Generating random numbers in C

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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 <stdlib.h>

int main ()
{
  srand ( 123 );
  int random_number = rand();
  return 0;
}

or call it with a changing sources, ie the time function

#include <stdlib.h>
#include <time.h>

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.

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