添加到Array时无法推断通用参数'Element'

Question

我有一个数组字典：

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要填充它，我尝试在正确的索引处向数组添加值。如果数组不存在，它将失败并在该索引处创建一个新数组：

var myDict : [String:[SomeObj]] = [:]

我想我应该能够将其缩短为：

if myDict[key]?.append(val) == nil {
    myDict[key] = [val]
}

但是，我收到错误：myDict[key]?.append(val) ?? myDict[key] = [val] 。为什么呢？

Answer 1

Swift 3.0

考虑一个简单的概念： -

在单行中使用if...else时，操作应该是单一的，否则我们需要在parenthesis下配对操作以使其成为单个操作，在我们的例子中{ {1}}是一项操作，但append(val)为多项（myDict[key] = [val]为1，myDict[key]分配为1，=为1）所以我们对它们进行分组使用[val] 进入单身。

以更简单的方式考虑以下parenthesis。

arithmatic operations

在这里，我们在//I need 10-5 = 5 let a = 2*4+2-4-3*5 print(a) // -9 //so we can seprate by () let b = ((2*4)+2)-(4-3)*5 print(b) //5 指示编译器不是预期的方式。

另见，

let a

这里let a:Int? = nil var b:Int? = nil let d = 10 let c = a ?? 10 * b ?? d 是错误的指令，错误是，

  

可选类型'Int？'的值没有打开;你的意思是用'！'   还是'？'？

如果我强制let c unwrapping，则错误将变为，

  

在解包可选值时意外发现nil

因此常量optionals a and b变为，

c

您应该在let c = a ?? 10 * (b ?? d) //100 值周围使用parenthesis。

default

Answer 2

最好这样做：

var array = myDict.removeValue(forKey: key) ?? []
array.append(item)
myDict[key] = array

它会在突变之前从array中删除myDict，因此array将是对数组内存的唯一引用。因为它是唯一引用的，所以可以对它执行可变操作，而不必复制数组的内存。

如果没有这个，每个myDict[key]?.append(val)调用都会导致写时复制操作。