Question posted in Xcode
Whether you're new to Xcode or an experienced developer, our archive has everything you need to know about this integrated development environment (IDE). From basic functionalities to advanced features, our archive covers a wide range of Xcode-related questions and answers. Browse our archive now and find solutions to your Xcode questions, and take your app development skills to the next level

Xcode – SwiftUI Can't Pass Variable to Higher Order Function Map

JohnSF
April 5, 2022
166 views
0 votes
2 Answers

I have an app with an array of models that contain number fields. I’d like to be
able to sum any given data field. I can do so with map by using the data field
name ($0.cups for example). However I would like to programmatically send in a
variable for "cups" and nothing I have tried works.

Here’s some example code. I’d like to pass the field name that I want to total into
the function sumOnAttribute

struct ContentView: View {

    @State private var answer: String = ""

    @State private var startingModels = [
        StartingModel(name: "abe", age: 1, bags: 2, cups: 3),
        StartingModel(name: "abe", age: 11, bags: 12, cups: 13),
        StartingModel(name: "abe", age: 21, bags: 22, cups: 23),
        StartingModel(name: "donna", age: 31, bags: 32, cups: 33),
        StartingModel(name: "elsie", age: 41, bags: 42, cups: 43),
        StartingModel(name: "farah", age: 51, bags: 52, cups: 53)
    ]//some data

    var body: some View {
    
        VStack(spacing: 10) {
            Text("Do Some Math")
                .font(.title)
            Text(answer)
                .font(.headline)
            Button {
                answer = String(sumOnAttribute(filterField: "abe", sumField: "cups"))
            } label: {
                Text("Make it So")
            }
            .font(.system(size: 20))
        }//v
    }//body

    func sumOnAttribute(filterField: String, sumField: String) -> Double {
    
        let filteredAttribute = startingModels.filter({ $0.name == filterField })
    
        //$0.sumField does not work, $0.value(sumField) does not work
        let nums = filteredAttribute.map({ $0.cups })

        let total = nums.reduce(0, +)
        return total
    }
}//struct

struct ContentView_Previews: PreviewProvider {
    static var previews: some View {
        ContentView()
    }
}

struct StartingModel: Identifiable {
    let id = UUID()
    let name: String
    let age: Double
    let bags: Double
    let cups: Double
}//model

Seems like this should be simple, but I have not found any answer in a lot of searches.
Any guidance would be appreciated. Xcode 13.3, iOS 15.4

Answers

One option is a KeyPath — it’s not a String like you’re using, but rather a compile-time-safe path to the property you’re looking for. An implementation might look like this:

struct ContentView: View {

    @State private var answer: String = ""

    @State private var startingModels = [
        StartingModel(name: "abe", age: 1, bags: 2, cups: 3),
        StartingModel(name: "abe", age: 11, bags: 12, cups: 13),
        StartingModel(name: "abe", age: 21, bags: 22, cups: 23),
        StartingModel(name: "donna", age: 31, bags: 32, cups: 33),
        StartingModel(name: "elsie", age: 41, bags: 42, cups: 43),
        StartingModel(name: "farah", age: 51, bags: 52, cups: 53)
    ]//some data

    var body: some View {
    
        VStack(spacing: 10) {
            Text("Do Some Math")
                .font(.title)
            Text(answer)
                .font(.headline)
            Button {
                answer = String(sumOnAttribute(filterField: "abe", sumField: .cups))
            } label: {
                Text("Make it So")
            }
            .font(.system(size: 20))
        }//v
    }//body

    func sumOnAttribute(filterField: String, sumField: KeyPath<StartingModel,Double>) -> Double {
        let filteredAttribute = startingModels.filter({ $0.name == filterField })
        return filteredAttribute.map({ $0[keyPath:sumField] }).reduce(0, +)
    }
}//struct

If you really wanted to use a String, you could consider using @dynamicMemberLookup: https://www.hackingwithswift.com/articles/55/how-to-use-dynamic-member-lookup-in-swift

not clear what you want, maybe you could try something like this, "…to pass the field name that I want to total into the function sumOnAttribute…":

struct ContentView: View {
    @State private var namefield: String = ""  // <-- here
    @State private var sumfield: String = ""   // <-- here
    @State private var answer: String = ""
    
    @State private var startingModels = [
        StartingModel(name: "abe", age: 1, bags: 2, cups: 3),
        StartingModel(name: "abe", age: 11, bags: 12, cups: 13),
        StartingModel(name: "abe", age: 21, bags: 22, cups: 23),
        StartingModel(name: "donna", age: 31, bags: 32, cups: 33),
        StartingModel(name: "elsie", age: 41, bags: 42, cups: 43),
        StartingModel(name: "farah", age: 51, bags: 52, cups: 53)
    ]
    
    var body: some View {
        
        VStack(spacing: 10) {
            TextField("name field", text: $namefield).border(.red) // <-- here
            TextField("sum field", text: $sumfield).border(.red)   // <-- here
            Text("Do Some Math")
                .font(.title)
            Text(answer)
                .font(.headline)
            Button {
                answer = String(sumOnAttribute(filterField: namefield, sumField: sumfield)) // <-- here
            } label: {
                Text("Make it So")
            }
            .font(.system(size: 20))
        }
    }
    
    func sumOnAttribute(filterField: String, sumField: String) -> Double {
        let filteredAttribute = startingModels.filter{ $0.name == filterField }
        // -- here
        var nums: [Double] = []
        switch sumField {
          case "cups": nums = filteredAttribute.map{ $0.cups }
          case "bags": nums = filteredAttribute.map{ $0.bags }
          case "age": nums = filteredAttribute.map{ $0.age }
          default: break
        }
        return nums.reduce(0, +)
    }
}

Please signup or login to give your own answer.

Click here to cancel reply.