Swift convert object that is NSNumber to Double

ios, swift

Solution

Update

Swift's behavior here has changed quite a bit since 1.0. Not that it was that easy before, but Swift has made it harder to convert between number types because it wants you to be explicit about what to do with precision loss. Your new choices now look like this:

var rating: NSNumber
var ratingDouble: Double

ratingDouble = rating as! Double // 1
ratingDouble = Double(exactly: rating)! // 2

ratingDouble = Double(truncating: rating) // 3
ratingDouble = rating.doubleValue // 4

if let x = rating as? Double { // 5
    ratingDouble = x
}

if let x = Double(exactly: rating) { // 6
    ratingDouble = x
}

This calls `Double._forceBridgeFromObjectiveC` which calls `Double(exactly:)` with `Double`, `Int64`, or `UInt64` based on the stored type in `rating`. It will fail and crash the app if the number isn't exactly representable as a `Double`. E.g. `UInt64.max` has more digits than `Double` can store, so it'll crash.

This is exactly the same as 1 except that it may also crash on `NaN` since that check isn't included.

This function always returns a `Double` but will lose precision in cases where 1 and 2 would crash. This literally just calls `doubleValue` when passing in an `NSNumber`.

Same as 3.

This is like 1 except that instead of crashing the app, it'll return nil and the inside of the statement won't be evaluated.

Same as 5, but like 2 will return nil if the value is `NaN`.

If you know your data source is dealing in doubles, 1-4 will probably all serve you about the same. 3 and 4 would be my first choices though.

Old Answer for Swift 1 and 2

There are several things you can do:

var rating: NSNumber
var ratingDouble: Double

ratingDouble = rating as Double   // 1
ratingDouble = Double(rating)     // 2
ratingDouble = rating.doubleValue // 3

- The first item takes advantage of `Objective-C`bridging which allows `AnyObject` and `NSNumber` to be cast as `Double|Float|Int|UInt|Bool`.

- The second item presumably goes through a constructor with the signature `init(_ number: NSNumber)`. I couldn't find it in the module or docs but passing `AnyObject` in generated an error that it cannot be implicitly downcast to `NSNumber` so it must be there and not just bridging.

- The third item doesn't employ language features in the same way. It just takes advantage of the fact that `doubleValue` returns a `Double`.

One benefit of 1 is that it also works for `AnyObject` so your code could be:

let ratingDouble = self.prodResult!.prodsInfo.prodList[indexPath.row].avgRating! as Double

Note that I removed the `?` from your function and moved the `!` in. Whenever you use ! you are eschewing the safety of `?` so there's no reason to do both together.

Problem

Original source