How can I simultaneously iterate over a Rust HashMap and modify some of its values?

Question

I\'m trying Advent of Code in Rust this year, as a way of learning the language. I\'ve parsed the input (from day 7) into the following structure:

struct Pro         


        

Answer 1

Yes, you can grant internal mutability to the HashMap's values using RefCell:

struct ProcessTree {
    processes: HashMap<String, RefCell<Process>>,  // change #1
}

impl ProcessTree {
    fn update_parents(&self) {
        for p in self.processes.values() {
            let p = p.borrow();                    // change #2
            for child_name in &p.children {
                let mut child = self.processes
                    .get(child_name)               // change #3
                    .expect("Child not found.")
                    .borrow_mut();                 // change #4
                child.parent = Some(p.name.clone());
            }
        }
    }
}

borrow_mut will panic at runtime if the child is already borrowed with borrow. This happens if a process is its own parent (which should presumably never happen, but in a more robust program you'd want to give a meaningful error message instead of just panicking).

I invented some names and made a few small changes (besides the ones specifically indicated) to make this code compile. Notably, p.name.clone() makes a full copy of p.name. This is necessary because both name and parent are owned Strings.