具有可选区域设置的扩展

斯威夫特2.2

extension String {
    func toDouble(locale: NSLocale? = nil) -> Double? {
        let formatter = NSNumberFormatter()
        if let locale = locale {
            formatter.locale = locale
        }
        return formatter.numberFromString(self)?.doubleValue
    }
}

斯威夫特3.1

extension String {
    func toDouble(_ locale: Locale) -> Double {
        let formatter = NumberFormatter()
        formatter.numberStyle = .decimal
        formatter.locale = locale
        formatter.usesGroupingSeparator = true
        if let result = formatter.number(from: self)?.doubleValue {
            return result
        } else {
            return 0
        }
    }
}

请到操场上检查!

let sString = "236.86"

var dNumber = NSNumberFormatter().numberFromString(sString)
var nDouble = dNumber!
var eNumber = Double(nDouble) * 3.7

顺便说一下，在我的Xcode中

.toDouble() -不存在

从非数值字符串中创建值0.0

另一个选项是将this转换为NSString并使用它:

let string = NSString(string: mySwiftString)
string.doubleValue

Swift 4.2+字符串到双

您应该使用新的类型初始化器在字符串和数值类型(Double, Float, Int)之间进行转换。它将返回一个可选类型(Double?)，它将有正确的值，如果字符串不是数字，则为nil。

注意:不建议使用NSString doubleValue属性，因为如果值不能转换(即:错误的用户输入)，它将返回0。

let lessPrecisePI = Float("3.14")

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

打开值，使用if/let来使用它们

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

可以使用NumberFormatter类转换格式化的数字和货币。

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

货币格式

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")

在我的博客文章中阅读更多关于将String转换为Double类型(和货币)的内容。

为了有一点Swift的感觉，使用NSFormatter()避免转换为NSString，并在字符串不包含Double值时返回nil(例如:"test"不会返回0.0)。

let double = NSNumberFormatter().numberFromString(myString)?.doubleValue

或者，扩展Swift的String类型:

extension String {
    func toDouble() -> Double? {
        return NumberFormatter().number(from: self)?.doubleValue
    }
}

并像toInt()一样使用它:

var myString = "4.2"
var myDouble = myString.toDouble()

这返回一个可选的Double?它必须被打开。

要么是强行打开:

println("The value is \(myDouble!)") // prints: The value is 4.2

或者使用if let语句:

if let myDouble = myDouble {
    println("The value is \(myDouble)") // prints: The value is 4.2
}

更新: 对于本地化，将locale应用到NSFormatter是非常容易的，如下所示:

let formatter = NSNumberFormatter()
formatter.locale = NSLocale(localeIdentifier: "fr_FR")
let double = formatter.numberFromString("100,25")

最后，你可以使用NSNumberFormatterCurrencyStyle在格式化器上，如果你正在处理包含货币符号的字符串的货币。

在某些情况下，使用Scanner从字符串中提取数字是一种非常方便的方法。当涉及到解码和处理不同的数字格式和地区时，它几乎和NumberFormatter一样强大。它可以提取具有不同小数和组分隔符的数字和货币。

import Foundation
// The code below includes manual fix for whitespaces (for French case)
let strings = ["en_US": "My salary is $9,999.99",
               "fr_FR": "Mon salaire est 9 999,99€",
               "de_DE": "Mein Gehalt ist 9999,99€",
               "en_GB": "My salary is £9,999.99" ]
// Just for referce
let allPossibleDecimalSeparators = Set(Locale.availableIdentifiers.compactMap({ Locale(identifier: $0).decimalSeparator}))
print(allPossibleDecimalSeparators)
for str in strings {
    let locale = Locale(identifier: str.key)
    let valStr = str.value.filter{!($0.isWhitespace || $0 == Character(locale.groupingSeparator ?? ""))}
    print("Value String", valStr)

    let sc = Scanner(string: valStr)
    // we could do this more reliably with `filter` as well
    sc.charactersToBeSkipped = CharacterSet.decimalDigits.inverted
    sc.locale = locale

    print("Locale \(locale.identifier) grouping separator: |\(locale.groupingSeparator ?? "")| . Decimal separator: \(locale.decimalSeparator ?? "")")
    while !(sc.isAtEnd) {
        if let val = sc.scanDouble() {
            print(val)
        }

    }
}

但是，将分隔符视为单词分隔符存在一些问题。

// This doesn't work. `Scanner` just ignores grouping separators because scanner tends to seek for multiple values
// It just refuses to ignore spaces or commas for example.
let strings = ["en_US": "$9,999.99", "fr_FR": "9999,99€", "de_DE": "9999,99€", "en_GB": "£9,999.99" ]
for str in strings {
    let locale = Locale(identifier: str.key)
    let sc = Scanner(string: str.value)
    sc.charactersToBeSkipped = CharacterSet.decimalDigits.inverted.union(CharacterSet(charactersIn: locale.groupingSeparator ?? ""))
    sc.locale = locale
    print("Locale \(locale.identifier) grouping separator: \(locale.groupingSeparator ?? "") . Decimal separator: \(locale.decimalSeparator ?? "")")
    while !(sc.isAtEnd) {
        if let val = sc.scanDouble() {
            print(val)
        }

    }
}
//     sc.scanDouble(representation: Scanner.NumberRepresentation) could help if there were .currency case

自动检测语言环境是没有问题的。注意，grouingseparator在法语区域设置中的字符串“Mon salaire est 9 999,99€”不是一个空格，尽管它可以完全呈现为空格(这里不是)。这就是为什么下面的代码没有!$0也能正常工作的原因。正在过滤的空白字符。

let stringsArr = ["My salary is $9,999.99",
                  "Mon salaire est 9 999,99€",
                  "Mein Gehalt ist 9.999,99€",
                  "My salary is £9,999.99" ]

let tagger = NSLinguisticTagger(tagSchemes: [.language], options: Int(NSLinguisticTagger.Options.init().rawValue))
for str in stringsArr {
    tagger.string = str
    let locale = Locale(identifier: tagger.dominantLanguage ?? "en")
    let valStr = str.filter{!($0 == Character(locale.groupingSeparator ?? ""))}
    print("Value String", valStr)

    let sc = Scanner(string: valStr)
    // we could do this more reliably with `filter` as well
    sc.charactersToBeSkipped = CharacterSet.decimalDigits.inverted
    sc.locale = locale

    print("Locale \(locale.identifier) grouping separator: |\(locale.groupingSeparator ?? "")| . Decimal separator: \(locale.decimalSeparator ?? "")")
    while !(sc.isAtEnd) {
        if let val = sc.scanDouble() {
            print(val)
        }

    }
}
// Also will fail if groupingSeparator == decimalSeparator (but don't think it's possible)