How do I sort a map by order of insertion?

Question

I have tried using HashMap and BTreeMap for this but neither have worked:

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

fn main() {
    let mut btreemap = BTreeMap::new();
    println!("BTreeMap");
    btreemap.insert("Z", "1");
    btreemap.insert("T", "2");
    btreemap.insert("R", "3");
    btreemap.insert("P", "4");
    btreemap.insert("K", "5");
    btreemap.insert("W", "6");
    btreemap.insert("G", "7");
    btreemap.insert("C", "8");
    btreemap.insert("A", "9");
    btreemap.insert("D", "0");
    for (key, value) in btreemap {
        println!("{} {}", key, value);
    }
    println!("Hash Map");
    let mut hashmap = HashMap::new();
    hashmap.insert("Z", "1");
    hashmap.insert("T", "2");
    hashmap.insert("R", "3");
    hashmap.insert("P", "4");
    hashmap.insert("K", "5");
    hashmap.insert("W", "6");
    hashmap.insert("G", "7");
    hashmap.insert("C", "8");
    hashmap.insert("A", "9");
    hashmap.insert("D", "0");
    for (key, value) in hashmap {
        println!("{} {}", key, value);
    }
}

When I run this via the Rust playground, I get a result that is not sorted by order of insertion; BTreeMap appears to be ordered alphabetically (prints A C D G K P R T W Z, along with the numbers) and HashMap seems to be ordered randomly (prints Z A C D R P T G WK ).

I've looked through the Rust standard library documentation and I don't see any other maps.

Answer 1

The default collections do not track order of insertion. If you wish to sort by that, you will need to either find a different collection that does track it, or track it yourself.