[See Plus Plus]

Hi everyone I am trying to write a program that calculates pi.
So far my program looks like this:

#include <iostream>
#include <cmath>
using namespace std;

int main ()

{

long double pi = 2.0;
int i = 1;

for (int n = 2; fabs(pi - 3.1416) > 0.0001; n+=2)
{
pi *= (n/(n-1.)) * (n/(n+1.));
i++;
cout << "i = " << i << "\tpi = " << pi << endl;
}

return 0;
}


Here's the catch - my professor only wants us to be as precise as 3.1416 - How the heck would I do that - I have tried everything and I am out of ideas???? He told us it would take between 600 and 700 iterations - I was going into the millions so I am doing something wrong...any ideas? Please help...I hate Pi.

[gab00n]

You can use iomanip,

cout<<fixed<<showpoint<<setprecision(4);

That way it will output the number to 4 sig figs.

[See Plus Plus]

Thanks for the reply, I actually did try that but it still goes up to maybe in the 900,000's because it is just the display on the screen that gets changed. Nonetheless, I did find something that sort of seems to work:

pi = (static_cast <double> (pi * 10000) + .0005) / 10000;

it seems to work, and keeps the iterations down to 400. I am just not sure how it works, and why the .0005 is needed so if anyone knows the answer and cares to explain, i am all ears.