Reduce array to set in Swift - arrays

Question

I am trying to reduce an array of objects to a set in Swift and this is my code:
objects.reduce(Set<String>()) { $0.insert($1.URL) }
However, I get an error:
Type of expression is ambiguous without more context.
I do not understand what the problem is, since the type of URL is definitely String. Any ideas?

Answer 1

You don't have to reduce an array to get it into a set; just create the set with an array: let objectSet = Set(objects.map { $0.URL }).

Answer 2

With Swift 5.1, you can use one of the three following examples in order to solve your problem.
#1. Using Array's map(_:) method and Set's init(_:) initializer
In the simplest case, you can map you initial array to an array of urls (String) then create a set from that array. The Playground below code shows how to do it:
struct MyObject {
let url: String
}
let objectArray = [
MyObject(url: "mozilla.org"),
MyObject(url: "gnu.org"),
MyObject(url: "git-scm.com")
]
let urlArray = objectArray.map({ $0.url })
let urlSet = Set(urlArray)
dump(urlSet)
// ▿ 3 members
// - "git-scm.com"
// - "mozilla.org"
// - "gnu.org"
#2. Using Array's reduce(into:_:) method
struct MyObject {
let url: String
}
let objectArray = [
MyObject(url: "mozilla.org"),
MyObject(url: "gnu.org"),
MyObject(url: "git-scm.com")
]
let urlSet = objectArray.reduce(into: Set<String>(), { (urls, object) in
urls.insert(object.url)
})
dump(urlSet)
// ▿ 3 members
// - "git-scm.com"
// - "mozilla.org"
// - "gnu.org"
As an alternative, you can use Array's reduce(_:_:) method:
struct MyObject {
let url: String
}
let objectArray = [
MyObject(url: "mozilla.org"),
MyObject(url: "gnu.org"),
MyObject(url: "git-scm.com")
]
let urlSet = objectArray.reduce(Set<String>(), { (partialSet, object) in
var urls = partialSet
urls.insert(object.url)
return urls
})
dump(urlSet)
// ▿ 3 members
// - "git-scm.com"
// - "mozilla.org"
// - "gnu.org"
#3. Using an Array extension
If necessary, you can create a mapToSet method for Array that takes a transform closure parameter and returns a Set. The Playground below code shows how to use it:
extension Array {
func mapToSet<T: Hashable>(_ transform: (Element) -> T) -> Set<T> {
var result = Set<T>()
for item in self {
result.insert(transform(item))
}
return result
}
}
struct MyObject {
let url: String
}
let objectArray = [
MyObject(url: "mozilla.org"),
MyObject(url: "gnu.org"),
MyObject(url: "git-scm.com")
]
let urlSet = objectArray.mapToSet({ $0.url })
dump(urlSet)
// ▿ 3 members
// - "git-scm.com"
// - "mozilla.org"
// - "gnu.org"

Answer 3

reduce() method expects a closure that returns a combined value, while insert() methods of Set value does not return anything but instead it inserts a new element into the existing set.
In order to make it work you would need to do something like:
objects.reduce(Set<String>()) {
$0.union(CollectionOfOne($1.URL))
}
But the above is a bit of an unnecessary complication. If you have a big array, that would mean quite a number of ever-growing sets to be created while Swift goes over all the elements from objects. Better follow the advice from #NRitH and use map() as that would make a resulting set in one go.

Answer 4

Swift 1.0-2.x ONLY:
If URL on your object is a strongly-typed String, you can create a new Set<String> object and use unionInPlace on the set with the mapped array:
var mySet = Set<String>()
mySet.unionInPlace(objects.map { $0.URL as String })