calculating pi with 4-4/3+4/5-

Question

I have written a code to calculate this to 100th sentence for pi but it shows "3" this the formula

#include <iostream>
using namespace std;

int main()
{
    int n = 1;
    double a, P = 0;
    for (n; n <= 100; n++) {
        a = 4 / (2 * n - 1);
        if (n % 2 == 0) {
            a = -a;
        }

        P = P + a;
    }

    cout << P;
    system("pause");

    return 0;
}

Wild guess - you don't know that [floating point math is broken](https://stackoverflow.com/questions/588004/is-floating-point-math-broken). — Fureeish, Dec 09 '18 at 21:58
"it does not work" is rarely a useful problem description. You will receive more positive responses if you post your expected and actual results, so that we don't have to guess about what you are expecting to see. — Claies, Dec 09 '18 at 22:02
4/3 is 1. 4/5 is 0. 4/7 is 0. 4/9 is 0. 4/11 is 0. 4/13 is 0. What's 3-1+0-0+0-0...? — user253751, Dec 10 '18 at 00:13

score 2 · Answer 1 · answered Dec 09 '18 at 22:05

2

4/(2*n-1) is evaluated in integer arithmetic since all the coefficients are integral types. This means that any decimal part is truncated.

4.0 / (2 * n - 1) is one fix.

answered Dec 09 '18 at 22:05

Bathsheba

231,907
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thanks it did the job – m reza Dec 09 '18 at 22:57

calculating pi with 4-4/3+4/5-

1 Answers1