how to use more precision in c++

Question

Hi i have built an algorithm to calculate pi but i use long float for it so i just get 3.14159 as a result, i need more precision. How? here is the code:

    #include <iostream>
#include <math.h>
using namespace std;

int main ()
{
  long double a, l, p, pi, r;
  long long int n, m;
  r = 100000;
  a = r * sqrt (3) / 2 ;
  n = 100000;
  m = 6;
  while (n > m)
  {
    a = sqrt (r / 2 * (r + a));
  m = m * 2 ;
  }
  l = sqrt (4 * (pow (r, 2) - pow (a, 2)));
  p = m * l;
  pi = p / (2 * r) ;
  cout << pi << endl;
  cout << "number of corners used: " << m << endl;
  return 0;
}

By the way there is a 24 core (12 dual core nodes) supercomputer at my high school, just in case

Answer 1

There's more precision in that number than you're displaying. You just have to ask for it:

cout << std::setprecision(40) << pi << endl;

That gives me 3.141592653543740176758092275122180581093 when run on Codepad.

As double should have way more than enough precision for basic calculations. If you need to compute millions of places, there isn't a standard floating point representation big enough to handle that.