Updating code to latest Swift 4 syntax results in a "Thread 1: Fatal error: Index out of range" error

Question

I'm currently in the process of updating some apps to the latest Swift 4 syntax and have run into a problem, and I'm sure I'm just missing something really obvious.

Initially my code was as follows:

    let indices : [Int] = [0,1,2,3]
    //let newSequence = shuffle(indices)
    let newSequence = indices.shuffle()
    var i : Int = 0
    for(i = 0; i < newSequence.count; i++)
    {
        let index = newSequence[i]
        if(index == 0)
        {
            // we need to store the correct answer index
            currentCorrectAnswerIndex =  i

        }

Initially I had three error with the above code, namely:

1. Value of type '[Int]' has no member 'shuffle'
2. C-style for statement has been removed in Swift 3
3. Unary operator '++' cannot be applied to an operand of type '@lvalue Int'

To address these I changed the code so that it's now as follows:

    let indices : [Int] = [0,1,2,3]
    //let newSequence = shuffle(indices)
    let newSequence = indices.shuffle()
    var i : Int = 0
    while i < newSequence.count {i += 1

        let index = newSequence[i]
        if(index == 0)
        {
            // we need to store the correct answer index
            currentCorrectAnswerIndex =  i

        }

After changing the code and running a Product > Clean within Xcode, I no longer have any errors. However, when I use the app in the iOS Simulator it hangs at a certain point and checking within Xcode gives me the Thread 1: Fatal error: Index out of range error on the following line of code:

let index = newSequence[I]

I've read through other questions about the same error in Swift (see: 1, 2, 3 and 4) but these don't seem to apply in my scenario.

I know I'm missing something really obvious here, but at present it's just escaping me.

Any thoughts?

Answer 1

Change position of (incrementing value) i as shown in below code. (Value of i is increment before it is used in loop operations, hence during final/last iteration of loop, value of i becomes larger than array indices)

let indices : [Int] = [0,1,2,3]
//let newSequence = shuffle(indices)
let newSequence = indices.shuffle()
var i : Int = 0
while i < newSequence.count {

    let index = newSequence[i]
    if(index == 0)
    {
        // we need to store the correct answer index
        currentCorrectAnswerIndex =  i

    }
    i += 1 // Update (increment) value for i at last
}

Update

(As suggested by MartinR) This would be better than you did:

let indices : [Int] = [0,1,2,3]
let newSequence = indices.shuffle()

for (i, newSeqIndex) in newSequence.enumerated() {
    if (newSeqIndex == 0) {
        currentCorrectAnswerIndex =  i
    }

}

Answer 2

The reason for crash is that you increment i, perform some operations with it, and only then check if it is not out of range. To get rid of this error just move i += 1 to the end of your closure.

But why not use fast enumeration? You're still trying to do a loop the old way, but with new syntax. Instead of doing it C way

var i : Int = 0
while i < newSequence.count {
    let index = newSequence[i]
    if(index == 0)
    {
        // we need to store the correct answer index
        currentCorrectAnswerIndex =  i

    }

    i += 1
}

do it the correct Swift way

for (index, value) in newSequence.enumerated() {
    if value == 0 {
        currentCorrectAnswerIndex =  index
    }
}

Answer 3

The Swift 3+ equivalent of the code is

let indices : [Int] = [0,1,2,3]
let newSequence = indices.shuffle() // this seems to be a custom function

for i in 0..<newSequence.count
{
    let index = newSequence[i]
    if index == 0 { // no parentheses in Swift
        // we need to store the correct answer index
        currentCorrectAnswerIndex = i
    }

}

Answer 4

You put a i += 1 in there that isn't in the original code.