Generating random numbers in Objective-C

Question

I m a Java head mainly  and I want a way to generate a pseudo-random number between 0 and 74  In Java I would use the method   Random nextInt 74    I m not interested in a discussion about seeds or true randomness  just how you accomplish the same task in Objective-C  I ve scoured Google  and there just seems to be lots of different and conflicting bits of information

User · Answer

The following example is going to generate a number between 0 and 73   int value  value    arc4random     74   NSLog   random number   i    value      In order to generate 1 to 73  do the following  int value1  value1    arc4random     73    1  NSLog   random number step 2   i    value1     Output    random number  72  random number step 2  52

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Generate random number between 0 to 99   int x   arc4random   100    Generate random number between 500 and 1000   int x    arc4random   501    500

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There are some great  articulate answers already  but the question asks for a random number between 0 and 74  Use   arc4random uniform 75

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According to the manual page for rand 3   the rand family of functions have been obsoleted by random 3   This is due to the fact that the lower 12 bits of rand   go through a cyclic pattern  To get a random number  just seed the generator by calling srandom   with an unsigned seed  and then call random    So  the equivalent of the code above would be   import  lt stdlib h gt   import  lt time h gt   srandom time NULL    random     74    You ll only need to call srandom   once in your program unless you want to change your seed  Although you said you didn t want a discussion of truly random values  rand   is a pretty bad random number generator  and random   still suffers from modulo bias  as it will generate a number between 0 and RAND MAX  So  e g  if RAND MAX is 3  and you want a random number between 0 and 2  you re twice as likely to get a 0 than a 1 or a 2

User · Answer

I wrote my own random number utility class just so that I would have something that functioned a bit more like Math random   in Java  It has just two functions  and it s all made in C   Header file     Random h void initRandomSeed long firstSeed   float nextRandomFloat      Implementation file     Random m static unsigned long seed   void initRandomSeed long firstSeed         seed   firstSeed     float nextRandomFloat         return    seed  1664525 seed   1013904223  gt  gt 16     float 0x10000       It s a pretty classic way of generating pseudo-randoms  In my app delegate I call    import  Random h   -  void applicationDidFinishLaunching  UIApplication   application       initRandomSeed   long    NSDate date  timeIntervalSince1970           Do other initialization junk      Then later I just say   float myRandomNumber   nextRandomFloat     74    Note that this method returns a random number between 0 0f  inclusive  and 1 0f  exclusive

User · Answer

For game dev use random   to generate randoms  Probably at least 5x faster than using arc4random    Modulo bias is not an issue  especially for games  when generating randoms using the full range of random    Be sure to seed first  Call srandomdev   in AppDelegate  Here s some helper functions   static inline int random range int low  int high   return  random    high-low 1   low   static inline CGFloat frandom    return  CGFloat random   UINT32 C 0x7FFFFFFF    static inline CGFloat frandom range CGFloat low  CGFloat high   return  high-low  frandom   low

User · Answer

Better to use arc4random uniform  However  this isn t available below iOS 4 3  Luckily iOS will bind this symbol at runtime  not at compile time  so don t use the  if preprocessor directive to check if it s available    The best way to determine if arc4random uniform is available is to do something like this    include  lt stdlib h gt   int r   0  if  arc4random uniform    NULL      r   arc4random uniform  74   else     r    arc4random     74

User · Answer

Use the arc4random uniform upper bound  function to generate a random number within a range    The following will generate a number between 0 and 73 inclusive   arc4random uniform 74    arc4random uniform upper bound  avoids modulo bias as described in the man page      arc4random uniform   will return a uniformly distributed random number less than upper bound   arc4random uniform   is recommended over constructions like   arc4random     upper bound   as it avoids  modulo bias  when the upper bound is not a power of two

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I thought I could add a method I use in many projects   -  NSInteger randomValueBetween  NSInteger min and  NSInteger max       return  NSInteger  min   arc4random uniform max - min   1        If I end up using it in many files I usually declare a macro as   define RAND FROM TO min  max   min   arc4random uniform max - min   1     E g   NSInteger myInteger   RAND FROM TO 0  74     0  1  2      73  74   Note  Only for iOS 4 3 OS nbsp X nbsp v10 7  Lion  and later

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Same as C  you would do   include  lt time h gt   include  lt stdlib h gt      srand time NULL    int r   rand     74     assuming you meant including 0 but excluding 74  which is what your Java example does   Edit  Feel free to substitute random   or arc4random   for rand    which is  as others have pointed out  quite sucky

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As of iOS 9 and OS X 10 11  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   NSInteger rand     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   NSInteger rand6     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   GKRandomDistribution  d6    GKRandomDistribution d6    d6 nextInt     Plus you can shape the random distribution by using things like GKShuffledDistribution

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This will give you a floating point number between 0 and 47  float low bound   0        float high bound   47  float rndValue      float arc4random   0x100000000   high bound-low bound  low bound     Or just simply  float rndValue      float arc4random   0x100000000  47     Both lower and upper bound can be negative as well  The example code below gives you a random number between -35 76 and  12 09  float low bound   -35 76        float high bound   12 09  float rndValue      float arc4random   0x100000000   high bound-low bound  low bound     Convert result to a rounder Integer value   int intRndValue    int  rndValue   0 5

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You should use the arc4random uniform   function  It uses a superior algorithm to rand  You don t even need to set a seed    include  lt stdlib h gt                int r   arc4random uniform 74     The arc4random man page    NAME      arc4random  arc4random stir  arc4random addrandom -- arc4 random number generator  LIBRARY      Standard C Library  libc  -lc   SYNOPSIS       include  lt stdlib h gt        u int32 t      arc4random void         void      arc4random stir void         void      arc4random addrandom unsigned char  dat  int datlen    DESCRIPTION      The arc4random   function uses the key stream generator employed by the arc4 cipher  which uses 8 8 8      bit S-Boxes   The S-Boxes can be in about  2  1700  states   The arc4random   function returns pseudo-      random numbers in the range of 0 to  2  32 -1  and therefore has twice the range of rand 3  and      random 3         The arc4random stir   function reads data from  dev urandom and uses it to permute the S-Boxes via      arc4random addrandom          There is no need to call arc4random stir   before using arc4random    since arc4random   automatically      initializes itself   EXAMPLES      The following produces a drop-in replacement for the traditional rand   and random   functions using      arc4random                 define foo4random    arc4random       unsigned RAND MAX   1

[objective-c] Generating random numbers in Objective-C

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