Pi approximation

[t48eagle]

Hello,
Im trying to make a program to approximate pi when a certain number of values is input by the user. The more values, the more accurate. The equation is 4 * (-1^input)/(2n-1). I always end up with some weird number as the approximation, nowhere close to pi. Is there something wrong with my for loop?

int _tmain(int argc, _TCHAR* argv[])
{
	int numberOfTerms;
	double sumOfTerms = 0, pi;
	char repeat;

	do{
		cout << "Enter number of terms that should be considered in calculation of pi: ";
		cin >> numberOfTerms;

		for(int i = 0; i <= numberOfTerms - 1; i++){
			sumOfTerms += (-1)^i / (2*i-1);
		}
		pi = 4 * sumOfTerms;

		cout << "*******************************************************" << endl;
		cout << "Considering " << numberOfTerms << " terms, the value of pi is: " << pi << endl;
		cout << "Do you want to repeat this program? (enter y to repeat): ";
		cin >> repeat;
	}while(repeat == 'y');

	return 0;
}

[Moschops]

^ is the XOR operator. Is that what you meant?

[t48eagle]

Thanks for that, no i need to use pow(-1,i)

[kbw]

We'll you could, but that expression just flips from 1 to -1 and back. So just add or subtract the term for althernate iterations, don't call an O(n) algorithm to work that out.