How do I use GMP properly to do this operation?

Question

My code in C++

   long long N=1000000000000LL;
   long long a = N;
   mpz_class v;
   mpz_mul(v, a, a); 
   cout<<v<<endl; //I want this to show 1000000000002000000000001
   long long U=((sqrt(4*N+v)-1)/4);  //not sure how to do this in GMP at all
   cout << U << endl; //should show 250000000000

This is a snippet that shows what kind of operations I want to do. But I am not experienced enough with GMP to get it down, and the documentation is unclear to me. How do I correct all this?

Answer 1

mpz_class has no constructor from long long (it only goes up to unsigned long), so you'd have to use an intermediate string:

#include <gmpxx.h>
#include <iostream>
#include <string>

int main()
{
    long long N = 1000000000000LL;
    mpz_class a(std::to_string(N).c_str());
    mpz_class v = a*a;
    std::cout << v << '\n'; // shows 1000000000000000000000000
    std::cout << (a+1) * (a+1) << '\n'; // THIS shows 1000000000002000000000001

    mpz_class U = ((sqrt(4*a+v)-1)/4);
    std::cout << U << '\n'; // shows 250000000000
}