Generating random numbers in C

Question

While searching for Tutorials on generating random numbers in C I found this topic  When I try to use the rand   function without parameters  I always get  0  When I try to use the rand   function with parameters  I always get the value 41  And whenever I try to use arc4random   and random   functions  I get a LNK2019 error   Here s what I ve done    include  lt stdlib h gt  int main       int x    x   rand 6     printf   d   x       This code always generates 41  Where am I going wrong  I m running Windows XP SP3 and using VS2010 Command Prompt as compiler

User · Answer

You need to seed your PRNG so it starts with a different value each time.

A simple but low quality seed is to use the current time:

srand(time(0));

This will get you started but is considered low quality (i.e. for example, don't use that if you are trying to generate RSA keys).

Background. Pseudo-random number generators do not create true random number sequences but just simulate them. Given a starting point number, a PRNG will always return the same sequence of numbers. By default, they start with the same internal state so will return the same sequence.

To not get the same sequence, you change the internal state. The act of changing the internal state is called "seeding".

User · Answer

include  lt stdlib h gt   int main         int x      x   rand 6       printf   d   x       Especially as a beginner  you should ask your compiler to print every warning about bad code that it can generate  Modern compilers know lots of different warnings which help you to program better  For example  when you compile this program with the GNU C Compiler     gcc -W -Wall rand c rand c  In function  main   rand c 5  error  too many arguments to function  rand  rand c 6  warning  implicit declaration of function  printf    You get two warnings here  The first one says that the rand function only takes zero arguments  not one as you tried  To get a random number between 0 and n  you can use the expression rand     n  which is not perfect but ok for small n  The resulting random numbers are normally not evenly distributed  smaller values are returned more often   The second warning tells you that you are calling a function that the compiler doesn t know at that point  You have to tell the compiler by saying  include  lt stdio h gt   Which include files are needed for which functions is not always simple  but asking the Open Group specification for portable operating systems works in many cases  http   www google com search q opengroup rand   These two warnings tell you much about the history of the C programming language  40 years back  the definition of a function didn t include the number of parameters or the types of the parameters  It was also ok to call an unknown function  which in most cases worked  If you want to write code today  you should not rely on these old features but instead enable your compiler s warnings  understand the warnings and then fix them properly

User · Answer

Also  linear congruential PRNGs tend to produce more randomness on the higher bits that on the lower bits  so to cap the result don t use modulo  but instead use something like   j   1    int   10 0    rand      RAND MAX   1 0        This one is from  Numerical Recipes in C   ch 7

User · Answer

You should call srand   before calling rand to initialize the random number generator   Either call it with a specific seed  and you will always get the same pseudo-random sequence   include  lt stdlib h gt   int main        srand   123      int random number   rand      return 0      or call it with a changing sources  ie the time function   include  lt stdlib h gt   include  lt time h gt   int main        srand   time NULL       int random number   rand      return 0      In response to Moon s Comment rand   generates a random number with an equal probability between 0 and RAND MAX  a macro pre-defined in stdlib h   You can then map this value to a smaller range  e g   int random value   rand      between 0 and RAND MAX    you can mod the result int N   33  int rand capped   random value   N     between 0 and 32 int S   50  int rand range   rand capped   S    between 50 and 82    you can convert it to a float float unit random   random value    float  RAND MAX    between 0 and 1  floating point    This might be sufficient for most uses  but its worth pointing out that in the first case using the mod operator introduces a slight bias if N does not divide evenly into RAND MAX 1   Random number generators are interesting and complex  it is widely said that the rand   generator in the C standard library is not a great quality random number generator  read  http   en wikipedia org wiki Random number generation for a definition of quality    http   en wikipedia org wiki Mersenne twister  source http   www math sci hiroshima-u ac jp  m-mat MT emt html   is a popular high quality random number generator   Also  I am not aware of arc4rand   or random   so I cannot comment

User · Answer

You first need to seed the generator because it doesn t generate real random numbers   Try this    include  lt stdlib h gt   include  lt time h gt  int main            random seed  time      srand  time NULL        hackish but gets the job done      int x      x   rand       everytime it is different because the seed is different      printf   d   x

User · Answer

int  generate randomnumbers int start  int end       int  res   malloc sizeof int   end-start        srand time NULL        for  int i  0  i  lt   end -start  1  i             int r   rand   end   start          int dup   0          for  int j   0  j  lt   end -start  1  j                 if  res j     r                   i--                  dup   1                  break                                  if   dup              res i    r            return res

User · Answer

Or  to get a pseudo-random int in the range 0 to 19  for example  you could use the higher bits like this   j     rand    gt  gt  15    20

[c] Generating random numbers in C

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