>>381
普通に平均を求めるだけではダメなのでしょうか？

fn main() {
assert_eq!(5.5, (1..=10).average());
assert_eq!(6.8, [2.3, 8.7, 9.4].average());
}

use num::ToPrimitive;
trait Average {
fn average(self) -> f64;
}
impl<I> Average for I
where I: IntoIterator, <I as IntoIterator>::Item: ToPrimitive,
{
fn average(self: I) -> f64 {
self.into_iter().fold((0.0, 1.0), |(ave, size), n| (ave + (n.to_f64().unwrap() - ave) / size, size + 1.0)).0
}
}