fn main() {
  let a = vec![1, 2, 3, 4, 5, 6];
  let b = a.iter().flat_map(|n| if n % 2 == 0 { Some(n) } else { None }).collect::<Vec<_>>();
  let c = a.iter().filter_map(|n| if n % 2 == 0 { Some(n) } else { None }).collect::<Vec<_>>();
  println!("{:?}", b);
  println!("{:?}", c);
}