How can I sort array with two kind of objects in Swift?

Question

This is my method when I sort elements by position property. Both DBSet and DBItem have that property.

@objc(DBCategory)
class DBCategory: NSManagedObject {

    @NSManaged var identifier: String
    @NSManaged var items: Set<DBItem>
    @NSManaged var sets: Set<DBSet>

}

And this is how I use it

private var elements = [AnyObject]()

private func prepareElements() {
    elements.removeAll(keepCapacity: false)
    if let items = currentCategory?.items {
        for item in items {
            elements.append(item)
        }
    }

    if let sets = currentCategory?.sets {
        for set in sets {
            elements.append(set)
        }
    }

    elements.sort {
        var previousPosition = 0
        var currentPosition = 0
        if let set = $0 as? DBSet {
            previousPosition = Int(set.position)
        }

        if let item = $0 as? DBItem {
            previousPosition = Int(item.position)
        }

        if let set = $1 as? DBSet {
            currentPosition = Int(set.position)
        }

        if let item = $1 as? DBItem {
            currentPosition = Int(item.position)
        }

        return previousPosition < currentPosition
    }
}

position is type of Int16

How can I simplify that?

Can you Also show the declaration of your two object types? (DBSet and DBItem) — Duncan C
– Duncan C, Commented May 29, 2015 at 11:51

Duncan C · Accepted Answer · 2015-05-29 11:30:53Z

2

Create a protocol that defines your position property. Let's call it HasPosition. Declare an array of [HasPosition]. You can then sort the array directly with any of the standard sorting functions.

answered May 29, 2015 at 11:30

Duncan C

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4 Comments

Airspeed Velocity Over a year ago

This has a big downside of type erasure…

Airspeed Velocity Over a year ago

Though, I guess the types are thoroughly erased in whatever he has here at the moment...

Duncan C Over a year ago

Wouldn't elements have to be type [AnyObject] in order to contain multiple types? Explain how my solution loses information? (Not being snide... You know more about Swift than I)

Airspeed Velocity Over a year ago

Yeah it’s hard to say since we don’t see the declaration of elements.

Airspeed Velocity · Accepted Answer · 2015-05-29 11:48:13Z

1

If .position returns the same type in both cases, and that type is comparable, you can simplify that to the following:

elements.sort {
    let l = ($0 as? DBSet)?.position ?? ($0 as? DBItem)?.position
    let r = ($1 as? DBSet)?.position ?? ($1 as? DBItem)?.position
    return l < r
}

The key to this working is that there is a version of < that works for optionals, where nil is defined as less than any non-nil value. So values in the array not of one of the two types will be sorted to the beginning.

edited May 29, 2015 at 11:48

answered May 29, 2015 at 11:39

Airspeed Velocity

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Comments

rintaro · Accepted Answer · 2015-05-29 12:52:06Z

When you have the array as [AnyObject] and the position is Int16, this should works:

elements.sort {
    ($0.position as Int16?) < ($1.position as Int16?)
}

This works because AnyObject has any @objc properties as Optional.

Demo:

class DBItem:NSObject {
    var position:Int16
    init(position: Int16) { self.position = position }
    override var description:String  { return "DBItem(\(position))" }
}

class DBSet:NSObject {
    var position:Int16
    init(position: Int16) { self.position = position }
    override var description:String  { return "DBSet(\(position))" }
}

var elements:[AnyObject] = [
    DBItem(position: 5),
    DBSet(position: 2),
    DBItem(position: 42),
    DBSet(position: 62),
    DBSet(position: 21),
    DBItem(position: 6),
    DBSet(position: 36),
    DBItem(position: 24),
]


elements.sort {
    ($0.position as Int16?) < ($1.position as Int16?)
}

println(elements)
// -> [DBSet(2), DBItem(5), DBItem(6), DBSet(21), DBItem(24), DBSet(36), DBItem(42), DBSet(62)]

And your prepareElements() can be simplified as:

private func prepareElements() {
    elements = []
    if let ct = currentCategory {
        elements += Array(ct.items) as [AnyObject]
        elements += Array(ct.sets) as [AnyObject]
        elements.sort {
            ($0.position as Int16?) < ($1.position as Int16?)
        }
    }
}

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