The calculation of the number pi

pi.rs

fn main(){
  let mut pi = 0f64;
  let mut n = 0u64;
  let mut a = 1f64;
  let mut b = 1f64;
  let mut k = 0f64;

  loop {
    a = inv(-1f64, n);
    b = inv(4f64, n);

    pi = pi + a/b*(2f64/(4f64*k+1f64)+2f64/(4f64*k+2f64)+1f64/(4f64*k+3f64));

    n = n + 1u64;

    if n > 100{
      break;
    }
    k = k + 1f64;
  }


  println!("{}", pi);
}


fn inv(a: f64, n: u64) -> f64 {
  let mut res = 1f64;
  let mut k = n;
  loop {
    if k < 1 {
      break;
    }
    res = res * a;
    k = k - 1;
  }
  return res;
}