Is there a Swift one-liner to remove an element from an array if present and append it if not?

Question

This is a construct I come across quite a lot. Is there a nice way to one-line it in Swift?

I could just write an extension on Sequence for it, but I feel like there's a "obvious" higher-order-function / set theory technique that is eluding me.

if array.contains(element) {
    array.removeObject(object: element)
}
else {
    array.append(element)
}

I don't think the solution will even necessarily be nicer per se, it's just something I think about every time I have to write this.

the simple answer is not using an Array , try to use Set . Set doesn't accept duplicates so it won't be added if already exist. — SafoineMoncefAmine, Sep 10 '19 at 11:24
@PGDev, let's say yes. I think I could colour in a Venn diagram of what I want, but I couldn't figure out how to code it using the options available in Swift's Set. — James Webster, Sep 10 '19 at 11:27
@SafoineMoncefAmine, that's not quite what that code is doing. It's swapping the presence of the element in the array.. Removing it if it's present and adding it if not. — James Webster, Sep 10 '19 at 11:28
The same in one line: ```array.contains(element) ? array.removeObject(object: element) : array.append(element)``` — Starsky, Sep 10 '19 at 11:30
@Starsky, my colleague suggested that. She suffixed it with `//Technically one line ` — James Webster, Sep 10 '19 at 11:31
@Starsky hahaha i was just answering that. Guess I should not anymore. — Galo Torres Sevilla, Sep 10 '19 at 11:32
`if array.contains(element) { array.removeObject(object: element) } else { array.append(element) }` — Adrian, Sep 10 '19 at 11:34
Rather than writing the shortest code try to write the most efficient code And there is no equivalent to NSArray's `removeObject` in Swift. — vadian, Sep 10 '19 at 11:45
@vadian, rather than writing the "most efficient" code, profile your code occasionally and decide which bits are worth optimising. "Premature optimisation is the route of all evil" after all. — James Webster, Sep 10 '19 at 11:48
@JamesWebster I generally agree, but there's obvious stuff that you should avoid in almost any case. Your proposed "append if doesn't exist" API would be `O(n)`, and undoubtedly called as part of some loop that's at least `O(n)`. `O(n^2)` explodes pretty fast, at very common real life data sets. People have thousands of songs, contacts, 10s/100s of thousands of files, etc. — Alexander, Sep 10 '19 at 13:02
@JamesWebster I know this looks funny and easy, but you actually asked for a one-liner )))) — Starsky, Sep 10 '19 at 14:13

James Webster · Accepted Answer · 2019-09-10T11:40:09.480

9

I've found the part of Set Theory that was eluding me! The result I want is the Symmetric Difference of the two arrays and this is included in Swift's set:

var element = Set([1])
var set = Set([1, 2, 3])

set = set.symmetricDifference(element) //2, 3
set = set.symmetricDifference(element) //1, 2, 3

edited Sep 10 '19 at 11:40

answered Sep 10 '19 at 11:36

James Webster

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You don't need to convert the element to a set, you can just write `set = set.symmetricDifference([1])` – Daniel Mar 11 '21 at 16:24
[formSymmetricDifference](https://developer.apple.com/documentation/swift/set/formsymmetricdifference(_:)-22p0m) – Aug 27 '22 at 17:58

score 1 · Answer 2 · answered Sep 10 '19 at 11:32

1

You can try using Set instead,

var set: Set<Int> = [1, 2, 4]
if !set.insert(4).inserted {
    set.remove(4)
}

answered Sep 10 '19 at 11:32

PGDev

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score -1 · Answer 3 · answered Aug 27 '22 at 17:28

Based on the great answer by. James Webster. We can enhance for reusability.

extension Set {
    /// Inserts given element if it doesn't exist in the set or removes it.
    ///
    mutating func withSymmetricDifference(_ element: Element) {
        self = symmetricDifference([element])
    }
}

Usage

var set = Set([1, 2, 3])
set.withSymmetricDifference(1) // [1, 2, 3]
set.withSymmetricDifference(1) // [2, 3]

Is there a Swift one-liner to remove an element from an array if present and append it if not?

3 Answers3