Swift - trying to use a shuffle function and get fatal error: swapping a location with itself is not supported

Question

I'm trying to use this function I found here for shuffling an array. Can anyone please tell me what I'm doing wrong here. I'm getting "fatal error: swapping a location with itself is not supported"

var sourceDays = [1,2,3,4,5,6,7]
var yourDays = [1,2,3,4,5,6,7]

func shuffle<C: MutableCollection>( list: C) -> C where C.Index == Int {
    var list = list
    let count = sourceDays.count
    for i in 0..<(count - 1) {
        let j = Int(arc4random_uniform(UInt32(count - i))) + i
        swap(&list[i], &list[j])
    }
    return list
}

@IBAction func go(_ sender: Any) {

    yourDays = shuffle(list: sourceDays)
    print(yourDays)

}

Thanks for any help.

edit: This was marked a duplicate to a question already asked but that one appears to be old and for swift 2, and doesn't work for me....thanks though.

I get "Declaration is only valid at file scope" with that code - looks like that link is for Swift 2, is that why? Thank you very much — Tommy Draughn, Jul 26 '17 at 00:35
I did try it and it did work - thank you very much. I marked it as the answer and upvoted and commented... Thanks again — Tommy Draughn, Jul 26 '17 at 01:30

score 1 · Accepted Answer · answered Jul 26 '17 at 00:07

1

From the error description, you should change your shuffle call as:

func shuffle<C: MutableCollection>( list: C) -> C where C.Index == Int {
    var list = list
    let count = sourceDays.count
    for i in 0..<(count - 1) {
        let j = Int(arc4random_uniform(UInt32(count - i))) + i
        if i != j {
            swap(&list[i], &list[j])
        }
    }
    return list
}

You are trying to shuffle an index with itself and in this case i and j both had same value. Hence the error.

answered Jul 26 '17 at 00:07

adev

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That worked - thank you very much and thanks for the explanation - I get it. – Tommy Draughn Jul 26 '17 at 01:10
Good to hear that. – adev Jul 26 '17 at 01:29

Swift - trying to use a shuffle function and get fatal error: swapping a location with itself is not supported

1 Answers1