How do you generate a random double uniformly distributed between 0 and 1 from C

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How do you generate a random double uniformly distributed between 0 and 1 from C     Of course I can think of some answers  but I d like to know what the standard practice is  to have    Good standards compliance Good randomness Good speed    speed is more important than randomness for my application    Thanks a lot   PS  In case that matters  my target platforms are Linux and Windows

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double randDouble       double out    out    double rand    RAND MAX   1     each iteration produces a number in  0  1    out    rand     out  RAND MAX    out    rand     out  RAND MAX    out    rand     out  RAND MAX    out    rand     out  RAND MAX    out    rand     out  RAND MAX     return out      Not quite as fast as double X   double rand    double RAND MAX    but with better distribution   That algorithm gives only RAND MAX evenly spaced choice of return values  this one gives RANDMAX 6  so its distribution is limited only by the precision of double   If you want a long double just add a few iterations   If you want a number in  0  1  rather than  0  1  just make line 4 read out    double rand    RAND MAX

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The C  11 standard  library contains a decent framework and a couple of serviceable generators  which is perfectly sufficient for homework assignments and off-the-cuff use    However  for production-grade code you should know exactly what the specific properties of the various generators are before you use them  since all of them have their caveats  Also  none of them passes standard tests for PRNGs like TestU01  except for the ranlux generators if used with a generous luxury factor   If you want solid  repeatable results then you have to bring your own generator   If you want portability then you have to bring your own generator    If you can live with restricted portability then you can use boost  or the C  11  framework in conjunction with your own generator s    More detail - including code for a simple yet fast generator of excellent quality and copious links - can be found in my answers to similar topics    General purpose random number generation Very fast uniform distribution random number generator   For professional uniform floating-point deviates there are two more issues to consider    open vs  half-open vs  closed range  i e   0 1    0  1  or  0 1  method of conversion from integral to floating-point  precision  speed    Both are actually two sides of the same coin  as the method of conversion takes care of the inclusion exclusion of 0 and 1  Here are three different methods for the half-open interval       exact values computed with bc   define POW2 M32   2 3283064365386962890625e-010  define POW2 M64   5 421010862427522170037264004349e-020  double random double a         double lo   random uint32     POW2 M64     return lo   random uint32     POW2 M32     double random double b         return random uint64     POW2 M64     double random double c         return int64 t random uint64      POW2 M64   0 5       random uint32   and random uint64   are placeholders for your actual functions and would normally be passed as template parameters   Method a demonstrates how to create a uniform deviate that is not biassed by excess precision for lower values  the code for 64-bit is not shown because it is simpler and just involves masking off 11 bits  The distribution is uniform for all functions but without this trick there would be more different values in the area closer to 0 than elsewhere  finer grid spacing due to the varying ulp    Method c shows how to get a uniform deviate faster on certain popular platforms where the FPU knows only a signed 64-bit integral type  What you see most often is method b but there the compiler has to generate lots of extra code under the hood to preserve the unsigned semantics   Mix and match these principles to create your own tailored solution   All this is explained in J  rgen Doornik s excellent paper Conversion of High-Period Random Numbers to Floating Point

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Well considering simplicity and speed as your primary criteria  you can add a small generic helper like this  -       C   rand generates random numbers between 0 and RAND MAX  This is quite a big range      Normally one would want the generated random number within a range to be really      useful  So the arguments have default values which can be overridden by the caller   int nextRandomNum int low   0  int high   100  const       int range    high - low    1         this modulo operation does not generate a truly uniformly distributed random number        in the span  since in most cases lower numbers are slightly more likely           but it is generally a good approximation for short spans  Use it if essential       int res     std  rand     high   low        int res   low   static cast lt int gt     range   std  rand       RAND MAX   1 0           return res        Random number generation is a well studied  complex and advanced topic  You can find some simple but useful algorithms here apart from the ones mentioned in other answers -  Eternally Confuzzled

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An old school solution like   double X   double rand    double RAND MAX     Should meet all your criteria  portable  standard and fast   obviously the random number generated has to be seeded the standard procedure is something like   srand  unsigned time NULL

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In C  11 and C  14 we have much better options with the random header  The presentation rand   Considered Harmful  by Stephan T  Lavavej  explains why we should eschew the use of rand   in C   in favor of the random header and N3924  Discouraging rand   in C  14 further reinforces this point   The example below is a modified version of the sample code on the cppreference site and uses the std  mersenne twister engine engine and the std  uniform real distribution which generates numbers in the  0 1  range  see it live     include  lt iostream gt   include  lt iomanip gt   include  lt map gt   include  lt random gt   int main         std  random device rd        std  mt19937 e2 rd          std  uniform real distribution lt  gt  dist 0  1        std  map lt int  int gt  hist      for  int n   0  n  lt  10000    n              hist std  round dist e2                for  auto p   hist            std  cout  lt  lt  std  fixed  lt  lt  std  setprecision 1   lt  lt  std  setw 2                     lt  lt  p first  lt  lt       lt  lt  std  string p second 200        lt  lt    n             output will be similar to the following   0                          1                             Since the post mentioned that speed was important then we should consider the cppreference section that describes the different random number engines  emphasis mine       The choice of which engine to use involves a number of tradeoffs   the     linear congruential engine is moderately fast and has a very small   storage requirement for state  The lagged Fibonacci generators are   very fast even on processors without advanced arithmetic instruction   sets  at the expense of greater state storage and sometimes less   desirable spectral characteristics  The Mersenne twister is slower and   has greater state storage requirements but with the right parameters   has the longest non-repeating sequence with the most desirable   spectral characteristics  for a given definition of desirable     So if there is a desire for a faster generator perhaps ranlux24 base or ranlux48 base are better choices over mt19937   rand    If you forced to use rand   then the C FAQ for a guide on How can I generate floating-point random numbers   gives us an example similar to this for generating an on the interval  0 1     include  lt stdlib h gt   double randZeroToOne         return rand      RAND MAX   1        and to generate a random number in the range from  M N    double randMToN double M  double N        return M    rand       RAND MAX    N-M

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You could try the Mersenne Twister algorithm     http   en wikipedia org wiki Mersenne twister  It has a good blend of speed and randomness  and a GPL implementation

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Here s how you d do it if you were using C   TR1

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The random real class from the Boost random library is what you need

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If speed is your primary concern  then I d simply go with  double r    double rand      double RAND MAX

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Returns a random number in the range  0 0f  1 0f      0111 1111 1111 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000    seee eeee eeee vvvv vvvv vvvv vvvv vvvv vvvv vvvv vvvv vvvv vvvv vvvv vvvv vvvv    sign        s     exponent    e     value       v  double DoubleRand       typedef unsigned long long uint64    uint64 ret   0    for  int i   0  i  lt  13  i           ret      uint64   rand     16   lt  lt  i   4         if  ret    0        return rand     2   1 0f   0 0f        uint64 retb   ret    unsigned int exp   0x3ff    retb   ret     uint64  exp  lt  lt  52     double  tmp    double    amp retb    double retval    tmp    while  retval  gt  1 0f    retval  lt  0 0f        retval     tmp    double    amp  retb   ret     uint64   exp--   lt  lt  52           if  rand     2        retval -  0 5f        return retval      This should do the trick  I used this Wikipedia article to help create this  I believe it to be as good as drand48

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First include stdlib h   include lt stdlib h gt    Then following can be a function to generate random double number between a range in C programming language   double randomDouble         double lowerRange   1 0      double upperRange   10 0      return   double rand      upperRange - lowerRange      double RAND MAX   lowerRange      Here RAND MAX is defined in stdlib h

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As I see it  there are three ways to go with this   1  The easy way   double rand easy void          return  double  rand      RAND MAX   1 0       2  The safe way  standard conforming    double rand safe void            double limit   pow 2 0  DBL MANT DIG           double denom   RAND MAX   1 0          double denom to k   1 0          double numer   0 0           for     denom to k  lt  limit  denom to k    denom              numer    rand     denom to k           double result   numer   denom to k          if  result    1 0             result -  DBL EPSILON 2          assert result    1 0           return result      3  The custom way   By eliminating rand   we no longer have to worry about the idiosyncrasies of any particular version  which gives us more leeway in our own implementation   Note  Period of the generator used here is  cong  1 8e 19    define RANDMAX  -1ULL  uint64 t custom lcg uint fast64 t  next          return  next    next   2862933555777941757ULL   3037000493ULL     uint fast64 t internal next  void seed fast uint64 t seed          internal next   seed     double rand fast void     define SHR BIT  64 -  DBL MANT DIG-1           union               double f  uint64 t i            u          u f   1 0          u i   u i    custom lcg  amp internal next   gt  gt  SHR BIT           return u f - 1 0      Whatever the choice  functionality may be extended as follows   double rand dist double min  double max          return rand fast      max - min    min     double rand open void          return rand dist DBL EPSILON  1 0      double rand closed void          return rand dist 0 0  1 0   DBL EPSILON       Final notes  The fast version - while written in C - may be adapted for use in C   to be used as a replacement for std  generate canonical  and will work for any generator emitting values with sufficient significant bits   Most 64 bit generators take advantage of their full width  so this can likely be used without modification  shift adjustment   e g  this works as-is with the std  mt19937 64 engine

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This is what I ended up using for my needs   int range upper bound   12345  int random number    double rand    double range upper bound

[c++] How do you generate a random double uniformly distributed between 0 and 1 from C++?

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