Build HashSet from a vector in Rust

Question

I want to build a HashSet<u8> from a Vec<u8>. I'd like to do this

1. in one line of code,
2. copying the data only once,
3. using only 2n memory,

but the only thing I can get to compile is this piece of .. junk, which I think copies the data twice and uses 3n memory.

fn vec_to_set(vec: Vec<u8>) -> HashSet<u8> {
    let mut victim = vec.clone();
    let x: HashSet<u8> = victim.drain(..).collect();
    return x;
}

I was hoping to write something simple, like this:

fn vec_to_set(vec: Vec<u8>) -> HashSet<u8> {
    return HashSet::from_iter(vec.iter());
}

but that won't compile:

error[E0308]: mismatched types
 --> <anon>:5:12
  |
5 |     return HashSet::from_iter(vec.iter());
  |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected u8, found &u8
  |
  = note: expected type `std::collections::HashSet<u8>`
  = note:    found type `std::collections::HashSet<&u8, _>`

.. and I don't really understand the error message, probably because I need to RTFM.

Solution 1

Because the operation does not need to consume the vector¹, I think it should not consume it. That only leads to extra copying somewhere else in the program:

use std::collections::HashSet;
use std::iter::FromIterator;

fn hashset(data: &[u8]) -> HashSet<u8> {
    HashSet::from_iter(data.iter().cloned())
}

Call it like hashset(&v) where v is a Vec<u8> or other thing that coerces to a slice.

There are of course more ways to write this, to be generic and all that, but this answer sticks to just introducing the thing I wanted to focus on.

¹This is based on that the element type u8 is Copy, i.e. it does not have ownership semantics.

Solution 2

The following should work nicely; it fulfills your requirements:

use std::collections::HashSet;
use std::iter::FromIterator;

fn vec_to_set(vec: Vec<u8>) -> HashSet<u8> {
    HashSet::from_iter(vec)
}

from_iter() works on types implementing IntoIterator, so a Vec argument is sufficient.

Additional remarks:

• you don't need to explicitly return function results; you only need to omit the semi-colon in the last expression in its body

• I'm not sure which version of Rust you are using, but on current stable (1.12) to_iter() doesn't exist

Solution 3

Moving data ownership

let vec: Vec<usize> = vec![1, 2, 3, 4];
let hash_set: HashSet<usize> = vec.into_iter().collect();

Cloning data

let vec: Vec<usize> = vec![1, 2, 3, 4];
let hash_set: HashSet<usize> = vec.iter().cloned().collect();

Comments

[Shepmaster]

collect uses FromIterator — this is the same as the above two answers except more verbose.

[Fuji]

fixed type error. This code does not require repeating HashSet in the conversion code

[Shepmaster]

There's no repeating of HashSet in the original answers either. Their code: let hash_set = HashSet::<_>::from_iter(vec); vs yours: let hash_set: HashSet<usize> = vec.into_iter().collect()

[Fuji]

You removed the type in your first example to hide the repeat of HashSet code 1: let hash_set: HashSet<usize> = HashSet::<_>::from_iter(vec); code 2: let hash_set: HashSet<usize> = vec.into_iter().collect() Example restored

[User]

This doesn't compile anymore.