Pi Approximation

[breezett93]

Hi Everyone,
For a couple weeks, I have been developing a pi approximation program and making little upgrades. Right now, the program asks for an input of iterations and then prints out the approximation. Now, I want to have every iteration printed out into an excel file with an approximation accurate to

 .000001 [50,000 iterations] 

However, I realized that the current setup of my program does not work well, so I think some rework is in order.
So my goal is to eliminate the input process; and instead, base the iterations on accuracy, not input. That is what I would like some help with. The problem is that I do not know how to set up a for loop to go until that accuracy point is reached. The ofstream part I am somewhat proficient at so I will add all that in later.

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

int main()
{
    //Declare variables
    float x;
    float y;
    int counter;
    int n;
    int swap;

    //Get input
    cout << "Please input the number of terms in the series to be calculated." << endl;
    cout << "(at least 700 for recognizable accuracy)" << endl;
    cin >> n;

    x = 0;
    swap = 1;
    counter = 0;    

    //Loop
    for (int counter = 0; counter != n; ++counter)
    {
        x += swap * (1.0 / (1 + counter * 2));
        swap *= -1;
    }

    x = 4 * (x);
    //Averaging most recent terms 
    counter = counter - 2;
    y = ((swap * (1 / (1 + counter * 2))) + x);

    cout << setprecision(15) << "My best approximation of PI is " << y << endl;

    cin.get();
    cin.get();

    //Exit program
    return 0;
}

[breezett93]

Ok. Updating that I figured everything out. Posting code to help others who experience similar problems.

#include<iostream>
#include<fstream>
#include<iomanip>
#include<cmath>

using namespace std;

int main()
{
    //Declare variables
    long double x;
    long double y;
    int counter;
    int n = 50000;
    int swap;
    int count;
    ofstream report;
    x = 0;
    swap = 1;
    counter = 0;

    //Open output file
    report.open("pi.csv");
    if (report.fail())
    {
        cerr << "Error opening input file.";
        cin.get();
        cin.get();
        exit(1);
    }
    else
        cout << "The report has been created in Microsoft Excel." << endl;

    //Loop
    for (int counter = 0; counter < n; ++counter)
    {
        x += 4 * (swap * (1.0 / (1 + counter * 2)));
        swap *= -1;
        report << x << endl;
    }
    //Averaging most recent terms 
    counter = counter - 2;
    y = ((swap * (1 / (1 + counter * 2))) + x);

    report << setprecision(15) << y << endl;

    cin.get();
    cin.get();

    //Exit program
    report.close();
    return 0;
}

[duskoKoscica]

There are manny aproximations for pi, try to find the fastest one to human kind.
Look for the best algorithm, and then try to ask the expert in numerical math...