Iterating an object array and changing values?

Question

Used this scenario as an example to be a bit more self explanatory. I have a struct which represents a character, and one of the structs attributes is another struct: Stats (where I used id to represent the name in a more simple way).

Besides that I have a view with a ForEach, where I iterate over some Characters and I need to be able to increase a specific stat.

The problem is: I'm trying to increase stat.points using a button, but I keep getting the message "Left side of mutating operator isn't mutable: 'product' is a 'let' constant".

struct Character: Identifiable {
   var id: String
   var name: String
   var stats: [Stats]
}

struct Stats: Identifiable {
   var id: String
   var points: Int
}


ForEach(characters.stats) { stat in
   HStack {
      Text("\(stat.id)")
      Button {
         stat.points += 1
      } label: {
         Text("Increase")
      }
   }
}

How could I make this work?

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– lorem ipsum, Commented Oct 29, 2022 at 23:19

workingdog support Ukraine · Accepted Answer · 2022-10-29 23:40:54Z

As you have found, structs in your stats: [Stats] does not allow changes especially in a ForEach where it is a let.

There are many ways to achieve what you want, depending on what you trying to do.

This example code shows the most basic way, using the array of stats: [Stats] directly:

struct ContentView: View {
    @State var characters = Character(id: "1",name: "name-1", stats: [Stats(id: "1", points: 1),Stats(id: "2", points: 1)])
    
    var body: some View {
        ForEach(characters.stats.indices, id: \.self) { index in
            HStack {
                Text("\(characters.stats[index].id)")
                Button {
                    characters.stats[index].points += 1
                } label: {
                    Text("Increase")
                }
                Text("\(characters.stats[index].points)")
            }
        }
    }
}

Here is another approach, using a function, to increase your stat points value:

struct ContentView: View {
    @State var characters = Character(id: "1", name: "name-1", stats: [Stats(id: "1", points: 1), Stats(id: "2", points: 1)])
    
    var body: some View {
        ForEach(characters.stats) { stat in
            HStack {
                Text("\(stat.id)")
                Button {
                    increase(stat: stat)
                } label: {
                    Text("Increase")
                }
                Text("\(stat.points)")
            }
        }
    }
    
    func increase(stat: Stats) {
        if let index = characters.stats.firstIndex(where: {$0.id == stat.id}) {
            characters.stats[index].points += 1
        }
    }
    
}

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