C++中隨機函數賦值

These functions generate pseudo-random numbers using the linear
congruential algorithm and 48-bit integer arithmetic.

   The drand48() and erand48() functions return nonnegative double-
   precision floating-point values uniformly distributed over the
   interval [0.0, 1.0).

   The lrand48() and nrand48() functions return nonnegative long
   integers uniformly distributed over the interval [0, 2^31).

   The mrand48() and jrand48() functions return signed long integers
   uniformly distributed over the interval [-2^31, 2^31).

   The srand48(), seed48() and lcong48() functions are initialization
   functions, one of which should be called before using drand48(),
   lrand48() or mrand48().  The functions erand48(), nrand48() and
   jrand48() do not require an initialization function to be called
   first.