struct SDFold {
   count: u32,
   mean: f32,
   var_mean: f32,
   std_dev: f32,
}

fn main() {
    let dataset = vec![1, 2, 3, 4, 5, 6, 7, 8, 9];
    let sd = dataset.fold(SDFold {
        count: 0,
        mean: 0,
        var_mean: 0,
        std_dev: 0,
    }, |acc, v| {
        let count = acc.count + 1;
        let mean = (acc.mean + v) / count;
        let var = (v - mean).pow(2);
        let var_mean = (acc.var_mean + var) / count;
        let std_dev = var_mean.sqrt();
        SDFold { count, mean, var_mean, std_dev }
        }
    )
}