How can I insert all values of one HashSet into another HashSet? - rust

Question

I have two HashSet<u16>s and I would like to implement a = a U b. If possible, I'd like to use HashSet::union rather than loops or other tweaks.
I tried the following:
use std::collections::HashSet;
let mut a: HashSet<u16> = [1, 2, 3].iter().cloned().collect();
let b: HashSet<u16> = [7, 8, 9].iter().cloned().collect();
// I can build a union object that contains &u16
let union: HashSet<&u16> = a.union(&b).collect();
// But I can't store the union into a
a = a.union(&b).collect(); // -> compile error
// of course I can do
for x in &b {
a.insert(*x);
}
// but I wonder if union could not be used to simply build a union
The error message is the following:
the trait bound
`std::collections::HashSet<u16>: std::iter::FromIterator<&u16>`
is not satisfied
How can I perform a = a U b?

Answer 1

You don't want union — as you said, it will create a new HashSet. Instead you can use Extend::extend:
use std::collections::HashSet;
fn main() {
let mut a: HashSet<u16> = [1, 2, 3].iter().copied().collect();
let b: HashSet<u16> = [1, 3, 7, 8, 9].iter().copied().collect();
a.extend(&b);
println!("{:?}", a); // {8, 3, 2, 1, 7, 9}
}
(Playground)
Extend::extend is also implemented for other collections, e.g. Vec. The result for Vec will differ because Vec does not honor duplicates in the same way a Set does.

Answer 2

// But I can't store the union into a
a = a.union(&b).collect(); // -> compile error
The error message is the following:
the trait bound `std::collections::HashSet<u16>:
std::iter::FromIterator<&u16>` is not satisfied
It's because a is a HashSet<u16>, but a.union(&b) is an Iterator<Item=&u16>. Converting a.union(&b) to Iterator<Item=u16> by using .copied() works:
a = a.union(&b).copied().collect(); // compiles
As the other mentioned, this will create a new HashSet. In some cases, this might be what you want, but it will make more sense if it's assigned to another variable:
let c: HashSet<u16> = a.union(&b).copied().collect();
// a is unchanged here

Answer 3

For collecting multiple hashsets:
use std::collections::HashSet;
fn main() {
let a: HashSet<u16> = [1, 2, 3].iter().copied().collect();
let b: HashSet<u16> = [1, 3, 7, 8, 9].iter().copied().collect();
let all = [a,b];
let combined = all.iter().flatten().collect::<HashSet<_>>();
println!("{:?}", combined);
}
Playground: https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=44cd73eb3a4e628378cbb7ff11a32649