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

User · Accepted Answer

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

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What also works      Init default Double variable var scanned  Double    let scanner   NSScanner string   String to Scan   scanner scanDouble  amp scanned      scanned has now the scanned value if something was found

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Extension with optional locale  Swift 2 2  extension String       func toDouble locale  NSLocale    nil  - gt  Double            let formatter   NSNumberFormatter           if let locale   locale               formatter locale   locale                   return formatter numberFromString self   doubleValue           Swift 3 1  extension String       func toDouble   locale  Locale  - gt  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

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var stringValue    55   var convertToDouble   Double  stringValue as NSString  doubleValue

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my problem was comma so i solve it this way   extension String       var doubleValue  Double           return Double  self replacingOccurrences of       with       as NSString  doubleValue

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Please check it on playground   let sString    236 86   var dNumber   NSNumberFormatter   numberFromString sString  var nDouble   dNumber  var eNumber   Double nDouble    3 7   By the way in my Xcode   toDouble   - doesn t exist   doubleValue create value 0 0 from not numerical strings

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Another option here is converting this to an NSString and using that   let string   NSString string  mySwiftString  string doubleValue

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Swift 4 0  try this   let str String    111 11   let tempString    str as NSString  doubleValue  print  String -  tempString

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Try this      var myDouble   myString bridgeToObjectiveC   doubleValue    println myDouble    NOTE  Removed in Beta 5  This no longer works

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This is building upon the answer by  Ryu  His solution is great as long as you re in a country where dots are used as separators  By default NSNumberFormatter uses the devices locale  Therefore this will fail in all countries where a comma is used as the default separator  including France as  PeterK  mentioned  if the number uses dots as separators  which is normally the case   To set the locale of this NSNumberFormatter to be US and thus use dots as separators replace the line  return NSNumberFormatter   numberFromString self   doubleValue   with  let numberFormatter   NSNumberFormatter   numberFormatter locale   NSLocale localeIdentifier   en US POSIX   return numberFormatter numberFromString self   doubleValue   Therefore the full code becomes  extension String       func toDouble   - gt  Double            let numberFormatter   NSNumberFormatter           numberFormatter locale   NSLocale localeIdentifier   en US POSIX           return numberFormatter numberFromString self   doubleValue           To use this  just call  Your text goes here  toDouble    This will return an optional Double   As  Ryu mentioned you can either force unwrap   println  The value is   myDouble        prints  The value is 4 2   or use an if let statement   if let myDouble   myDouble       println  The value is   myDouble       prints  The value is 4 2

User · Answer

For a little more Swift feeling  using NSFormatter   avoids casting to NSString  and returns nil when the string does not contain a Double value  e g   test  will not return 0 0    let double   NSNumberFormatter   numberFromString myString   doubleValue   Alternatively  extending Swift s String type   extension String       func toDouble   - gt  Double            return NumberFormatter   number from  self   doubleValue           and use it like toInt     var myString    4 2  var myDouble   myString toDouble     This returns an optional Double  which has to be unwrapped   Either with forced unwrapping   println  The value is   myDouble        prints  The value is 4 2   or with an if let statement   if let myDouble   myDouble       println  The value is   myDouble       prints  The value is 4 2     Update  For localization  it is very easy to apply locales to the NSFormatter as follows   let formatter   NSNumberFormatter   formatter locale   NSLocale localeIdentifier   fr FR   let double   formatter numberFromString  100 25     Finally  you can use NSNumberFormatterCurrencyStyle on the formatter if you are working with currencies where the string contains the currency symbol

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Swift 2 Update There are new failable initializers that allow you to do this in more idiomatic and safe way  as many answers have noted  NSString s double value is not very safe because it returns 0 for non number values  This means that the doubleValue of  foo  and  0  are the same    let myDouble   Double myString    This returns an optional  so in cases like passing in  foo  where doubleValue would have returned 0  the failable intializer will return nil  You can use a guard  if-let  or map to handle the Optional lt Double gt   Original Post  You don t need to use the NSString constructor like the accepted answer proposes  You can simply bridge it like this    swiftString as NSString  doubleValue

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we can use CDouble value which will be obtained by myString doubleValue

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Here s an extension method that allows you to simply call doubleValue   on a Swift string and get a double back  example output comes first   println  543 29  doubleValue    println  543  doubleValue    println   29  doubleValue    println  0 29  doubleValue     println  -543 29  doubleValue    println  -543  doubleValue    println  - 29  doubleValue    println  -0 29  doubleValue       prints 543 29 543 0 0 29 0 29 -543 29 -543 0 -0 29 -0 29   Here s the extension method   extension String       func doubleValue   - gt  Double               let minusAscii  UInt8   45         let dotAscii  UInt8   46         let zeroAscii  UInt8   48          var res   0 0         let ascii   self utf8          var whole    Double            var current   ascii startIndex          let negative   current    ascii endIndex  amp  amp  ascii current     minusAscii         if  negative                        current   current successor                      while current    ascii endIndex  amp  amp  ascii current     dotAscii                       whole append Double ascii current  - zeroAscii               current   current successor                        whole number         var factor  Double   1         for var i   countElements whole  - 1  i  gt   0  i--                       res    Double whole i     factor             factor    10                      mantissa         if current    ascii endIndex                       factor   0 1             current   current successor               while current    ascii endIndex                               res    Double ascii current  - zeroAscii    factor                 factor    0 1                 current   current successor                                   if  negative                        res    -1                     return res           No error checking  but you can add it if you need it

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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  quot -1 0 quot   let f   Double  quot 0x1c 6 quot   let i   Double  quot inf quot      etc

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Swift   4 and 5  There are possibly two ways to do this    String -  Int -  Double   let strNumber    314  if let intFromString   Int strNumber       let dobleFromInt   Double intFromString      print dobleFromInt     String -  NSString -  Double  let strNumber1    314  let NSstringFromString   NSString string  strNumber1  let doubleFromNSString   NSstringFromString doubleValue print doubleFromNSString     Use it anyway you like according to you need of the code

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Or you could do   var myDouble   Double  mySwiftString text as NSString  doubleValue

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As already pointed out  the best way to achieve this is with direct casting    myString as NSString  doubleValue   Building from that  you can make a slick native Swift String extension   extension String       var doubleValue  Double           return  self as NSString  doubleValue           This allows you to directly use   myString doubleValue   Which will perform the casting for you  If Apple does add a doubleValue to the native String you just need to remove the extension and the rest of your code will automatically compile fine

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In Swift 2 0 the best way is to avoid thinking like an Objective-C developer  So you should not  convert a String to a Double  but you should  initialize a Double from a String   Apple doc over here  https   developer apple com library ios  documentation Swift Reference Swift Double Structure index html   apple ref swift structctr Double s FSdcFMSdFSSGSqSd   It s an optional init so you can use the nil coalescing operator      to set a default value  Example   let myDouble   Double  1 1      0 0

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Use this code in Swift 2 0 let strWithFloat    quot 78 65 quot  let floatFromString   Double strWithFloat

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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

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As of Swift 1 1  you can directly pass String to const char   parameter   import Foundation  let str    123 4567  let num   atof str     - gt  123 4567  atof  123 4567fubar      - gt  123 4567   If you don t like deprecated atof   strtod  765 4321   nil     - gt  765 4321   One caveat  the behavior of conversion is different from NSString doubleValue   atof and strtod accept 0x prefixed hex string   atof  0xffp-2      - gt  63 75 atof  12 3456e 2      - gt  1 234 56 atof  nan      - gt   not a number  atof  inf      - gt    infinity    If you prefer  doubleValue behavior  we can still use CFString bridging   let str    0xff  atof str                          - gt  255 0 strtod str  nil                   - gt  255 0 CFStringGetDoubleValue str        - gt  0 0  str as NSString  doubleValue     - gt  0 0

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SWIFT 4  extension String       func toDouble   - gt  Double            let numberFormatter   NumberFormatter           numberFormatter locale   Locale identifier   en US POSIX           return numberFormatter number from  self   doubleValue

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On SWIFT 3  you can use   if let myDouble   NumberFormatter   number from  yourString   doubleValue      print  My double    myDouble        Note    - If a string contains any characters other than numerical digits or locale-appropriate group or decimal separators  parsing will fail   - Any leading or trailing space separator characters in a string are ignored  For example  the strings     5        5      and    5    all produce the number 5   Taken from the documentation  https   developer apple com reference foundation numberformatter 1408845-number

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Using Scanner in some cases is a very convenient way of extracting numbers from a string  And it is almost as powerful as NumberFormatter when it comes to decoding and dealing with different number formats and locales  It can extract numbers and currencies with different decimal and group separators   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                       However  there are issues with separators that could be conceived as word delimiters      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   There is no problem to auto detect locale  Note that groupingSeparator in French locale in string  Mon salaire est 9  999 99     is not a space  though it may render exactly as space  here it doesn t   Thats why the code below works fine without   0 isWhitespace characters being filtered out   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

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Swift 4  extension String       func toDouble   - gt  Double           let nsString   self as NSString         return nsString doubleValue

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I find more readable to add an extension to String as follow   extension String       var doubleValue  Double           return  self as NSString  doubleValue           and then you just could write your code   myDouble   myString doubleValue

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You can use StringEx  It extends String with string-to-number conversions including toDouble     extension String       func toDouble   - gt  Double      It verifies the string and fails if it can t be converted to double   Example   import StringEx  let str    123 45678  if let num   str toDouble         println  Number    num      else       println  Invalid string

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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 a specific format   Swift 3  extension String       var toDouble  Double           return Double self     0 0

[swift] Swift - How to convert String to Double

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