Swift language: nil check, and if so instantiate new object

Question

Is there any way I can simplify this:

var unloadedImagesRows = [String:[Int]]()

private func addToUnloadedImagesRow(row: Int, forLocation:String!) {
    if unloadedImagesRows[forLocation] == nil {
                    unloadedImagesRows[forLocation] = [Int]()
    }
    unloadedImagesRows[forLocation]!.append(row)
}

Doesn't Swift have an easy way to check for nil, and if so, create a new object, and all subsequent uses refers to the object?

Pretty sure your answer is the most simple swift has to offer. All the others here seem to be less simple and/or efficient. I dug for more than an hour on this and do not think there is a simpler way to express this. — Eric Mentele, Apr 12 '16 at 08:31
Do you know of an example from another language that does this better? I am curious to know what inspired this question and if there is another way from another language, maybe propose a similar feature addition to Swift open source. — Eric Mentele, Apr 12 '16 at 08:36

score 8 · Accepted Answer · answered Apr 12 '16 at 06:03

8

You can simplify it into just one line:

private func addToUnloadedImagesRow(row: Int, forLocation:String!) {
    unloadedImagesRows[forLocation] = (unloadedImagesRows[forLocation] ?? []) + [row]
}

answered Apr 12 '16 at 06:03

Eendje

8,815
1
29
31

So sweet! I was trying so hard to figure that out! Though it had to be +=... Nice work! – Eric Mentele Apr 12 '16 at 17:27

Eric Mentele · Answer 2 · 2016-04-12T08:07:38.713

var unloadedImagesRows = [String:[Int]]()

// if let
private func addToUnloadedImagesRow(row: Int, forLocation:String!) {
    if let _ = unloadedImagesRows[forLocation] {

    } else {
        unloadedImagesRows[forLocation] = [Int]()
    }
    unloadedImagesRows[forLocation]!.append(row)
}

// guard let
private func addToUnloadedImagesRow(row: Int, forLocation:String!) {
    guard let _ = unloadedImagesRows[forLocation] else {
        unloadedImagesRows[forLocation] = [Int]()
        return addToUnloadedImagesRow(row, forLocation: forLocation)
    }

    unloadedImagesRows[forLocation]!.append(row)
}

// nil coalescing
private func addToUnloadedImagesRow(row: Int, forLocation:String!) {
    var b = unloadedImagesRows[forLocation] ?? [Int]()
    b.append(row)
    unloadedImagesRows[forLocation] = b
}

It looks like nil coalescing can only be used in one way because someone got the same answer I have for that one. Apparently he beat me to it while I was editing/working it out in a playground. Upvote! — Eric Mentele, Apr 12 '16 at 08:15

Rahul Katariya · Answer 3 · 2016-04-12T05:46:03.650

You can create a helper operator for nil checks and use it like below.

infix operator ?= { associativity left precedence 160 }

func ?=<T: Any>(inout left: T?, right: T) -> T {
    if let left = left {
        return left
    } else {
        left = right
        return left!
    }
}

Here you will be able to use it like unloadedImagesRows[forLocation] ?= [Int]() if empty

var unloadedImagesRows = [String:[Int]]()

private func addToUnloadedImagesRow(row: Int, forLocation:String!) {
    unloadedImagesRows[forLocation] ?= [Int]()
    unloadedImagesRows[forLocation]!.append(row)
}

addToUnloadedImagesRow(1, forLocation: "This is something")

print(unloadedImagesRows) // "["This is something": [1]]\n"

score 2 · Answer 4 · answered Apr 12 '16 at 05:50

If you want to avoid if or guard you could try the nil coalescing operator (??).

private func addToUnloadedImagesRow(row: Int, forLocation:String!) {
    var rowsForLocation = unloadedImagesRows[forLocation] ?? [Int]();
    rowsForLocation.append(row)
    unloadedImagesRows[forLocation] = rowsForLocation
}

Note: This might not be very efficient since you have to re-assign the array to the dictionary. I'm not sure if this will result in a full copy of the array.

score 1 · Answer 5 · edited May 23 '17 at 12:01

you can use if let or guard statement

  private func addToUnloadedImagesRow(row: Int, forLocation:String!) {
      if let a = unloadedImagesRows[forLocation] as? [Int] {
           //...success block
      }
  }

or use guard statement

  private func addToUnloadedImagesRow(row: Int, forLocation:String!) {
      guard let a = unloadedImagesRows[forLocation] else {
           return
      }
      //...
  }

for more info. check this link

score 1 · Answer 6 · answered Apr 12 '16 at 05:04

1

You can check it as follow.

private func addToUnloadedImagesRow(row: Int, forLocation:String!) {
    if let image = unloadedImagesRows[forLocation] {
                //it is not nil
    } else {
        //it is nil
    }

}

answered Apr 12 '16 at 05:04

Mahendra

8,448
3
33
56

Swift language: nil check, and if so instantiate new object

6 Answers6