Check if string is a valid double value in Swift

Question

In Swift, how can one check if a string is a valid double value? I have been using the following extension from this question (but as a float) but if the value cannot be converted, it simply returns "0":

extension String {
    var doubleValue:Double? {
        return (self as NSString).doubleValue
    }
}

Ideally, I would like it to return nil so it can be caught in an if-let, like so:

if let i = str.doubleValue {
    object.price = i
} else {
    // Tell user the value is invalid
}

Answer 1

It is indeed more efficient not to create a number formatter every time we do a conversion:

extension String {
     struct NumFormatter {
         static let instance = NumberFormatter()
     }

     var doubleValue: Double? {
         return NumFormatter.instance.number(from: self)?.doubleValue
     }

     var integerValue: Int? {
         return NumFormatter.instance.number(from: self)?.intValue
     }
}

Answer 2

edit/update: Xcode 11 or later • Swift 5.1 or later

You can use Double initializer init?<S>(_ text: S) where S : StringProtocol to create an instance property on StringProtocol and use it to check if a String or Substring is a valid Double:

extension StringProtocol {
    var double: Double? { Double(self) }
    var float: Float? { Float(self) }
    var integer: Int? { Int(self) }
}

---

Testing

let str = "2.9"
if let value = str.double  {
    print(value)           // "2.9\n"
} else {
    print("invalid input")
}

str.prefix(1).integer  // 2
str.suffix(1).integer  // 9