rand between 0 and 1

Question

So the following code makes 0  lt  r  lt  1   r     double  rand      RAND MAX     Why does having  r     double  rand      RAND MAX   1   make -1  lt  r  lt  0   Shouldn t adding one to RAND MAX make 1  lt  r  lt  2   Edit  I was getting a  warning  integer overflow in expression  on that line  so that might be the problem  I just did cout  lt  lt  r  lt  lt  endl and it definitely gives me values between -1 and 0

User · Answer

No  because RAND MAX is typically expanded to MAX INT  So adding one  apparently  puts it at MIN INT  although it should be undefined behavior as I m told   hence the reversal of sign   To get what you want you will need to move the  1 outside the computation   r     double  rand      RAND MAX     1

User · Answer

My guess is that RAND MAX is equal to INT MAX and so you re overflowing it to a negative   Just do this   r     double  rand      RAND MAX     1    Or even better  use C  11 s random number generators

User · Answer

It doesn t  It makes 0  lt   r  lt  1  but your original is 0  lt   r  lt   1   Note that this can lead to undefined behavior if RAND MAX   1 overflows

User · Answer

This is the right way   double randd       return  double rand       double RAND MAX   1       or  double randd       return  double rand      RAND MAX   1 0

User · Answer

rand     double RAND MAX  generates a floating-point random number between 0  inclusive  and 1  inclusive   but it s not a good way for the following reasons  because RAND MAX is usually 32767     The number of different random numbers that can be generated is too small  32768  If you need more different random numbers  you need a different way  a code example is given below  The generated numbers are too coarse-grained  you can get 1 32768  2 32768  3 32768  but never anything in between  Limited states of random number generator engine  after generating RAND MAX random numbers  implementations usually start to repeat the same sequence of random numbers    Due to the above limitations of rand    a better choice for generation of random numbers between 0  inclusive  and 1  exclusive  would be the following snippet  similar to the example at http   en cppreference com w cpp numeric random uniform real distribution      include  lt iostream gt   include  lt random gt   include  lt chrono gt   int main         std  mt19937 64 rng         initialize the random number generator with time-dependent seed     uint64 t timeSeed   std  chrono  high resolution clock  now   time since epoch   count        std  seed seq ss uint32 t timeSeed  amp  0xffffffff   uint32 t timeSeed gt  gt 32        rng seed ss          initialize a uniform distribution between 0 and 1     std  uniform real distribution lt double gt  unif 0  1          ready to generate random numbers     const int nSimulations   10      for  int i   0  i  lt  nSimulations  i                  double currentRandomNumber   unif rng           std  cout  lt  lt  currentRandomNumber  lt  lt  std  endl            return 0      This is easy to modify to generate random numbers between 1  inclusive  and 2  exclusive  by replacing unif 0  1  with unif 1  2

User · Answer

In my case  I m using VS 2017  works fine the following simple code    include  pch h   include  lt iostream gt   include  lt stdlib h gt   include  lt time h gt   int main         srand time NULL         for  int i   1000  i  gt  0  i--    try it thousand times               int randnum    double rand       double RAND MAX   1           std  cout  lt  lt    rnum     lt  lt  rand   2

User · Answer

This is entirely implementation specific  but it appears that in the C   environment you re working in  RAND MAX is equal to INT MAX   Because of this  RAND MAX   1 exhibits undefined  overflow  behavior  and becomes INT MIN  While your initial statement was dividing  random   between 0 and INT MAX   INT MAX  and generating a value 0  lt   r  lt  1  now it s dividing  random   between 0 and INT MAX   INT MIN   generating a value -1  lt  r  lt   0  In order to generate a random number 1  lt   r  lt  2  you would want  r     double  rand      RAND MAX     1

[c++] rand() between 0 and 1

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