Swift：AnyObject强制转换为Float失败 [英] Swift: AnyObject cast to Float failed

问题描述

let json = [
    "left" : 18,
    "deadline" : "May 10",
    "progress" : 0.6
] as [String: AnyObject]

let ss = json["progress"] as? Float
let sss = json["progress"] as? Double
print("ss = \(ss)\n  sss = \(sss)")

我不知道为什么 ss 显示为nil，而 sss 为什么显示 0.599999998 。为什么强制转换为 Float 无效？你们有一些方法可以使我得到正确的结果吗？

I have no idea why the ss shows nil while sss shows 0.599999998. Why does casting to Float get nil? Do you guys have some methods so that I can get the correct result?

推荐答案

0.6 是 Double 文字价值。因此，您不能将其转换为 Float （需要对其进行转换）。

The 0.6 is a Double literal value. As such, you can't cast it to Float (you need to convert it).

尝试以下操作：

let f = Float(json["progress"] as! Double)

或者，如果您不确定如何确定数字的类型这个 AnyObject 成立，更安全的方法是：

Or, if you aren't really sure what type of number this AnyObject holds, the safer approach would be:

let f = (json["progress"] as! NSNumber).floatValue

当然，那些<$ c $如果json值丢失 或您误判了预期的类型，则上述c> as！将会崩溃 hard 。否则，请使用 as？运算符:)

Of course, those as! above will crash hard if the json value is missing or you misjudge the expected type. Use the as? operator instead if you feel otherwise :)

铸造速成班。将已知的 Double 值转换为 Float 时，编译器会为我们提供一个很好的提示：

Casting crash course. When casting a known Double value to a Float, the compiler gives us a nice heads up about this:

let d = 0.6
let f = d as? Float

警告：从 Double转换为无关类型 Float总是失败

warning: cast from 'Double' to unrelated type 'Float' always fails

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