How to iterate through the keys of a HashMap in order - rust

Question

I'd like to iterate through the keys of a HashMap in order. Is there an elegant way to do this? The best I can think of is this:
use std::collections::HashMap;
fn main() {
let mut m = HashMap::<String, String>::new();
m.insert("a".to_string(), "1".to_string());
m.insert("b".to_string(), "2".to_string());
m.insert("c".to_string(), "3".to_string());
m.insert("d".to_string(), "4".to_string());
let mut its = m.iter().collect::<Vec<_>>();
its.sort();
for (k, v) in &its {
println!("{}: {}", k, v);
}
}
I'd like to be able to do something like this:
for (k, v) in m.iter_sorted() {
}
for (k, v) in m.iter_sorted_by(...) {
}
Obviously I can write a trait to do that, but my question is does something like this already exist?
Edit: Also, since people are pointing out that BTreeMap is already sorted I should probably note that while this is true, it isn't actually as fast as a HashMap followed by sort() (as long as you only sort it once of course). Here are some benchmark results for random u32->u32 maps:
Additionally, a BTreeMap only allows a single sort order.

Answer 1

HashMap doesn't guarantee a particular order of iteration. Simplest way to achieve consistent order is to use BTreeMap which is based on B-tree, where data is sorted.
You should understand that any implementation will do this in O(n) memory, particularly storing references to all items and at least O(n * log(n)) time to sort data out.
If you understand cost of doing this you can use IterTools::sorted from itertools crate.
use itertools::Itertools; // 0.8.2
use std::collections::HashMap;
fn main() {
let mut m = HashMap::<String, String>::new();
m.insert("a".to_string(), "1".to_string());
m.insert("b".to_string(), "2".to_string());
m.insert("c".to_string(), "3".to_string());
m.insert("d".to_string(), "4".to_string());
println!("{:#?}", m.iter().sorted())
}
Playground link

Answer 2

Based on what #Inline wrote, a more generic solution using HashMap, allowing for sorting by value and changing values. (Note that the content of the HashMap was adjusted in order to make the distinction of sorting by key and value visible.)
use itertools::Itertools; // itertools = "0.10"
use std::collections::HashMap;
fn main() {
let mut m = HashMap::<String, String>::new();
m.insert("a".to_string(), "4".to_string());
m.insert("b".to_string(), "3".to_string());
m.insert("c".to_string(), "2".to_string());
m.insert("d".to_string(), "1".to_string());
// iterate (sorted by keys)
for (k, v) in m.iter().sorted_by_key(|x| x.0) {
println!("k={}, v={}", k, v);
}
println!();
// iterate (sorted by values)
for (k, v) in m.iter().sorted_by_key(|x| x.1) {
println!("k={}, v={}", k, v);
}
println!();
// iterate (sorted by keys), write to values
for (k, v) in m.iter_mut().sorted_by_key(|x| x.0) {
*v += "v"; // append 'v' to value
println!("k={}, v={}", k, v);
}
}
Playground link