Count frequency of an element in a Swift array of elements-CodePudding

I want to count the frequency of false m's in an array of elements:

    import UIKit

struct aW: Hashable, Codable {
    var day: Int
    var w: Double
    var m: Bool
}

let array = [
    aW(day: 0, w: 32.62, m: true),
    aW(day: 1, w: 32.40, m: true),
    aW(day: 2, w: 32.16, m: true),
    aW(day: 3, w: 32.01, m: false),
    aW(day: 4, w: 31.81, m: true),
    aW(day: 5, w: 31.61, m: false),
    aW(day: 6, w: 31.41, m: false),
    aW(day: 7, w: 31.20, m: true),
    aW(day: 8, w: 31.01, m: true),
    aW(day: 9, w: 30.83, m: true)]

Also I want to select the first element with a false occurence.

I found a lot of examples for frequencies for arrays of strings or int's. But how do you do this with an array of such elements?

CodePudding user response：

You can simply use reduce. If the element's m property is true return the same partial result otherwise return the partial result plus one:

let count = array.reduce(0) { $1.m ? $0 : $0   1 }
count // 3

To get the first element that is false you can use first where predicate:

if let element = array.first(where: { !$0.m }) {
    print(element)  // AW(day: 3, w: 32.01, m: false)
}

Note that it is Swift naming convention to name your structs starting with an uppercase letter. You should also use better descriptive naming.

CodePudding user response：

Map the items in an array of tuples with the property you want to count as key:

let mappedItems = array.map { ($0.m, 1) }

Then use this Dictionary initializer to add the values if it finds duplicate keys.

let counts = Dictionary(mappedItems, uniquingKeysWith:  )

this prints:

[true: 7, false: 3]

CodePudding user response：

Just in case if you’re interested. Below is the playground combine version for @Leo Dabus answer.

var subscriptions = Set<AnyCancellable>()

struct aW: Hashable, Codable {
    var day: Int
    var w: Double
    var m: Bool
}

let array = [
     aW(day: 0, w: 32.62, m: true),
     aW(day: 1, w: 32.40, m: true),
     aW(day: 2, w: 32.16, m: true),
     aW(day: 3, w: 32.01, m: false),
     aW(day: 4, w: 31.81, m: true),
     aW(day: 5, w: 31.61, m: false),
     aW(day: 6, w: 31.41, m: false),
     aW(day: 7, w: 31.20, m: true),
     aW(day: 8, w: 31.01, m: true),
     aW(day: 9, w: 30.83, m: true)].publisher
   
      array
     .reduce(0) {$1.m ? $0 : $0   1}
     .sink(receiveValue: { print($0) }) // 3
     .store(in: &subscriptions)