How to find X in a HashMap of HashMaps?

Question

Given that I know ParentId and ChildId, how would I find the UserId if the hashmap is:

HashMap<ParentId, HashMap<ChildId, HashMap<UserId, Foobar>>>

As my knowledge about Rust is pretty basic, I found that the following works but it's quite verbose, as I don't know any better:

match foobar.get(&pid) {
  Some(data1) => {
    println!("Found 1: {:?}", data1);

   match data1.get(&cid) {
     Some(data2) => {
      println!("Found 2: {:?}", data2);       
      
      ...and so on
     }, 
     _ => println!("Not found")     
   }
  },
  _ => println!("Not found")
}

I've also attempted chained get but it's tricky and did not find how to do it correctly:

foobar
        .get(pid)?
        .get(cid)?        
        .get(to_find)

What can I try next?

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Dogbert · Accepted Answer · 2022-04-26T07:57:20.657

1

You can use Option::and_then to chain operations that return Option<_>:

let _: Option<&Foobar> = foobar.get(&pid).and_then(|map| map.get(&cid)).and_then(|map| map.get(&to_find));

Example:

use std::collections::HashMap;

fn main() {
    let map: HashMap<i32, HashMap<bool, HashMap<String, bool>>> = HashMap::new();
    let _: Option<&bool> = map
        .get(&123)
        .and_then(|map| map.get(&true))
        .and_then(|map| map.get("foo"));
}

Playground

Your try with ? is also correct but it'll only work in a function that returns an Option as it returns None from the function the expression is in if any value is None, which is probably the error you're getting.

fn get(map: &HashMap<i32, HashMap<bool, HashMap<String, bool>>>) -> Option<bool> {
    map.get(&123)?.get(&true)?.get("foo").cloned()
}

Edit: As @Jmb pointed out in a comment below, another option is to create and immediately call a closure so you can use the ? operator which could be more readable in certain cases:

let _: Option<&bool> = (|| map.get(&123)?.get(&true)?.get("foo"))();

edited Apr 26 '22 at 07:57

answered Apr 25 '22 at 22:14

Dogbert

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thanks a lot for your time and knowledge! I'll make sure I read about Option::and_then, as it seems easier to read than my attempt with `get`. – punkbit Apr 25 '22 at 22:21
There's also a proposal for [try blocks](https://doc.rust-lang.org/beta/unstable-book/language-features/try-blocks.html) which will let you do `let value = try { map.get(&123)?.get(&true)?.get("foo") };` but it's going to be a while before it's stabilized. – Dogbert Apr 25 '22 at 22:25
For readability, you can also use a crate like [`map_for`](https://crates.io/crates/map_for) (which I wrote before `?` made it into the language) or use an inner closure like in [this answer](https://stackoverflow.com/a/55756926/5397009). – Jmb Apr 26 '22 at 06:53
@Jmb thanks, I've used the closure approach multiple times but forgot to mention that. I've edited that into the answer! – Dogbert Apr 26 '22 at 07:58

How to find X in a HashMap of HashMaps?

1 Answers1