Reduce array to set in Swift

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.

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#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" 

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#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" 

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#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"