Basic question (I think) about types of variables and User inputs

Hello. I am fairly new to SwiftUI, and I am currently making a timetable app, to help keep me organised, next academic year, so I still have a fair amount of time to complete it, before September.

I am making this app specifically for my college, and its timetabling. There are 6 periods, A-F, and every A period, you have the same lesson, every B period, etc. Parts of this project come from tutorials I have worked through, and real life examples I have looked at.

It is not at all elegant, or working, currently.

The view I am having a problem with is the so-called 'TimetableEditView'. I want a user to choose a subject from the list, for all of their periods, and then save them. These will then be used in the actual timetable.

The error I am getting, is at both lines 21 & 22.
Line 21 error: 'Cannot assign value of type 'Int' to type 'String''
Line 22 error: 'Cannot assign value of type 'String' to type 'Int''

Code Block 
import SwiftUI
	struct TimetableEditView: View {
    	var subjects = ["Free", "Maths", "English", "Geography"]
    	@State var selectedSubjectA = 0
			...
    	@State var retrievedSubjectA = ""
			...
    	var body: some View {
        	Form {
            	Section {
                	Picker(selection: $selectedSubjectA, label: Text("A Periods").font(.body)) {
                    	ForEach(0 ..< subjects.count) {
Text(self.subjects[$0]).font(.body)
                    }
                }
            }
            Button(action: {
				UserDefaults.standard.set(self.selectedSubjectA, forKey: "SelectedSubjectA")
                self.retrievedSubjectA = self.selectedSubjectA
                self.selectedSubjectA = ""
                UserDefaults.standard.set(self.selectedSubjectB, forKey: "SelectedSubjectB")
                ...
            }) {
                Text("Save")
                    .font(.headline)
                    .fontWeight(.bold)
                    .foregroundColor(.blue)
            }
            .onAppear {
                guard let retrievedSubjectAa = UserDefaults.standard.value(forKey: "SelectedSubjectA") else {return}
                self.retrievedSubjectA = retrievedSubjectAa as! String
            }
        }
        .navigationBarTitle("Select your subjects", displayMode: .inline)
    }
}

I understand that this is because:

Code Block  
selectedSubjectA = 0

for the purposes of choosing the name from the list of subjects, and because:

Code Block 
retrievedSubjectA = ""

which I believe means that it is a string.

I want to assign the users chosen subject to a variable, that I can then use in different view.

I understand that this is quite basic compared to most questions on here, but any and all help is appreciated.
Thanks.