Question

In java random number can get like

protected final static Random RANDOM = new Random(System.currentTimeMillis()); 

In c++ using GMP Library how it possible to generate random number?

I used code like

    gmp_randstate_t s;
unsigned long seed;
seed = time(NULL); 
gmp_randinit_default(s);
gmp_randseed_ui(s, seed);

mpz_class ran;
gmp_randclass rr(s);
ran =rr.get_z_bits(125);
long int random=ran.get_ui();

But i dont get random number.

Please help me.

Answer 1

First, there is no gmp_randclass constructor that takes a gmp_randstate instance, so your code didn’t compile for me. The recommended way to construct a gmp_randclass instance is using gmp_randinit_default, like this:

gmp_randclass rr(gmp_randinit_default);

The first part of your code is seeding the gmp_randstate_t s, but that random state variable s is not used in the second part of your code (after the above change). Unless seeded otherwise, the default GMP random number generator always starts with the same seed, which means the same sequence of random numbers will be generated each time you run the program. You can seed an instance of gmp_randclass using the gmp_randclass::seed function.

The following code is similar to yours but seeds the random number generator based on the current time.

mpz_class ran;
gmp_randclass rr(gmp_randinit_default);
rr.seed(time(NULL));
ran =rr.get_z_bits(125);
long int random=ran.get_ui();

Note that as discussed in Random State Seeding, using a low-resolution current time is usually a poor choice for a random number generator seed.