Nested HashMaps

help
1

Hi,

I have been having a hard time trying to get a HashMap which includes a HashMap to work as expected (assuming it is at all possible). It could very well be a gap in my knowledge regarding ownership which I'm still very much getting my head around.

I am trying to create a structure that looks something like...

{
  "Thing": { 123: [4,5,6] },
  "Other": { 789: [10,11,12] }
}

I have written an example of my current attempt which has errors.

use std::collections::HashMap;

struct Test {
  nested_hashmap: HashMap<String, HashMap<i32, Vec<(i32)>>>,
}

fn main() {
  let mut test = Test { nested_hashmap: HashMap::new() };

  test.nested_hashmap.entry("Foo".to_string()).or_insert(HashMap::new());
  
  for (id, &thing) in test.nested_hashmap.iter() {
    thing.entry(1).or_insert(Vec::new());
  }
}

which produces the following error

test.rs:20:9: 20:14 error: cannot borrow immutable local variable `thing` as mutable
test.rs:20         thing.entry(1).or_insert(Vec::new());
                   ^~~~~
note: in expansion of for loop expansion
test.rs:19:5: 21:6 note: expansion site
test.rs:19:14: 19:20 error: cannot move out of borrowed content
test.rs:19     for (id, &thing) in test.nested_hashmap.iter() {
                        ^~~~~~
note: in expansion of for loop expansion
test.rs:19:5: 21:6 note: expansion site
test.rs:19:15: 19:20 note: attempting to move value to here
test.rs:19     for (id, &thing) in test.nested_hashmap.iter() {
                        ^~~~~
note: in expansion of for loop expansion
test.rs:19:5: 21:6 note: expansion site
test.rs:19:15: 19:20 help: to prevent the move, use `ref thing` or `ref mut thing` to capture value by reference

Thanks in advance any help is greatly appreciated as well as any recommendations for things I should be reading in order to close some gaps in my knowledge (especially if you see any big incorrect assumptions above).

2

It seems to work with iter_mut: Playpen.

1 Like
3

Oh wow! I had used mut_iter() from an extremely old example but it said it wasn't a method. I guess it was changed or it was a typo.

Thank you!

4

You can also shorthand this from for (_, mut thing) in test.nested_hashmap.iter_mut() to for (_, thing) in &mut test.nested_hashmap. Playpen