Precision String Format Specifier In Swift

Question

Below is how I would have previously truncated a float to two decimal places  NSLog      02f   02f   02f   r  g  b     I checked the docs and the eBook but haven t been able to figure it out  Thanks

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a simple way is   import Foundation    required for String format         print String format   hex string   X   123456   print String format   a float number    5f   1 0321

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Swift 4 Xcode 10 Update  extension Double       var asNumber String           if self  gt   0               let formatter   NumberFormatter               formatter numberStyle    none             formatter percentSymbol                  formatter maximumFractionDigits   2             return    formatter string from  NSNumber value  self                             return

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A more elegant and generic solution is to rewrite ruby   python   operator      Updated for beta 5 func   format String  args  CVarArgType   - gt  String       return NSString format format  arguments getVaList args       Hello     This is pi     2f      World   M PI

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import Foundation  extension CGFloat       var string1  String           return String format     1f   self            var string2  String           return String format     2f   self            Usage  let offset   CGPoint 1 23  4 56  print  offset    offset x string1  x   offset y string1       offset  1 2 x 4 6

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Why make it so complicated  You can use this instead   import UIKit  let PI   3 14159265359  round  PI      3 0 rounded to the nearest decimal round  PI   100     100   3 14 rounded to the nearest hundredth round  PI   1000     1000    3 142 rounded to the nearest thousandth   See it work in Playground   PS  Solution from  http   rrike sh xcode rounding-various-decimal-places-swift

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Swift2 example  Screen width of iOS device formatting the Float removing the decimal  print NSString format   Screen width     0f pixels   CGRectGetWidth self view frame

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Details   Xcode 9 3  Swift 4 1 Xcode 10 2 1  10E1001   Swift 5   Solution 1  func rounded   -  Double   5 2  rounded      5 0  5 5  rounded      6 0  -5 2  rounded      -5 0  -5 5  rounded      -6 0   func rounded   rule  FloatingPointRoundingRule  -  Double  let x   6 5     Equivalent to the C  round  function  print x rounded  toNearestOrAwayFromZero      Prints  7 0      Equivalent to the C  trunc  function  print x rounded  towardZero      Prints  6 0      Equivalent to the C  ceil  function  print x rounded  up      Prints  7 0      Equivalent to the C  floor  function  print x rounded  down      Prints  6 0    mutating func round    var x   5 2 x round      x    5 0 var y   5 5 y round      y    6 0 var z   -5 5 z round      z    -6 0   mutating func round   rule  FloatingPointRoundingRule      Equivalent to the C  round  function  var w   6 5 w round  toNearestOrAwayFromZero     w    7 0     Equivalent to the C  trunc  function  var x   6 5 x round  towardZero     x    6 0     Equivalent to the C  ceil  function  var y   6 5 y round  up     y    7 0     Equivalent to the C  floor  function  var z   6 5 z round  down     z    6 0   Solution 2  extension Numeric        private func  precision number  NSNumber  formatter  NumberFormatter  - gt  Self            if  let formatedNumString   formatter string from  number               let formatedNum   formatter number from  formatedNumString                    return formatedNum as  Self                   return nil            private func toNSNumber   - gt  NSNumber            if let num   self as  NSNumber   return num           guard let string   self as  String  let double   Double string  else   return nil           return NSNumber value  double             func precision   minimumFractionDigits  Int                     roundingMode  NumberFormatter RoundingMode   NumberFormatter RoundingMode halfUp  - gt  Self            guard let number   toNSNumber   else   return nil           let formatter   NumberFormatter           formatter minimumFractionDigits   minimumFractionDigits         formatter roundingMode   roundingMode         return  precision number  number  formatter  formatter             func precision with numberFormatter  NumberFormatter  - gt  String            guard let number   toNSNumber   else   return nil           return numberFormatter string from  number            Usage      123 44 precision 2      123 44 precision 3  roundingMode   up   let numberFormatter   NumberFormatter   numberFormatter minimumFractionDigits   1 numberFormatter groupingSeparator       let num   222 3333     num precision 2    Full sample  func option1 lt T  Numeric gt  value  T  numerFormatter  NumberFormatter    nil        print  Type    type of  value         print  Original Value    value        let value1   value precision 2      print  value1     value1    nil      value1       nil         let value2   value precision 5      print  value2     value2    nil      value2       nil         if let value1   value1  let value2   value2           print  value1   value2     value1   value2              print        func option2 lt T  Numeric gt  value  T  numberFormatter  NumberFormatter        print  Type    type of  value         print  Original Value    value        let value1   value precision with  numberFormatter      print  formated value     value1    nil      value1       nil   n      func test with double  Double        print                              nTest with    double  n       let float   Float double      let float32   Float32 double      let float64   Float64 double      let float80   Float80 double      let cgfloat   CGFloat double          Exapmle 1     print  -- Option1 n       option1 value  double      option1 value  float      option1 value  float32      option1 value  float64      option1 value  float80      option1 value  cgfloat          Exapmle 2      let numberFormatter   NumberFormatter       numberFormatter formatterBehavior    behavior10 4     numberFormatter minimumIntegerDigits   1     numberFormatter minimumFractionDigits   4     numberFormatter maximumFractionDigits   9     numberFormatter usesGroupingSeparator   true     numberFormatter groupingSeparator           numberFormatter groupingSize   3      print  -- Option 2 n       option2 value  double  numberFormatter  numberFormatter      option2 value  float  numberFormatter  numberFormatter      option2 value  float32  numberFormatter  numberFormatter      option2 value  float64  numberFormatter  numberFormatter      option2 value  float80  numberFormatter  numberFormatter      option2 value  cgfloat  numberFormatter  numberFormatter     test with  123 22  test with  1234567890987654321 0987654321    Output                              Test with  123 22  -- Option1  Type  Double Original Value  123 22 value1   123 22 value2   123 22 value1   value2   246 44  Type  Float Original Value  123 22 value1   nil value2   nil  Type  Float Original Value  123 22 value1   nil value2   nil  Type  Double Original Value  123 22 value1   123 22 value2   123 22 value1   value2   246 44  Type  Float80 Original Value  123 21999999999999886 value1   nil value2   nil  Type  CGFloat Original Value  123 22 value1   123 22 value2   123 22 value1   value2   246 44  -- Option 2  Type  Double Original Value  123 22 formatted value   123 2200  Type  Float Original Value  123 22 formatted value   123 220001221  Type  Float Original Value  123 22 formatted value   123 220001221  Type  Double Original Value  123 22 formatted value   123 2200  Type  Float80 Original Value  123 21999999999999886 formatted value   nil  Type  CGFloat Original Value  123 22 formatted value   123 2200                              Test with  1 2345678909876544e 18  -- Option1  Type  Double Original Value  1 2345678909876544e 18 value1   1 23456789098765e 18 value2   1 23456789098765e 18 value1   value2   2 4691357819753e 18  Type  Float Original Value  1 234568e 18 value1   nil value2   nil  Type  Float Original Value  1 234568e 18 value1   nil value2   nil  Type  Double Original Value  1 2345678909876544e 18 value1   1 23456789098765e 18 value2   1 23456789098765e 18 value1   value2   2 4691357819753e 18  Type  Float80 Original Value  1234567890987654400 0 value1   nil value2   nil  Type  CGFloat Original Value  1 2345678909876544e 18 value1   1 23456789098765e 18 value2   1 23456789098765e 18 value1   value2   2 4691357819753e 18  -- Option 2  Type  Double Original Value  1 2345678909876544e 18 formatted value   1 234 567 890 987 650 000 0000  Type  Float Original Value  1 234568e 18 formatted value   1 234 567 939 550 610 000 0000  Type  Float Original Value  1 234568e 18 formatted value   1 234 567 939 550 610 000 0000  Type  Double Original Value  1 2345678909876544e 18 formatted value   1 234 567 890 987 650 000 0000  Type  Float80 Original Value  1234567890987654400 0 formatted value   nil  Type  CGFloat Original Value  1 2345678909876544e 18 formatted value   1 234 567 890 987 650 000 0000

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Christian Dietrich   instead of   var k   1 0     for i in 1   right 1           k   10 0   k       let n   Double Int left k     Double k  return    n     it could also be   let k   pow 10 0  Double right   let n   Double Int left k     k return    n      correction   Sorry for confusion  - Of course this works with Doubles  I think  most practical  if you want digits to be rounded  not cut off  it would be something like that   infix operator   gt     func   gt   left  Double  right  Int  - gt  Double       if right  lt   0           return round left            let k   pow 10 0  Double right       return round left k    k     For Float only  simply replace Double with Float  pow with powf and round with roundf  Update  I found that it is most practical to use return type Double instead of String  It works the same for String output  i e      println  Pi is roughly   3 1415926   gt  3      prints  Pi is roughly 3 142 So you can use it the same way for Strings  you can even still write  println d    2    but additionally you can also use it to round values directly  i e      d   Double slider value    gt  2   or whatever you need

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My best solution so far  following from David s response   import Foundation  extension Int       func format f  String  - gt  String           return String format      f d   self           extension Double       func format f  String  - gt  String           return String format      f f   self           let someInt   4  someIntFormat    03  println  The integer number   someInt  formatted with     someIntFormat    looks like   someInt format someIntFormat        The integer number 4 formatted with  03  looks like 004  let someDouble   3 14159265359  someDoubleFormat     3  println  The floating point number   someDouble  formatted with     someDoubleFormat    looks like   someDouble format someDoubleFormat        The floating point number 3 14159265359 formatted with   3  looks like 3 142   I think this is the most Swift-like solution  tying the formatting operations directly to the data type  It may well be that there is a built-in library of formatting operations somewhere  or maybe it will be released soon  Keep in mind that the language is still in beta

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Swift 4  let string   String format     2f   locale  Locale current  arguments  15 123

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What about extensions on Double and CGFloat types    extension Double       func formatted   decimalPlaces  Int   - gt  String         let theDecimalPlaces   Int       if decimalPlaces    nil            theDecimalPlaces   decimalPlaces                else            theDecimalPlaces   2               let theNumberFormatter   NumberFormatter         theNumberFormatter formatterBehavior    behavior10 4       theNumberFormatter minimumIntegerDigits   1       theNumberFormatter minimumFractionDigits   1       theNumberFormatter maximumFractionDigits   theDecimalPlaces       theNumberFormatter usesGroupingSeparator   true       theNumberFormatter groupingSeparator             theNumberFormatter groupingSize   3        if let theResult   theNumberFormatter string from  NSNumber value self              return theResult               else            return    self                    Usage    let aNumber  Double   112465848348508 458758344 Swift print  The number    aNumber formatted 2       prints  112 465 848 348 508 46

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use below method  let output   String localizedStringWithFormat     02f   02f   02f   r  g  b   println output

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I don t know about two decimal places  but here s how you can print floats with zero decimal places  so I d imagine that can be 2 place  3  places      Note  you must convert CGFloat to Double to pass to String format   or it will see a value of zero   func logRect r  CGRect    title  String             println String format        0f    0f      0f    0f                 Double r origin x   Double r origin y   Double r size width   Double r size height   title

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You can also create an operator in this way   operator infix  lt -     func  lt -  format  String  args  CVarArg   - gt  String       return String format  format  arguments  args     let str     d   1f   lt -  1453  1 123

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less typing way   func fprint format  String    args  CVarArgType           print NSString format  format  arguments  getVaList args

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Also with rounding   extension Float       func format f  String  - gt  String               return NSString format      f f   self            mutating func roundTo f  String                self   NSString format      f f   self  floatValue          extension Double       func format f  String  - gt  String               return NSString format      f f   self            mutating func roundTo f  String                self   NSString format      f f   self  doubleValue          x   0 90695652173913 x roundTo   2   println x    0 91

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A version of Vincent Guerci s ruby   python   operator  updated for Swift 2 1   func   format String  args  CVarArgType   - gt  String     return String format format  arguments args      Hello     This is pi     2f      World   M PI

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use   CGFloat    or   Float roundTo places 2

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It will more help  by specify how much decimal Point you want  let decimalPoint   2 let floatAmount   1 10001 let amountValue   String format    0  f   decimalPoint  floatAmount

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I found String localizedStringWithFormat to work quite well   Example   let value  Float   0 33333 let unit  String    mph   yourUILabel text   String localizedStringWithFormat    2f      value  unit

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extension Double     func formatWithDecimalPlaces decimalPlaces  Int  - gt  Double        let formattedString   NSString format       decimalPlaces f   self  as String      return Double formattedString               1 3333 formatWithDecimalPlaces 2

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You can still use NSLog in Swift as in Objective-C just without the   sign   NSLog    02f   02f   02f   r  g  b    Edit  After working with Swift since a while I would like to add also this variation      var r 1 2     var g 1 3     var b 1 4     NSLog    r    g    b      Output   2014-12-07 21 00 42 128 MyApp 1626 60b  1 2 1 3 1 4

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The answers given so far that have received the most votes are relying on NSString methods and are going to require that you have imported Foundation   Having done that  though  you still have access to NSLog   So I think the answer to the question  if you are asking how to continue using NSLog in Swift  is simply   import Foundation

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This is a very fast and simple way who doesn t need complex solution   let duration   String format     01f   3 32323242     result   3 3

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here a  pure  swift solution   var d   1 234567 operator infix   gt      infix func   gt   left  Double  right  Int  - gt  String       if right    0           return    Int left              var k   1 0     for i in 1  right 1           k   10 0   k           let n   Double Int left k     Double k      return    n     println    d  gt 2    println    d  gt 1    println    d  gt 0

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infix func   left Double  right  Int  - gt  NSNumber       let nf   NSNumberFormatter       nf maximumSignificantDigits   Int right      return  nf numberFromString nf stringFromNumber left       let r   0 52264 let g   0 22643 let b   0 94837  println  this is a color    r 3    g 3    b 3        this is a color  0 523 0 226 0 948

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Most answers here are valid  However  in case you will format the number often  consider extending the Float class to add a method that returns a formatted string  See example code below  This one achieves the same goal by using a number formatter and extension   extension Float       func string fractionDigits Int  - gt  String           let formatter   NSNumberFormatter           formatter minimumFractionDigits   fractionDigits         formatter maximumFractionDigits   fractionDigits         return formatter stringFromNumber self        self            let myVelocity Float   12 32982342034  println  The velocity is   myVelocity string 2     println  The velocity is   myVelocity string 1       The console shows   The velocity is 12 33 The velocity is 12 3   SWIFT 3 1 update  extension Float       func string fractionDigits Int  - gt  String           let formatter   NumberFormatter           formatter minimumFractionDigits   fractionDigits         formatter maximumFractionDigits   fractionDigits         return formatter string from  NSNumber value  self         self

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You can t do it  yet  with string interpolation   Your best bet is still going to be NSString formatting   println NSString format    2f   sqrt 2 0      Extrapolating from python  it seems like a reasonable syntax might be    infix func    value Double  format String  - gt  String       return NSString format format  value      Which then allows you to use them as   M PI     5 3f                     3 142    You can define similar operators for all of the numeric types  unfortunately I haven t found a way to do it with generics   Swift 5 Update  As of at least Swift 5  String directly supports the format  initializer  so there s no need to use NSString and the  infix attribute is no longer needed which means the samples above should be written as   println String format    2f   sqrt 2 0     func   value Double  format String  - gt  String       return String format format  value     Double pi     5 3f              3 142

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Power of extension  extension Double       var asNumber String           if self  gt   0               var formatter   NSNumberFormatter               formatter numberStyle    NoStyle             formatter percentSymbol                  formatter maximumFractionDigits   1             return    formatter stringFromNumber self                       return             let velocity Float   12 32982342034  println  The velocity is   velocity toNumber      Output  The velocity is 12 3

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Plenty of good answers above  but sometimes a pattern is more appropriate than the    3f  sort of gobbledygook  Here s my take using a NumberFormatter in Swift 3   extension Double     func format   pattern  String  - gt  String       let formatter   NumberFormatter       formatter format   pattern     return formatter string from  NSNumber value  self               let n1   0 350  n2   0 355 print n1 format  0 00        0 35 print n2 format  0 00        0 355   Here I wanted 2 decimals to be always shown  but the third only if it wasn t zero

[swift] Precision String Format Specifier In Swift

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