Using firstIndex(of: Element) in Swift

Question

I have the following code in Swift 4, on Xcode 10

        var newCard = deck.randomElement()!

        deck.remove(at: deck.firstIndex(of: newCard)!)
        dealtCards.append(newCard)

I'm trying to take a random element from deck: [SetCard] and place it into dealtCards: [SetCard]

However, Xcode gives me the following error:

I've been looking through the documentation, but as far as I can tell, func firstIndex(of element: Element) -> Int? is a method that exists in Array, so I don't understand why Xcode wants me to change 'of' to 'where', when the function has the exact behaviour I want.

What am I missing here?

See developer.apple.com/documentation/swift/array/…: “Available when Element conforms to Equatable.” — Martin R
– Martin R, Commented Dec 30, 2018 at 18:02

Martin R · Accepted Answer · 2018-12-30 19:19:02Z

3

The problem has been already explained in the other answers. Just for the sake of completeness: If you choose a random index instead of a random element in the array then the problem is avoided and the code simplifies to

if let randomIndex = deck.indices.randomElement() {
    let newCard = deck.remove(at: randomIndex)
    dealtCards.append(newCard)
} else {
    // Deck is empty.
}

edited Dec 30, 2018 at 19:19

answered Dec 30, 2018 at 18:24

Martin R

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1 Comment

konsolas Over a year ago

Good to know - I suppose it also has better efficiency as well since I don't have to search through the array again to find the index of element.

Bappaditya · Accepted Answer · 2018-12-30 18:07:48Z

2

Your deck array elements should conform to Equatable protocol,

For example as String conforms to Equatable protocol, the below code snippet works,

var arr = ["1", "2", "3"]
let newCard = arr.randomElement()!
arr.remove(at: arr.firstIndex(of: newCard)!)

answered Dec 30, 2018 at 18:07

Bappaditya

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Comments

matt · Accepted Answer · 2018-12-30 18:26:40Z

1

The problem is that your SetCard type has not been declared Equatable. Thus there is no way to know if a card is present or what its index is, because the notion of a card that “matches” your newCard is undefined.

(The compiler error message is not as helpful on this point as it could be. That is a known issue. Bug reports have been filed.)

edited Dec 30, 2018 at 18:26

answered Dec 30, 2018 at 18:08

matt

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3 Comments

konsolas Over a year ago

That seems to have been the issue. Thanks :)

matt Over a year ago

bugs.swift.org/browse/SR-8934 — I’m surprised this isn’t fixed, since the bug is marked resolved.

Martin R Over a year ago

Seems to be fixed for Swift 5. With a current snapshot the error message is "Argument type '...' does not conform to expected type 'Equatable'”

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