Having trouble with while loop in main

[monicam]

I am trying to compute pi with a certain level of precision using the trapezoid method. However, I am encountering an error within my while loop, it is never ending. Does anyone know why?

#include <iostream>
#include <stdlib.h>
#include <fstream>
#include <iomanip>
#include <math.h>
#include <cmath>
using namespace std;


//******************************************************************************
// Limits of integration, number of steps, and the function to be integrated
const double A = 0;
const double B = 1;  


double f(double x)
{
	return sqrt(1 - x * x);
}
//******************************************************************************

int main()
{
	double pidiff;
	double prec;
	double pi = 4 * atan(1);
	cout << "What precision would you like?" << endl;
	cin >> prec;
	double answer; //final answer
	double changex;
	double xI;
	double N = 1;
	changex = (B - A) / N; //change in x formula
	double sum = 0;
	int i = 0;
	int mintries = 1000;
	while (true)
	{
		if (i == 0 || i==N)
		{
			xI = A + (i * changex); //xi formula
			sum += f(xI); //add up all functions with xI
		}

		else
		{
			xI = A + (i * changex);
			sum += 2 * f(xI); // same as above but need to multiply by 2.
		}
		i=i++;
		answer = 2*changex * sum;  // final answer
		cout << answer << endl;
		pidiff = abs(pi - answer);
		cout << pidiff << endl;
		N = N * 10;
		if (pidiff <= prec && N>mintries)
		{
			break; 
	    }
	}
	cout << answer << endl;
	cout << N << endl;
	cout << pi << endl;

	system("pause");
	return 0;
} 

[jonnin]

while (true)
you rely on the break to stop it.

so you need to see what these values are:
if (pidiff <= prec && N>mintries)

to see why it did not stop.

print them out every iteration.

you can also force it to stop with a counter -- change the while loop to a for loop and stop after 100 iterations or something, with your printed debug values, to see what is going on without hassle. you can set it back to while-true later or you can keep the for loop to force it to stop after a while once you get a handle on # of iterations needed.

[lastchance]

Well, each of the while loops should do one numerical integration ... but it doesn't.

Each numerical integration should start by resetting sum to 0, computing a new value for changex - you don't do either of those.

Each numerical integration should loop through i values from 0 to N - you don't do that.

N should be an integer of some sort, not a double.

If you integrate that f(x) between those limits you will get pi/4 - just saying; it's not clear whether your code is on the way to that or not.

There's just too many errors, @monicam. Why don't you start with a single numerical integration with the trapezium rule and get that right. You can sling a while() loop round it afterwards.