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

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

extension String {

    func isInt() -> Bool {

        if let intValue = Int(self) {

            if intValue >= 0 {
                return true
            }
        }

        return false
    }

    func isFloat() -> Bool {

        if let floatValue = Float(self) {

            if floatValue >= 0 {
                return true
            }
        }

        return false
    }

    func isDouble() -> Bool {

        if let doubleValue = Double(self) {

            if doubleValue >= 0 {
                return true
            }
        }

        return false
    }

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

Answer 2

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 3

Here is my function:

func je_numerik(text:Character)->Bool //en znak
{
    if((text=="0")||(text=="1")||(text=="2")||(text=="3")||(text=="4")||(text=="5")||(text=="6")||(text=="7")||(text=="8")||(text=="9")){
        return true
    }
    else
    {
        return false
    }
}

func je_stevilka(text: String)->Bool
{
    var pika:Character
    pika="."

    var je_stevilka=true
    //var znaki = Character(text)
    var znaki=Array(text)
    var stevilo_znakov=znaki.count

    if(stevilo_znakov==0)
    {
        je_stevilka=false
    }
    else
    {
        if(stevilo_znakov==1)
        {
            if(je_numerik(znaki[0]))
            {
                je_stevilka=true
            }
            else
            {
                je_stevilka=false
            }
        }
        else
        {

            if((je_numerik(znaki[0])) && (!((znaki[0]=="0")&&((znaki[1]=="0"))))&&(!((znaki[0]=="0")&&((je_numerik(znaki[1]))))))
            {

                var je_pika=false
                for var i = 0; i < stevilo_znakov; i++
                {
                    if(je_numerik(znaki[i])||(znaki[i]==pika))
                    {
                        if(znaki[i]==pika)
                        {
                            if(je_pika)
                            {
                                je_stevilka=false
                            }
                            else
                            {
                                je_pika=true
                                if(stevilo_znakov==(i+1))
                                {
                                    je_stevilka=false
                                }
                            }
                        }
                    }
                    else
                    {
                        je_stevilka=false
                    }
                }
            }
            else
            {
                je_stevilka=false
            }
        }
    }

    return je_stevilka
}

Just call it like this:

var check_if_is_number=je_stevilka(numberText)

Answer 4

edit/update: Xcode 9 or later • Swift 4 or later

You can use Double initialiser StringProtocol to create an extension and use it to check if your string is a valid double as follow:

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