这是基于@Ryu的回答

只要你在一个用圆点作为分隔符的国家，他的解决方案就很好。默认情况下NSNumberFormatter使用设备区域设置。因此，在所有使用逗号作为默认分隔符的国家(包括法国的@PeterK)，这将失败。如果数字使用点作为分隔符(通常情况下)。要将这个NSNumberFormatter的区域设置为US，并使用圆点作为分隔符替换行

return NSNumberFormatter().numberFromString(self)?.doubleValue

with

let numberFormatter = NSNumberFormatter()
numberFormatter.locale = NSLocale(localeIdentifier: "en_US_POSIX")
return numberFormatter.numberFromString(self)?.doubleValue

因此，完整的代码变成

extension String {
    func toDouble() -> Double? {
        let numberFormatter = NSNumberFormatter()
        numberFormatter.locale = NSLocale(localeIdentifier: "en_US_POSIX")
        return numberFormatter.numberFromString(self)?.doubleValue
    }
}

要使用它，只需调用"Your text goes here".toDouble()

这将返回一个可选的Double?

正如@Ryu提到的，你可以强制展开:

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
}

斯威夫特:4和5

可能有两种方法:

String -> Int -> Double: let strNumber = "314" if let intFromString = Int(strNumber){ let dobleFromInt = Double(intFromString) 打印(dobleFromInt) ｝ String -> NSString -> Double 让strNumber1 = "314" 让NSstringFromString = NSString(字符串:strNumber1) let doubleFromNSString = NSstringFromString.doubleValue 打印(doubleFromNSString)

使用它，无论你喜欢根据你需要的代码。

你可以使用StringEx。它用字符串到数字的转换扩展了String，包括toDouble()。

extension String {
    func toDouble() -> Double?
}

它验证字符串，如果不能转换为double则失败。

例子:

import StringEx

let str = "123.45678"
if let num = str.toDouble() {
    println("Number: \(num)")
} else {
    println("Invalid string")
}

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)

在字符串包含其他字符的情况下:“27.8°C”，“52.523553 kM”或“Total: 349.0”。

这适用于Swift 4:

let anyString = "52.523553 kM"
let digitsCharacterSet = CharacterSet.init(charactersIn: "0123456789.")
let doubleResult = Double(anyString.components(separatedBy:digitsCharacterSet.inverted).joined())

谨慎!这对包含多个的字符串无效。比如“27.8°C 3.5 kM”

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。