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

in one line of code,
copying the data only once,
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.

FYI: in your first code, you don't need to clone the vector, you just need to declare it as mutable. Either by let mut victim = vec; or in the argument list by: fn vec_to_set(mut vec: Vec<u8>). — Lukas Kalbertodt
– Lukas Kalbertodt, Commented Oct 1, 2016 at 9:52
You're not using the O notation correctly. O(n) = O(2n) = O(3n) = O(c*n). The point is that constants don't matter. I think that it is clear what you mean but you should probably say just "2n memory" or something instead. — Lii
– Lii, Commented Oct 1, 2016 at 10:05

bluss · Accepted Answer · 2019-09-25 08:53:12Z

96

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.

edited Sep 25, 2019 at 8:53

answered Oct 1, 2016 at 7:52

bluss

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2 Comments

ixx Over a year ago

This doesn't compile anymore.

Crispy13 Over a year ago

Why do we need to use from_iter instead of iter? Could you explain the difference between them?

Shepmaster · Accepted Answer · 2018-04-30 15:46:10Z

64

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

edited Apr 30, 2018 at 15:46

Shepmaster

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answered Oct 1, 2016 at 5:43

ljedrz

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Comments

Fuji · Accepted Answer · 2023-01-29 01:32:57Z

40

Converting Vec to HashSet

Moving data ownership

let vec: Vec<u8> = vec![1, 2, 3, 4];
let hash_set: HashSet<u8> = vec.into_iter().collect();

Cloning data

let vec: Vec<u8> = vec![1, 2, 3, 4];
let hash_set: HashSet<u8> = vec.iter().cloned().collect();

edited Jan 29, 2023 at 1:32

answered Apr 1, 2019 at 4:09

Fuji

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4 Comments

Shepmaster Over a year ago

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

Fuji Over a year ago

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

Shepmaster Over a year ago

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 Over a year ago

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

Zukaru · Accepted Answer · 2025-05-27 10:15:10Z

0

using generic method with Eq + Has Trait works for me:

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

fn vec_to_set<T>(vec: Vec<T>) -> HashSet<T> where T: Eq + Hash {
    HashSet::from_iter(vec)
}

also tested with strings:

let hashset = vec_to_set(["foo", "bar"].to_vec());

answered May 27 at 10:15

Zukaru

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