Combine HashMap keys

Consider a Rust program that has 2 HashMaps, and we want to combine the keys from both into a third collection. We can remove duplicates from the final vector.

We can place the keys from both HashMaps into an empty vector, and then perform duplicate removal. This reliably gets the union of keys.

Example program

To start, we create 2 HashMaps. The types of the keys and values do not matter, but we should have the same key types for each HashMap.

use std::collections::HashMap;

fn main() {
    // First HashMap.
    let mut values1 = HashMap::new();
    values1.insert("cat", 1);
    values1.insert("dog", 2);

    // Second HashMap.
    let mut values2 = HashMap::new();
    values2.insert("cat", 3);
    values2.insert("bird", 4);

    // Get keys from both HashMaps.
    // ... Place in single vector.
    let mut vector: Vec<&str> = vec![];
    for k in values1.keys() {
        vector.push(k);
    }
    for k in values2.keys() {
        vector.push(k);
    }
    // Sort and dedup, removing all duplicate keys.
    vector.sort();
    vector.dedup();
    println!("{:?}", vector);
}
["bird", "cat", "dog"]

Another approach

It would be possible to place all the keys() from the first 2 HashMaps into a third HashMap. Then the keys() of that HashMap would be a combined key list.

Getting a merged list of keys from 2 HashMaps is useful in many Rust programs. Other approaches are possible, but this code is simple and effective.