Thread: pi program using arbitrary precision (GMP)

HI everyone,

I'm trying to make a program to compute pi to n places. I'm using the Gauss Legendre (Gauss-Legendre Algorithm -- from Harry J. Smith).
I'm using the GMP MP package for arbitrary precision. The variables correspond to those in that link

Code:

#include <string>
#include <iostream>
#include <fstream>
#include <vector>
#include <numeric>
#include <ctime>
#include <iomanip>
#include <cmath>


#include <gmp.h>
#include <gmpxx.h>


using namespace std;






int main()


{
//set a//


        mpf_set_default_prec(100000);              //bit precision


    mpf_t a;
    mpf_init (a);
    mpf_set_ui (a,1);




//set b//
    mpf_t b;
    mpf_init (b);


    mpf_t b1;
    mpf_init (b1);
    mpf_sqrt_ui (b1,2);


    mpf_ui_div(b,1,b1);




//set t//


    mpf_t t;
    mpf_init (t);
    mpf_set_d (t,0.25);




//set x//


    mpf_t x;
    mpf_init (x);
    mpf_set_ui(x,1);


//calc//
    for (int f=0;f<1000;f++)                    //number of iterations
    {




        //set y//
        mpf_t y;
        mpf_init (y);
        mpf_set (y,a);




        //change a//


        mpf_t a1;
        mpf_init (a1);
        mpf_add(a1,a,b);


        mpf_div_ui (a,a1,2);




        // change b//


        mpf_t b1;
        mpf_init (b1);
        mpf_mul(b1,b,y);




        mpf_sqrt (b,b1);






        //set t//


        mpf_t t1;
        mpf_init(t1);
        mpf_sub(t1,y,a);


        mpf_t t2;
        mpf_init (t2);
        mpf_pow_ui (t2,t1,2);


        mpf_t t3;
        mpf_init(t3);
        mpf_mul(t3,x,t2);


        mpf_sub(t,t,t2);




        //change x//












    }








        // calculate pi//


        mpf_t piup1;
        mpf_init(piup1);
        mpf_add(piup1,a,b);


        mpf_t piup2;
        mpf_init (piup2);
        mpf_pow_ui(piup2,piup1,2);         //final up


        mpf_t pidown;                   //final down
        mpf_init (pidown);
        mpf_mul_ui(pidown,t,4);


        mpf_t pifinal;
        mpf_init (pifinal);
        mpf_div(pifinal,piup2,pidown);




    cout << setprecision(100)<<pifinal;  //output pi//






}

The problem I'm having is that despite many iterations the number is incorrect at 3.1410420453.

Any help is most appreciated.

Ah, sorry about that. Still doesn't work with mpf_mul_ui (x,x,2);