generate random double numbers in c

Question

How to generate random numbers between two doubles in c     these numbers should look like xxxxx yyyyy

User · Accepted Answer

Here s how  double fRand double fMin  double fMax        double f    double rand     RAND MAX      return fMin   f    fMax - fMin       Remember to call srand   with a proper seed each time your program starts    Edit  This answer is obsolete since C   got it s native non-C based random library  see Alessandro Jacopsons answer   But  this still applies to C

User · Answer

This should be performant  thread-safe and flexible enough for many uses    include  lt random gt   include  lt iostream gt   template lt typename Numeric  typename Generator   std  mt19937 gt  Numeric random Numeric from  Numeric to        thread local static Generator gen std  random device            using dist type   typename std  conditional      lt          std  is integral lt Numeric gt   value           std  uniform int distribution lt Numeric gt            std  uniform real distribution lt Numeric gt       gt   type       thread local static dist type dist       return dist gen  typename dist type  param type from  to       int main int  char           for auto i   0U  i  lt  20    i          std  cout  lt  lt  random lt double gt  0 0  0 3   lt  lt    n

User · Answer

This solution requires C  11  or TR1     include  lt random gt   int main        double lower bound   0     double upper bound   10000     std  uniform real distribution lt double gt  unif lower bound upper bound      std  default random engine re     double a random double   unif re       return 0      For more details see John D  Cook s  Random number generation using C   TR1    See also Stroustrup s  Random number generation

User · Answer

something like this      include  lt iostream gt   include  lt time h gt   using namespace std   int main         const long max rand   1000000L      double x1   12 33  x2   34 123  x       srandom time NULL         x   x1     x2 - x1     random     max rand    max rand       cout  lt  lt  x1  lt  lt     lt      lt  lt  x  lt  lt     lt      lt  lt  x2  lt  lt  endl       return 0

User · Answer

This snippet is straight from Stroustrup s The C   Programming Language  4th Edition     40 7  it requires C  11    include  lt functional gt   include  lt random gt   class Rand double   public      Rand double double low  double high       r std  bind std  uniform real distribution lt  gt  low high  std  default random engine            double operator      return r       private      std  function lt double   gt  r       include  lt iostream gt      int main            create the random number generator      Rand double rd 0 0 5           print 10 random number between 0 and 0 5     for  int i 0 i lt 10   i           std  cout  lt  lt  rd    lt  lt                 return 0

User · Answer

If accuracy is an issue here you can create random numbers with a finer graduation by randomizing the significant bits  Let s assume we want to have a double between 0 0 and 1000 0   On MSVC  12   Win32  RAND MAX is 32767 for example    If you use the common rand   RAND MAX scheme your gaps will be as large as  1 0   32767 0     1000 0 - 0 0    0 0305       In case of IEE 754 double variables  53 significant bits  and 53 bit randomization the smallest possible randomization gap for the 0 to 1000 problem will be  2 -53    1000 0 - 0 0    1 110e-13   and therefore significantly lower   The downside is that 4 rand   calls will be needed to obtain the randomized integral number  assuming a 15 bit RNG    double random range  double const range min  double const range max      static unsigned long long const mant mask53 9007199254740991     static double const i to d53 1 0 9007199254740992 0     unsigned long long const r   unsigned long long rand       unsigned long long rand     lt  lt  15     unsigned long long rand     lt  lt  30     unsigned long long rand     lt  lt  45    amp  mant mask53      return range min   i to d53 double r   range max-range min       If the number of bits for the mantissa or the RNG is unknown the respective values need to be obtained within the function    include  lt limits gt  using namespace std  double random range p  double const range min  double const range max      static unsigned long long const num mant bits numeric limits lt double gt   digits   ll one 1        mant limit ll one  lt  lt  num mant bits     static double const i to d 1 0 double mant limit      static size t num rand calls  rng bits    if  num rand calls    0    rng bits    0          size t const rand max RAND MAX   one 1       while  rand max  gt   one  lt  lt  rng bits                 rng bits            num rand calls   size t ceil double num mant bits  double rng bits           unsigned long long r 0     for  size t i 0  i lt num rand calls    i          r     unsigned long long rand     lt  lt   i rng bits          r   r  amp   mant limit-ll one     return range min   i to d double r   range max-range min       Note  I don t know whether the number of bits for unsigned long long  64 bit  is greater than the number of double mantissa bits  53 bit for  IEE 754  on all platforms or not  It would probably be  smart  to include a check like if  sizeof unsigned long long  8  gt  num mant bits      if this is not the case

[c++] generate random double numbers in c++

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