Swift - How to convert String to Double

Question

I'm trying to write a BMI program in swift language. And I got this problem: how to convert a String to a Double?

In Objective-C, I can do like this:

double myDouble = [myString doubleValue];

But how can I achieve this in Swift language?

Answer 1

Swift 4.2+ String to Double

You should use the new type initializers to convert between String and numeric types (Double, Float, Int). It'll return an Optional type (Double?) which will have the correct value or nil if the String was not a number.

Note: The NSString doubleValue property is not recommended because it returns 0 if the value cannot be converted (i.e.: bad user input).

let lessPrecisePI = Float("3.14")

let morePrecisePI = Double("3.1415926536")
let invalidNumber = Float("alphabet") // nil, not a valid number

Unwrap the values to use them using if/let

if let cost = Double(textField.text!) {
    print("The user entered a value price of \(cost)")
} else {
    print("Not a valid number: \(textField.text!)")
}

You can convert formatted numbers and currency using the NumberFormatter class.

let formatter = NumberFormatter()
formatter.locale = Locale.current // USA: Locale(identifier: "en_US")
formatter.numberStyle = .decimal
let number = formatter.number(from: "9,999.99")

Currency formats

let usLocale = Locale(identifier: "en_US")
let frenchLocale = Locale(identifier: "fr_FR")
let germanLocale = Locale(identifier: "de_DE")
let englishUKLocale = Locale(identifier: "en_GB") // United Kingdom
formatter.numberStyle = .currency

formatter.locale = usLocale
let usCurrency = formatter.number(from: "$9,999.99")

formatter.locale = frenchLocale
let frenchCurrency = formatter.number(from: "9999,99€")
// Note: "9 999,99€" fails with grouping separator
// Note: "9999,99 €" fails with a space before the €

formatter.locale = germanLocale
let germanCurrency = formatter.number(from: "9999,99€")
// Note: "9.999,99€" fails with grouping separator

formatter.locale = englishUKLocale
let englishUKCurrency = formatter.number(from: "£9,999.99")

Read more on my blog post about converting String to Double types (and currency).

Answer 2

SWIFT 3

To clear, nowadays there is a default method:

public init?(_ text: String) of Double class.

It can be used for all classes.

let c = Double("-1.0") let f = Double("0x1c.6") let i = Double("inf") , etc.

Answer 3

I haven't seen the answer I was looking for. I just post in here mine in case it can help anyone. This answer is valid only if you don't need an specific format.

Swift 3

extension String {
    var toDouble: Double {
        return Double(self) ?? 0.0
    }
}

Answer 4

Use this code in Swift 2.0

let strWithFloat = "78.65" let floatFromString = Double(strWithFloat)

Answer 5

1.

let strswift = "12"
let double = (strswift as NSString).doubleValue

2.

var strswift= "10.6"
var double : Double = NSString(string: strswift).doubleValue 

May be this help for you.

Answer 6

var stringValue = "55"

var convertToDouble = Double((stringValue as NSString).doubleValue)