Sort HashMap

Programs use HashMaps to store look up data—keys matched to values. This optimizes performance, but it eliminates the ability to sort and order items.

By converting the HashMap and its tuple data to a Vector, we can sort the data by the keys or values. This creates a copy of the collection, but is needed for sorting.

Example input, output

Consider these requirements: we have a HashMap that has 2 keys—the 1-character strings "x" and "a." We want to loop over them in alphabetical order.

x, xyz
a, abc

SORTED:
a, abc
x, xyz

Example program

Here we implement the conversion method and invoke sort_by_key in Rust code. Please notice that we have a "use" statement at the top.

use std::collections::HashMap;

fn main() {
    // Step 1: create HashMap.
    let mut source = HashMap::new();
    source.insert("x", "xyz");
    source.insert("a", "abc");

    // Step 2: get Vector from HashMap.
    let mut sorted: Vec<_> = source.iter().collect();
    println!("{:?}", sorted);

    // Step 3: sort Vector by key from HashMap.
    // ... This sorts by HashMap keys.
    //     Each tuple is sorted by its first item.
    sorted.sort_by_key(|a| a.0);
    println!("{:?}", sorted);

    // Step 4: loop over sorted vector.
    for (key, value) in sorted.iter() {
        println!("KEY, VALUE: {} {}", key, value);
    }
}
[("a", "abc"), ("x", "xyz")]
[("a", "abc"), ("x", "xyz")]
KEY, VALUE: a abc
KEY, VALUE: x xyz

A review

With this code we can sort the keys (or values of a HashMap) using the sort_by_key function. We must specify 0 or 1 for the key or value.