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
}

Solution 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
     }
}

Solution 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

Solution 3

Why not let it return false? Or true of course.

extension String {

    func isInt() -> Bool {

        if let intValue = Int(self) {
            return true
        }

        return false
    }

    func isFloat() -> Bool {

        if let floatValue = Float(self) {
            return true
        }

        return false
    }

    func isDouble() -> Bool {

        if let doubleValue = Double(self) {
            return true
        }

        return false
    }

    func numberOfCharacters() -> Int {
        return self.characters.count
    }
}

Or even better, as suggested by @LeoDabus:

extension String {
    var isInteger: Bool { return Int(self) != nil }
    var isFloat: Bool { return Float(self) != nil }
    var isDouble: Bool { return Double(self) != nil }
} 

Comments

[EmilioPelaez]

I would suggest you cache NSNumberFormatter, if you use this method a lot (for example on a tableViewCell) you would benefit from caching it.

[Blake Merryman]

If going this route, any reason to not make the structure a first-class member of the entire class extension? Then you can easily extend string to also support integerValue, boolValue, etc. and share the same Formatter.

[Roman Safin]

it is a bad pattern to use snake case for swift developer, check out the camel case.

[Hedylove]

Note for others: NSNumberFormatter() has been renamed to NumberFormatter in Swift 3 so the struct name will give you errors. All you have to do is change it's name.

[Fengson]

@EmilioPelaez How would you go about caching it?

[Leo Dabus]

@Fengson his question was done when the answer was in Swift 1 and at the time there was no Double(_ string:) initializer. this answer shows how to do it in Swift 3.x and later stackoverflow.com/a/27705739/2303865

[Michael Voccola]

That is definitely cleaner!

[Leo Dabus]

Why would youl return FALSE for all negative numbers?

[Leo Dabus]

You can simply return not equal to nil and there is no need to make it a method. If there is no need to pass any value to it. You can make them all computed properties with a getter but no setter extension StringProtocol { var isInteger: Bool { return Int(self) != nil } var isFloat: Bool { return Float(self) != nil } var isDouble: Bool { return Double(self) != nil } }

[Leo Dabus]

Note that this is no pure Swift. It requires you to import Foundation.