Convert HashMap, vec

We often need to translate from one type to another, and this is one of the more difficult tasks at first. In Rust we can change from a HashMap to a vector.

Sometimes finding a useful module is worth more than lots of looping code. With FromIterator we can call from_iter to translate an iterator from HashMap.

HashMap to vec

This code example goes from a HashMap to 3 different kinds of vectors. We can place the keys, values, or both keys and values at once into a vector.

use std::collections::HashMap;
use std::iter::FromIterator;

fn main() {
    let mut map = HashMap::new();
    map.insert("cat", 800);
    map.insert("frog", 200);

    // Version 1: get vector of pairs.
    let vec1 = Vec::from_iter(map.iter());
    println!("{:?}", vec1);

    // Version 2: get vector of keys.
    let vec2 = Vec::from_iter(map.keys());
    println!("{:?}", vec2);

    // Version 3: get vector of values.
    let vec3 = Vec::from_iter(map.values());
    println!("{:?}", vec3);
}
[("cat", 800), ("frog", 200)]
["cat", "frog"]
[800, 200]

Convert to HashMap

One way we can go from a Vector to a HashMap is by using a simple for-loop. This code benefits from being clear and direct, but is not particularly clever.

use std::collections::HashMap;

fn main() {
    // Create vec of 2 strings.
    let animals = vec!["dog", "lizard"];

    // Create empty HashMap.
    let mut map = HashMap::new();

    // Add each animal from the vector to the HashMap.
    for animal in &animals {
        map.insert(animal, 1);    
    }
    println!("{:?}", map);
}
{"dog": 1, "lizard": 1}

Summary

Conversion between types is paramount for program development. If we could not convert types, we could not do much. These examples help with HashMap Rust conversions.