comments

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related