How do I get the unique list of values from a hashmap in rust?

Question

I have the following code which finds all the unique values from a hashmap of String to Vec. I cobbled it together (eventually as I was getting various borrow errors along the way), but it looks a mess to me and I'm looking for a more idiomatic way of doing it in rust.

fn main() {
    let mut my_map: HashMap<String, Vec<String>> = HashMap::new();
    my_map.insert("option1".to_string(), vec![String::from("sarah"), String::from("john")]);
    my_map.insert("option2".to_string(), vec![String::from("john"), String::from("mark")]);

    let x: Vec<String> = my_map.values().flat_map(|x| x).cloned().collect();
    // or use flatten() instead of flat_map(|x| x)
    let y: HashSet<&String> = HashSet::from_iter(x.iter());
    let mut z = y.iter().collect::<Vec<_>>();
    z.sort();
    println!("{:?}", z);
}

My idea was to flatmap the values from the map down into a simple vector, and then find the unique values (putting everything into a hashset), then converting back to a vector of strings.

The output is the sorted list of all unique names in the Vectors:

["john", "mark", "sarah"]

What would be a better implementation of this in rust?

Answer 1

You can collect into a different collection type, such as HashSet or BTreeSet. Both of these will eliminate duplicates, but entries in a HashSet will be in a random order (because the hash is randomized) while entries in a BTreeSet will be ordered (based on the Ord implementation for the element type).

use std::collections::{BTreeSet, HashMap};

fn main() {
    let mut my_map: HashMap<String, Vec<String>> = HashMap::new();
    my_map.insert("option1".to_string(), vec![String::from("sarah"), String::from("john")]);
    my_map.insert("option2".to_string(), vec![String::from("john"), String::from("mark")]);

    let b: BTreeSet<_> = my_map.values().flatten().collect();
    println!("{:?}", b);
}

Note that instead of annotating the variable type, you can use the "turbofish" to specify the collection type to collect into:

let b = my_map.values().flatten().collect::<BTreeSet<_>>();

itertools is a crate that provides lots of extension methods for iterators. Relevant here are methods such as unique and sorted_unstable.

use std::collections::HashMap;
use itertools::Itertools; // 0.10.3

fn main() {
    let mut my_map: HashMap<String, Vec<String>> = HashMap::new();
    my_map.insert("option1".to_string(), vec![String::from("sarah"), String::from("john")]);
    my_map.insert("option2".to_string(), vec![String::from("john"), String::from("mark")]);

    let x: Vec<_> = my_map.values().flatten().unique().sorted_unstable().collect();
    println!("{:?}", x);
}