SwiftUI - Index out of range

Question

I start by adding some integers to an array in onAppear for my outermost stack. But when I try to display the contents of the array using ForEach, I get an index out of range error.

struct MyView: View {
    @State private var answers = [Int]()
    var body: some View {
        VStack {
            ForEach(0..<4) { number in 
            Text("\(answers[number])")
            }
        }
        .onAppear {
           for _ in (0..<4) {
               answerArray.append(Int.random(in: 1...10))
           }
        }

That stopped it from crashing, but only because it thinks there are no elements in the array. — squarehippo10
– squarehippo10, Commented May 25, 2021 at 20:08
It's right, there are no elements in the array, at least at first... — aheze
– aheze, Commented May 25, 2021 at 20:22

vadian · Accepted Answer · 2021-05-25 21:02:48Z

1

Never retrieve items by hard-coded indices in a ForEach expression.

Do count the array, the loop is skipped (safely) if the array is empty.

ForEach(0..<answers.count) { number in

Or - still simpler – enumerate the items rather than the indices

ForEach(answers, id: \.self) { answer in 
    Text("\(answer)")
}

answered May 25, 2021 at 21:02

vadian

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Comments

aheze · Accepted Answer · 2021-05-25 20:22:01Z

onAppear is called after MyView loads for the first time, and at that moment, answers is still empty. That's why your program crashes at ForEach(0..<4), because answers's doesn't have 4 elements yet.

ForEach(0..<4) { number in 
    Text("\(answers[number])") /// answers is still empty.
}

Instead, you should look over answers.indices, so that answers[number] is guaranteed to exist. Make sure to also provide an id (id: \.self) to satisfy ForEach's Identifiable requirement.

struct MyView: View {
    @State private var answers = [Int]()
    var body: some View {
        VStack {
            ForEach(answers.indices, id: \.self) { number in
                Text("\(answers[number])")
            }
        }
        .onAppear {
            for _ in (0..<4) {
                answers.append(Int.random(in: 1...10)) /// you probably meant `answers.append`, not `answerArray.append`
            }
        }
    }
}

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