Fatal error: Index out of range: file Swift/ContiguousArrayBuffer.swift when removing element from array

Question

I have a form to select some users and assign them a int value.

The model:

class ReadingTime: Identifiable, Hashable {
    var id: Int
    @State var user: User
    @Published var value: Int

    func hash(into hasher: inout Hasher) {
        hasher.combine(id)
    }
    
    static func == (lhs: ReadingTime, rhs: ReadingTime) -> Bool {
        lhs.id == rhs.id
    }
    
    init(id: Int, user: User, value: Int) {
        self.id = id
        self._user = State(wrappedValue: user)
        self.value = value
    }
}

The view:

@Binding var times: [ReadingTime]
@State var newUser: User?

func didSelect(_ user: User?) {
    if let user = user {
        readingTime.append(ReadingTime(id: readingTime.nextMaxId,
                                       user: user,
                                       value: 0))
    }
}

// In the body:
VStack(alignment: .leading, spacing: 0) {
    HStack {
        Picker("Select a user", selection: $newUser.onChange(didSelect)) {
                    ForEach(users) {
                        Text($0.name).tag(Optional($0))
                    }
                }
                .id(users)
            }
            VStack(spacing: 8) {
                ForEach(0..<times.count, id: \.self) { i in
                    HStack(spacing: 0) {                            
                        Text(times[i].user.name)
                        TextField("ms", value: $times[i].value, formatter: NumberFormatter())
                        Button(action: {
                            NSApp.keyWindow?.makeFirstResponder(nil)
                            if let index = times.firstIndex(where: { $0 == times[i] }) {
                                times.remove(at: index)
                            }
                            newUser = nil
                        }, label: {
                            Text("REMOVE")
                        })
                    }
                }
            }
        }
    }
}

It looks like this:

However, when deleting an entry in the list, I get this error:

Fatal error: Index out of range: file Swift/ContiguousArrayBuffer.swift

What's going on here?

Does this answer your question stackoverflow.com/a/61431459/12299030? Also pay attention at stackoverflow.com/a/58911168/12299030 description about ForEach? — Asperi
– Asperi, Commented Nov 19, 2020 at 11:39
@Asperi Thank you, I followed what you described in the 2nd link and it works perfectly fine — Another Dude
– Another Dude, Commented Nov 19, 2020 at 15:32

vadian · Accepted Answer · 2020-11-19 11:39:46Z

7

Modifying the number of items of an array while being enumerated is an evil trap.

0..<times.count creates a temporary static range.

If you remove the first item in the array index 1 becomes 0, index 2 becomes 1 and so on.

Unfortunately there is no index times.count-1 anymore and you get the Index out of range crash when the loop reaches the last index.

You can avoid the crash if you enumerate the array reversed.

answered Nov 19, 2020 at 11:39

vadian

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3 Comments

Another Dude Over a year ago

Thank you for your answer. I perfectly understand the issue, but can you explain how to "enumerate the array reversed"?

vadian Over a year ago

Instead of starting at index 0 start at the last index and count backwards.

mmk Over a year ago

@vadian in 2023 also facing this issue. I think using reversed() worked for me if I delete the last element from the array. But If I delete the first element, crash happens.

Tom Hakemann · Accepted Answer · 2022-01-24 14:40:03Z

1

Since there is no accepted answer:

Don't use the index of your array to get the specific object inside your ForEach. Like you did before, you're able to access your objects without using count. Using count causes your Index out of range error.

Do it like this:

ForEach(times) { time in
   // ...
   Text(time.user.name)
   // ...
}

answered Jan 24, 2022 at 14:40

Tom Hakemann

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