添加到数组时无法推断通用参数“元素" [英] Generic parameter 'Element' could not be inferred when adding to Array

问题描述

我有一个数组字典:

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

要填充它，我尝试在正确的索引处向数组添加一个值.如果该数组不存在，它将失败，并在该索引处创建一个新数组:

To populate it, I try to add a value to the array at the correct index. If the array does not exist, it fails and I make a new array at that index:

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

我认为我应该可以将其简化为:

I think I should be able to shorten this to:

myDict[key]?.append(val) ?? myDict[key] = [val]

但是，相反，我得到了错误:Generic parameter 'Element' could not be inferred.为什么?

However, instead I get the error: Generic parameter 'Element' could not be inferred. Why?

推荐答案

Swift 3.0

考虑简单的概念:-

Consider the simple concept :-

在单行中使用if...else时，操作应该是单个操作，否则我们需要在parenthesis下配对操作以使其成为单个操作，在我们的情况下，append(val)是一个单个操作，但是myDict[key] = [val]是多个操作(myDict[key]是一个，并且=分配是一个，而[val]是1)，所以我们使用parenthesis 将它们分组为单个.

While using if...else in single line the operations should be single or else we need to mate operations under parenthesis to make it as a single operation, in our case append(val) is a single operation but myDict[key] = [val] is multiple (myDict[key] is one and = assignment is one and [val] is one ) so we are grouping them into single using parenthesis.

以更简单的方式考虑以下arithmatic operations.

At more simple way consider the following arithmatic operations.

//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

在这里，我们在let a上指示编译器不是预期的方式.

Here, we are instructing the compiler not a expected way at let a.

也可以看到

let a:Int? = nil
var b:Int? = nil
let d = 10

let c = a ?? 10 * b ?? d

这里let c是错误的指令，错误是

Here let c is wrong instruction, error is,

可选类型'Int?'的值没有包装;你的意思是使用'！' 还是?"?

Value of optional type 'Int?' not unwrapped; did you mean to use '!' or '?'?

如果我强制unwrapping optionals a and b，则错误将变为

If i force unwrapping the optionals a and b, then the error will become,

在展开可选值时意外发现nil

unexpectedly found nil while unwrapping an Optional value

所以常数c变为

let c = a ?? 10 * (b ?? d) //100

那是您应该在default值附近使用parenthesis.

That's you should use parenthesis around the default value.

myDict[key]?.append(val) ?? (myDict[key] = [val])

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