sin function

[cash]

Why isn't my program outputting the correct sin value?

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

int main()
{
    double accuracy=1,n,sinx,x1;
    cout << "Enter value of n" << endl;
    cin >> n;

    n = n * (3.142 / 180.0);
    sinx = n;
    int i = 1;
    do
    {
        x1 = n;
        x1 = -x1 * n * n / (2 * i * (2 * i + 1));
        sinx = sinx + x1;
        i = i + 1;
    } while (abs(accuracy)<=+1e-15);
    cout << setprecision(5)<< fixed << showpoint<< sinx;
    return 0;
} 

[dutch]

accuracy never changes. And you presumably want to keep the loop going while it's > your limit. As it is, since it equals 1, which is not <= your (somewhat excessive!) limit, the loop exits immediately.

What formula are you using? Do you have a link?

[TheIdeasMan]

Also, if pi is only 3dp, don't expect your answer to be any better than 3dp either. And you only have setprecision(5), that also makes the limit excessive.

[lastchance]

x - x^3/3! + x^5/5! - x^7/7!+ ...

- Use a more accurate value of pi.

- Set x1 to n BEFORE the loop, not on line 17. It is the first term of the series.

- If anything one would expect >= on line 21.

- You should be looking at abs(x1) on line 21; possibly you meant
while (abs(x1)>=accuracy);, with accuracy previously set to 1e-15.

- To improve convergence it would be a good idea to subtract integer multiples of 2.pi before evaluating the power series.

- The whole thing would be better in a function, where it could be reused.

- The code would be easier to read and debug if you used better names for variables.