数组 – 不可变值只有变异成员

我已经使用var声明了一个数组，并将其填充在一个init()中。然而，当我尝试改变该数组时，我发现一堆错误告诉我的数组是不可变的。我在这里缺少什么？

struct Deck {
    var cards: Card[] = []

    init () {
        for i in 1...4 {
            for ii in 1...13 {
                self.cards.append(Card(rank: Rank.fromraw(ii)!,suit: Suit.fromraw(i)!))
            }
        }
    }

    func shuffle () {
        var shuffledDeck: Card[] = []
        var count = self.cards.count

        for i in 1...52 {
            var limit = count - i
            var key = Int(arc4random_uniform(UInt32(limit)));
            shuffledDeck.append(self.cards[key])
            self.cards.removeAtIndex(key)
        }

        self.cards = shuffledDeck
    }
}

我得到的错误：

Playground execution Failed: error:
<REPL>:75:22: error: immutable value of type 'Card[]' only has mutating members named 'removeAtIndex'
            self.cards.removeAtIndex(key)
                 ^     ~~~~~~~~~~~~~
<REPL>:78:24: error: cannot assign to 'cards' in 'self'
            self.cards = shuffledDeck

一个struct被认为是一个值类型，所以默认情况下它是不可变的。如果要使用方法更改它，则必须声明方法变异。引用Swift书：

Structures and enumerations are value types. By default,the
properties of a value type cannot be modified from within its instance
methods.

However,if you need to modify the properties of your structure or
enumeration within a particular method,you can opt in to mutating
behavior for that method. The method can then mutate (that is,change)
its properties from within the method,and any changes that it makes
are written back to the original structure when the method ends. The
method can also assign a completely new instance to its implicit self
property,and this new instance will replace the existing one when the
method ends.

You can opt in to this behavior by placing the mutating keyword before
the func keyword for that method.