Find first element matching condition in Swift arr

问题:

I would like to find the first EKSource of type EKSourceType.Local with a "single"-line expression in Swift. Here is what I currently have:

let eventSourceForLocal = 
    eventStore.sources[eventStore.sources.map({ $0.sourceType })
        .indexOf(EKSourceType.Local)!]

Is there a better way of doing this (such as without mapping and/or with a generic version of find)?

回答1:

There's a version of indexOf that takes a predicate closure - use it to find the index of the first local source (if it exists), and then use that index on eventStore.sources:

if let index = eventStore.sources.indexOf({ $0.sourceType == .Local }) {
    let eventSourceForLocal = eventStore.sources[index]
}

Alternately, you could add a generic find method via an extension on SequenceType:

extension SequenceType {
    func find(@noescape predicate: (Self.Generator.Element) throws -> Bool) rethrows -> Self.Generator.Element? {
        for element in self {
            if try predicate(element) {
                return element
            }
        }
        return nil
    }
}

let eventSourceForLocal = eventStore.sources.find({ $0.sourceType == .Local })

(Why isn't this there already?)

回答2:

Alternatively in Swift3 you could use:

let local = eventStore.sources.first(where: {$0.sourceType == .Local}) 

回答3:

I don't understand why you're using map at all. Why not use filter? You will then end up with all the local sources, but in actual fact there will probably be only one, or none, and you can readily find out by asking for the first one (it will be nil if there isn't one):

let local = eventStore.sources.filter{$0.sourceType == .Local}.first

回答4:

Swift 4 solution that also handles the situation when there are no elements in your array that match your condition:

if let firstMatch = yourArray.first{$0.id == lookupId} {
  print("found it: \(firstMatch)")
} else {
  print("nothing found :(")
}

回答5:

Let's try something more functional:

let arr = [0,1,2,3]
let result = arr.lazy.map{print("