Can somebody give a snippet of "append if not exists" method in swift array?

Question

Because I use this routine a lot, can somebody create an extension method of Swift array which will detect whether if the data that is going to be appended already exists, then it's not appended? I know that it's only a matter of few code like this:

var arr = [Int]()
for element in inputArr {
    if !arr.contains(element) { arr.append(element); }
}

Becomes:

var arr = [Int]()
for element in inputArr { arr.appendUnique(element); }

Or:

var arr = [String]()
for element in inputArr {
    if !arr.contains(element) { arr.append(element); }
}

Becomes:

var arr = [String]()
for element in inputArr { arr.appendUnique(element); }

Same method for different element types. Frankly, from this simple code, I also want to learn on how to extend the Collection with variable types. It fascinates me how Array's methods can have different parameter types whenever the object was initialized with different parameter types. Array and Dictionary are two things that I still don't get how to extend them properly. Thanks.

why not use Set instead?

koropok
– koropok

2017-10-02 03:16:22 +00:00
Commented Oct 2, 2017 at 3:16 — koropok
– koropok, Commented Oct 2, 2017 at 3:16
A set has no order

Leo Dabus
– Leo Dabus

2017-10-02 03:17:08 +00:00
Commented Oct 2, 2017 at 3:17 — Leo Dabus
– Leo Dabus, Commented Oct 2, 2017 at 3:17
How about OrderedSet then?

malhal
– malhal

2025-05-09 14:36:43 +00:00
Commented May 9 at 14:36 — malhal
– malhal, Commented May 9 at 14:36

Leo Dabus · Accepted Answer · 2019-03-15 21:34:06Z

26

You can extend RangeReplaceableCollection, constrain its elements to Equatable and declare your method as mutating. If you want to return Bool in case the appends succeeds you can also make the result discardable. Your extension should look like this:

extension RangeReplaceableCollection where Element: Equatable {
    @discardableResult
    mutating func appendIfNotContains(_ element: Element) -> (appended: Bool, memberAfterAppend: Element) {
        if let index = firstIndex(of: element) {
            return (false, self[index])
        } else {
            append(element)
            return (true, element)
        }
    }
}

edited Mar 15, 2019 at 21:34

answered Oct 2, 2017 at 3:29

Leo Dabus

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2 Comments

Alexander Over a year ago

If Element is Equatable, you can just use !contains(element)

Chen Li Yong Over a year ago

Oh wow, @discardableResult, Equatable, Element, I'm learning so much things in just one post! I've been thinking about how to make a result discardable so I don't need to use _ = ... but didn't know that such thing exists so I never look it up. Thanks!!!

Rand · Accepted Answer · 2019-03-15 15:37:38Z

2

I needed to prepend a unique element (removing it first if it already exists).

extension RangeReplaceableCollection where Element: Equatable
{
    mutating func prependUnique(_ element: Element) {
        if let index = firstIndex(of: element) {
            remove(at: index)
        }
        insert(element, at: startIndex)
    }
}

answered Mar 15, 2019 at 15:37

Rand

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2 Comments

Chen Li Yong Over a year ago

The difference between this approach and the accepted answer approach is that the accepted answer approach preserves the existing object index, while this approach uses the new object index instead. But my question is specifically about appending data. In my case, it's better if we preserve the existing object index instead. But thanks for your input.

Medhi Over a year ago

Useful to track order of seen posts! Thanks

Nitin · Accepted Answer · 2024-07-04 11:25:52Z

1

    var arr = [String]()
    if !arr.contains(element) {
          arr.append(element)
    }

or 
extension Array where Element: Equatable {
    mutating func appendIfNotExists(_ element: Element) {
        if !self.contains(element) {
            self.append(element)
        }
    }
}

// Usage example
var myArray = [1, 2, 3]
myArray.appendIfNotExists(2) // Will not append since 2 already exists
myArray.appendIfNotExists(4) // Will append since 4 does not exist

print(myArray) // Output: [1, 2, 3, 4]

edited Jul 4, 2024 at 11:25

answered May 21, 2024 at 10:20

Nitin

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1 Comment

Jeremy Caney Over a year ago

Thank you for your interest in contributing to the Stack Overflow community. This question already has a few answers—including one that has been extensively validated by the community. Are you certain your approach hasn’t been given previously? If so, it would be useful to explain how your approach is different, under what circumstances your approach might be preferred, and/or why you think the previous answers aren’t sufficient. Can you kindly edit your answer to offer an explanation?

Asad Jamil · Accepted Answer · 2019-07-17 14:05:00Z

0

In my case I was filtering results and appending on clicking search button with api response but appending uniquely slow down the process as it has to check every index for unique, I basically made my local array empty or simply arr.removeAll().

answered Jul 17, 2019 at 14:05

Asad Jamil

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