How does one generate a random number in Apple s Swift language

Question

I realize the Swift book provided an implementation of a random number generator   Is the best practice to copy and paste this implementation in one s own program   Or is there a library that does this that we can use now

User · Answer

jstn s answer is good  but a bit verbose  Swift is known as a protocol-oriented language  so we can achieve the same result without having to implement boilerplate code for every class in the integer family  by adding a default implementation for the protocol extension   public extension ExpressibleByIntegerLiteral       public static func arc4random   - gt  Self           var r  Self   0         arc4random buf  amp r  MemoryLayout lt Self gt  size          return r           Now we can do   let i   Int arc4random   let j   UInt32 arc4random     and all other integer classes are ok

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You can use GeneratorOf like this   var fibs   ArraySlice  1  1   var fibGenerator   GeneratorOf        - gt  Int  in     fibs append fibs reduce 0  combine         return fibs removeAtIndex 0     println fibGenerator next    println fibGenerator next    println fibGenerator next    println fibGenerator next    println fibGenerator next    println fibGenerator next

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I use this code to generate a random number              FactModel swift     Collection        Created by Ahmadreza Shamimi on 6 11 16      Copyright    2016 Ahmadreza Shamimi  All rights reserved      import GameKit  struct FactModel        let fun      I love swift   My name is Ahmadreza   I love coding    I love PHP   My name is ALireza   I love Coding too         func getRandomNumber   - gt  String            let randomNumber    GKRandomSource sharedRandom   nextIntWithUpperBound fun count           return fun randomNumber

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Example for random number in between 10  0-9    import UIKit  let randomNumber   Int arc4random uniform 10     Very easy code - simple and short

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Use arc4random uniform   Usage  arc4random uniform someNumber  UInt32  - gt  UInt32 This gives you random integers in the range 0 to someNumber - 1  The maximum value for UInt32 is 4 294 967 295  that is  2 32 - 1   Examples   Coin flip   let flip   arc4random uniform 2     0 or 1   Dice roll   let roll   arc4random uniform 6    1    1   6   Random day in October   let day   arc4random uniform 31    1    1   31   Random year in the 1990s   let year   1990   arc4random uniform 10     General form  let number   min   arc4random uniform max - min   1   where number  max  and min are UInt32  What about    arc4random   You can also get a random number by using arc4random    which produces a UInt32 between 0 and 2 32-1  Thus to get a random number between 0 and x-1  you can divide it by x and take the remainder  Or in other words  use the Remainder Operator      let number   arc4random     5    0   4  However  this produces the slight modulo bias  see also here and here   so that is why arc4random uniform   is recommended  Converting to and from Int Normally it would be fine to do something like this in order to convert back and forth between Int and UInt32  let number  Int   10 let random   Int arc4random uniform UInt32 number     The problem  though  is that Int has a range of -2 147 483 648   2 147 483 647 on 32 bit systems and a range of -9 223 372 036 854 775 808   9 223 372 036 854 775 807 on 64 bit systems  Compare this to the UInt32 range of 0   4 294 967 295  The U of UInt32 means unsigned  Consider the following errors  UInt32 -1     negative numbers cause integer overflow error UInt32 4294967296     numbers greater than 4 294 967 295 cause integer overflow error  So you just need to be sure that your input parameters are within the UInt32 range and that you don t need an output that is outside of that range either

User · Answer

I ve been able to just use rand   to get a random CInt  You can make it an Int by using something like this    let myVar  Int   Int rand      You can use your favourite C random function  and just convert to value to Int if needed

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I would like to add to existing answers that the random number generator example in the Swift book is a Linear Congruence Generator  LCG   it is a severely limited one and shouldn t be except for the must trivial examples  where quality of randomness doesn t matter at all  And a LCG should never be used for cryptographic purposes    arc4random   is much better and can be used for most purposes  but again should not be used for cryptographic purposes   If you want something that is guaranteed to be cryptographically secure  use SecCopyRandomBytes     Note that if you build a random number generator into something  someone else might end up  mis -using it for cryptographic purposes  such as password  key or salt generation   then you should consider using SecCopyRandomBytes   anyway  even if your need doesn t quite require that

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Edit  Updated for Swift 3 0  arc4random works well in Swift  but the base functions are limited to 32-bit integer types  Int is 64-bit on iPhone 5S and modern Macs   Here s a generic function for a random number of a type expressible by an integer literal   public func arc4random lt T  ExpressibleByIntegerLiteral gt    type  T Type  - gt  T       var r  T   0     arc4random buf  amp r  MemoryLayout lt T gt  size      return r     We can use this new generic function to extend UInt64  adding boundary arguments and mitigating modulo bias   This is lifted straight from arc4random c   public extension UInt64       public static func random lower  UInt64   min  upper  UInt64   max  - gt  UInt64           var m  UInt64         let u   upper - lower         var r   arc4random UInt64 self           if u  gt  UInt64 Int64 max                m   1    u           else               m     max -  u   2     1    u                    while r  lt  m               r   arc4random UInt64 self                     return  r   u    lower           With that we can extend Int64 for the same arguments  dealing with overflow   public extension Int64       public static func random lower  Int64   min  upper  Int64   max  - gt  Int64           let  s  overflow    Int64 subtractWithOverflow upper  lower          let u   overflow   UInt64 max - UInt64  s    UInt64 s          let r   UInt64 random upper  u           if r  gt  UInt64 Int64 max                 return Int64 r -  UInt64  lower    1             else               return Int64 r    lower                     To complete the family     private let  wordSize     WORDSIZE  public extension UInt32       public static func random lower  UInt32   min  upper  UInt32   max  - gt  UInt32           return arc4random uniform upper - lower    lower          public extension Int32       public static func random lower  Int32   min  upper  Int32   max  - gt  Int32           let r   arc4random uniform UInt32 Int64 upper  - Int64 lower            return Int32 Int64 r    Int64 lower            public extension UInt       public static func random lower  UInt   min  upper  UInt   max  - gt  UInt           switch   wordSize                case 32  return UInt UInt32 random UInt32 lower   upper  UInt32 upper                case 64  return UInt UInt64 random UInt64 lower   upper  UInt64 upper                default  return lower                    public extension Int       public static func random lower  Int   min  upper  Int   max  - gt  Int           switch   wordSize                case 32  return Int Int32 random Int32 lower   upper  Int32 upper                case 64  return Int Int64 random Int64 lower   upper  Int64 upper                default  return lower                     After all that  we can finally do something like this   let diceRoll   UInt64 random lower  1  upper  7

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I used this code   var k  Int   random     10

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Swift 4 2  Bye bye to import Foundation C lib  arc4random uniform       1   let digit   Int random in  0   lt 10      2 if let anotherDigit    0   lt 10  randomElement       print anotherDigit    else     print  Empty range          3 let double   Double random in  0   lt 1  let float   Float random in  0   lt 1  let cgFloat   CGFloat random in  0   lt 1  let bool   Bool random      You use random in   to generate random digits from ranges  randomElement   returns nil if the range is empty  so you unwrap the returned Int  with if let  You use random in   to generate a random Double  Float or CGFloat and random   to return a random Bool    More   Official

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let MAX   UInt32   9  let MIN   UInt32   1      func randomNumber         var random number   Int arc4random uniform MAX    MIN      print   random      random number

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Swift 4 2   Swift 4 2 shipped with Xcode 10 introduces new easy-to-use random functions for many data types  You can call the random   method on numeric types   let randomInt   Int random in  0   lt 6  let randomDouble   Double random in  2 71828   3 14159  let randomBool   Bool random

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Swift 4 2  Swift 4 2 has included a native and fairly full-featured random number API in the standard library   Swift Evolution proposal SE-0202   let intBetween0to9   Int random in  0   9   let doubleBetween0to1   Double random in  0   1    All number types have the static random in   which takes the range and returns the random number in the given range

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var randomNumber   Int arc4random uniform UInt32 5      Here 5 will make sure that the random number is generated through zero to four  You can set the value accordingly

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In Swift 4 2 you can generate random numbers by calling the random   method on whatever numeric type you want  providing the range you want to work with  For example  this generates a random number in the range 1 through 9  inclusive on both sides  let randInt   Int random in  1   lt 10    Also with other types   let randFloat   Float random in  1   lt 20  let randDouble   Double random in  1   30  let randCGFloat   CGFloat random in  1   40

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Updated  August 06  2020  Swift 5 3 Let s assume we have an array  let numbers   Int     1  2  3  4  5  6  7  8  9  10    For iOS and macOS you can use system-wide random source in Xcode s framework GameKit  Here you can find GKRandomSource class with its sharedRandom   class method  import GameKit  private func randomNumberGenerator   - gt  Int       let random   GKRandomSource sharedRandom   nextInt upperBound  numbers count      return numbers random     randomNumberGenerator     Also you can use a randomElement   method that returns a random element of a collection  let randomNumber   numbers randomElement    print randomNumber    Or use arc4random uniform    Pay attention that this method returns UInt32  let generator   Int arc4random uniform 10   print generator    And  of course  we can use a makeIterator   method that returns an iterator over the elements of the collection  let iterator  Int    1   10  makeIterator   shuffled   first  print iterator    The final example you see here returns a random value within the specified range with a help of static func random in range  ClosedRange lt Int gt   - gt  Int  let randomizer   Int random in  1   10  print randomizer

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Since Swift 4 2  There is a new set of APIs   let randomIntFrom0To10   Int random in  0    lt  10  let randomDouble   Double random in  1     10     All numeric types now have the random in   method that takes range  It returns a number uniformly distributed in that range      TL DR  Well  what is wrong with the  good  old way    You have to use imported C APIs  They are different between platforms   And moreover         What if I told you that the random is not that random    If you use arc4random    to calculate the remainder  like arc4random     aNumber  the result is not uniformly distributed between the 0 and aNumber  There is a problem called the Modulo bias   Modulo bias  Normally  the function generates a random number between 0 and MAX  depends on the type etc    To make a quick  easy example  let s say the max number is 7 and you care about a random number in the range 0    lt  2  or the interval  0  3  if you prefer that    The probabilities for individual numbers are    0  3 8   37 5  1  3 8   37 5  2  2 8   25    In other words  you are more likely to end up with 0 or 1 than 2    Of course  bare in mind that this is extremely simplified and the MAX number is much higher  making it more  fair       This problem is addressed by SE-0202 - Random unification in Swift 4 2

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As of iOS 9  you can use the new GameplayKit classes to generate random numbers in a variety of ways   You have four source types to choose from  a general random source  unnamed  down to the system to choose what it does   linear congruential  ARC4 and Mersenne Twister  These can generate random ints  floats and bools   At the simplest level  you can generate a random number from the system s built-in random source like this   GKRandomSource sharedRandom   nextInt     That generates a number between -2 147 483 648 and 2 147 483 647  If you want a number between 0 and an upper bound  exclusive  you d use this   GKRandomSource sharedRandom   nextIntWithUpperBound 6    GameplayKit has some convenience constructors built in to work with dice  For example  you can roll a six-sided die like this   let d6   GKRandomDistribution d6   d6 nextInt     Plus you can shape the random distribution by using things like GKShuffledDistribution  That takes a little more explaining  but if you re interested you can read my tutorial on GameplayKit random numbers

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The following code will produce a secure random number between 0 and 255   extension UInt8     public static var random  UInt8       var number  UInt8   0         SecRandomCopyBytes kSecRandomDefault  1   amp number      return number         You call it like this   print UInt8 random    For bigger numbers it becomes more complicated  This is the best I could come up with   extension UInt16     public static var random  UInt16       let count   Int UInt8 random   2    1     var numbers    UInt8  repeating  0  count  2          SecRandomCopyBytes kSecRandomDefault  count   amp numbers      return numbers reversed   reduce 0     0  lt  lt  8   UInt16  1           extension UInt32     public static var random  UInt32       let count   Int UInt8 random   4    1     var numbers    UInt8  repeating  0  count  4          SecRandomCopyBytes kSecRandomDefault  count   amp numbers      return numbers reversed   reduce 0     0  lt  lt  8   UInt32  1            These methods use an extra random number to determine how many UInt8s are going to be used to create the random number  The last line converts the  UInt8  to UInt16 or UInt32   I don t know if the last two still count as truly random  but you can tweak it to your likings

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You can do it the same way that you would in C   let randomNumber   arc4random     randomNumber is inferred to be of type UInt32  a 32-bit unsigned integer

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Use arc4random uniform n  for a random integer between 0 and n-1   let diceRoll   Int arc4random uniform 6    1    Cast the result to Int so you don t have to explicitly type your vars as UInt32  which seems un-Swifty

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Details  xCode 9 1  Swift 4  Math oriented solution  1   import Foundation  class Random        subscript lt T gt    min  T    max  T  - gt  T where T   BinaryInteger           get               return rand min-1  max 1                     let rand   Random    func rand lt T gt    min  T    max  T  - gt  T where T   BinaryInteger       let  min   min   1     let difference   max -  min     return T arc4random uniform UInt32 difference       min     Usage of solution  1   let x   rand -5  5           x    -4  -3  -2  -1  0  1  2  3  4  let x   rand 0  10           x    0  1  2  3  4  5  6  7  8  9  10    Programmers oriented solution  2      Do not forget to add Math oriented solution  1  code here   import Foundation  extension CountableRange where Bound   BinaryInteger        var random  Bound           return rand lowerBound-1  upperBound           extension CountableClosedRange where Bound   BinaryInteger        var random  Bound           return rand lowerBound  upperBound            Usage of solution  2   let x    -8   lt 2  random              x    -8  -7  -6  -5  -4  -3  -2  -1  0  1  let x    0   lt 10  random              x    0  1  2  3  4  5  6  7  8  9  let x    -10     -2  random          x    -10  -9  -8  -7  -6  -5  -4  -3  -2    Full Sample     Do not forget to add solution  1  and solution  2  codes here   private func generateRandNums closure   - gt  Int          var allNums   Set lt Int gt        for   in 0   lt 100           allNums insert closure              print allNums sorted   0  lt   1       generateRandNums        -8   lt 2  random    generateRandNums        0   lt 10  random    generateRandNums        -10     -2  random    generateRandNums       rand -5  5    generateRandNums       rand 0  10      Sample result

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Without arc4Random uniform   in some versions of Xcode in 7 1 it runs but doesn t autocomplete for me   You can do this instead    To generate a random number from 0-5  First  import GameplayKit   Then  let diceRoll   GKRandomSource sharedRandom   nextIntWithUpperBound 6

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Here is a library that does the job well https   github com thellimist SwiftRandom  public extension Int           SwiftRandom extension     public static func random lower  Int   0    upper  Int   100  - gt  Int           return lower   Int arc4random uniform UInt32 upper - lower   1             public extension Double           SwiftRandom extension     public static func random lower  Double   0    upper  Double   100  - gt  Double           return  Double arc4random      0xFFFFFFFF     upper - lower    lower          public extension Float           SwiftRandom extension     public static func random lower  Float   0    upper  Float   100  - gt  Float           return  Float arc4random      0xFFFFFFFF     upper - lower    lower          public extension CGFloat           SwiftRandom extension     public static func random lower  CGFloat   0    upper  CGFloat   1  - gt  CGFloat           return CGFloat Float arc4random      Float UINT32 MAX      upper - lower    lower

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Edit for Swift 4 2  Starting in Swift 4 2  instead of using the imported C function arc4random uniform    you can now use Swift   s own native functions      Generates integers starting with 0 up to  and including  10 Int random in  0     10    You can use random in   to get random values for other primitive values as well  such as Int  Double  Float and even Bool   Swift versions  lt  4 2  This method will generate a random Int value between the given minimum and maximum  func randomInt min  Int  max  Int  - gt  Int       return min   Int arc4random uniform UInt32 max - min   1

[swift] How does one generate a random number in Apple's Swift language?

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