Swift Collection extension: pick every other item - swift

Question

I have written the following extension for Collection in Swift that returns a new Collection of every other element from a starting index.
extension Collection {
func everyOtherElement(from theIndex: Int = 0) -> [Element] {
if theIndex >= self.count { return self as! [Element] }
let start = self.index(startIndex, offsetBy: theIndex)
let end = self.endIndex
var everyOther = [Element]()
var iter = start
while iter != end {
everyOther.append(self[iter])
let next = index(after: iter)
if next == end { break }
iter = index(after: next)
}
return everyOther
}
}
There are probably ways to improve the code but my issue is when the Collection is a Dictionary. The extension works but returns an Array of Tuples [(key: key, value: value)]
I would like the extension to return a Dictionary.
I have tried Dictionary(uniqueKeysWithValuesMethod) which works fine once applied to the return of the everyOtherElement method but I can't seem to find a way to make it so directly.
var myDictionary = ["EN" : "Cheers", "SV" : "Skåll", "ES" : "Salud" ].everyOtherElement()
// returns [(key: "EN", value: "Cheers"),(key: "ES", value: "Salud")]
Dictionary(uniqueKeysWithValues: myDictionary.map { ($0.key, $0.value)})
// returns ["EN" : "Cheers", "ES" : "Salud" ]
Thanks for your help!

Answer 1

As other have mentioned a Dictionary is an unordered collection. If you would like to return a Dictionary you can simply extend dictionary and filter every other element. Note that there is no guarantee that the dictionary will keep the order that the key value pairs were enterer. That being said you can accomplish what you want as follow:
extension Dictionary {
var everyOtherElements: Dictionary {
var bool = true
return filter { _ in
defer { bool = !bool }
return bool
}
}
}
let dict = ["EN" : "Cheers", "SV" : "Skåll", "ES" : "Salud" ]
// returns [(key: "EN",
let everyOtherElements = dict.everyOtherElements // ["ES": "Salud", "SV": "Skåll"]
Regarding your comment
I understand from your reply that I can't extend all collection with
the same code, I need to break it down per type?
You don't need to extend every single element type, you can for instance extend RangeReplaceableCollection protocol and return Self which will englobe Strings as well:
extension RangeReplaceableCollection {
mutating func removeEvenIndexElements() {
var bool = true
removeAll { _ in
defer { bool = !bool }
return bool
}
}
mutating func removeOddIndexElements() {
var bool = false
removeAll { _ in
defer { bool = !bool }
return bool
}
}
func evenIndexElements() -> Self {
var bool = true
return filter { _ in
defer { bool = !bool }
return bool
}
}
func oddIndexElements() -> Self {
var bool = false
return filter { _ in
defer { bool = !bool }
return bool
}
}
}
var alphabet = "abcdefghijklmnopqrstuvwxyz"
alphabet.removeEvenIndexElements()
alphabet // "bdfhjlnprtvxz"
var arr = [1,2,3,4,5,6,7,8,9,0]
arr.removeOddIndexElements()
arr // [1, 3, 5, 7, 9]