Random Numbers

• To complete our tour of basic C library functions, I should mention random numbers. int rand(); returns a random number between 0 and 2^15 - 1.

• How would I get a random integer between 1 and 6? A random float between 0 and 1?

• Of course, int rand(); isn't very random in the sense that it will output the same stream of numbers every time you run your program. To get a different sequence of random numbers every time you run your program, you must seed the random number generator (give it a different starting point) with the void srand(unsigned int seed) function.

• Popular choices for seeds are:
  • Your program's process id number, which is different from every process and can be obtained by the int getpid() call.
  • The time since midnight, GMT, January 1, 1970 which can be obtained by the time(NULL) call. time is defined in time.h and returns a long int.

• rand isn't a very good random number generator, and there are better ones (such as random()) on most systems. It is, on the other hand, fast and easy to implement:

  static unsigned int next = 1;
    
  int rand( )
    {
    next = next * 1103515245 + 12345;
    return ((next >>16) & 32767);
    }