how do I implement func calculation inside struc?

Question

I am trying to create an apple watch app that calculates Karvonen value.

When I inserted func calculation into struct, I kept receiving the Swift Compiler Error that says:

[Type "FourthView"does not conform to protocol "View"]

I understood that I should not be calculating inside the func, but I don't know how to solve this problem.

struct FourthView: View{

    @Binding var fourthScreenShown:Bool
    //@State var RHR:Int
    //@State var weight:Int
    @State var Age:Int
    @State var ExerciseIT = 0.3
    @State var ConstantNumber = 220
    @State var RHR = 68
    
    func karvonen(cn: Int, rhr: Int, age: Int, ei:Double) -> Double {
        return Double((cn-age-rhr)) * ei + Double(rhr)
        let output = karvonen(cn: ConstantNumber, rhr: RHR, age: Age, ei: ExerciseIT)
        
        var body: some View {
            VStack{
                Text("Your Kar is")
                    .font(.system(size: 14))
                Text("\(output)")
                    .font(.system(size: 40))
            
            }
        }
    }
}

Answer 1

The var body has to exist at the top level of the struct for it to conform to View. Right now, you have var body inside your func.

struct FourthView: View{
    
    @Binding var fourthScreenShown:Bool
    @State var Age:Int
    @State var ExerciseIT = 0.3
    @State var ConstantNumber = 220
    @State var RHR = 68
    
    func karvonen(cn: Int, rhr: Int, age: Int, ei:Double) -> Double {
        return Double((cn-age-rhr)) * ei + Double(rhr)
    }
    
    var body: some View {
        VStack{
            Text("Your Kar is")
                .font(.system(size: 14))
            Text("\(karvonen(cn: ConstantNumber, rhr: RHR, age: Age, ei: ExerciseIT))")
                .font(.system(size: 40))
        }
    }
    
}

Another equivalent version (maybe useful if you have to use the result in multiple places):

var body: some View {
    let output = karvonen(cn: ConstantNumber, rhr: RHR, age: Age, ei: ExerciseIT)
    VStack{
        Text("Your Kar is")
            .font(.system(size: 14))
        Text("\(output)")
            .font(.system(size: 40))
    }
}

---

Unrelated to your issue, but a hint for the future: in Swift, generally, variables/parameters are camel cased. So, your Age, ExerciseIT, etc would be age, exerciseIT, etc.

Also, unless you're changing the values, you don't need @State in front of all of the parameters. ConstantNumber, for example (if the name is accurate), probably doesn't need to be @State.