Want to add to HashMap using pattern match, get borrow mutable more than once at a time

Question

I am trying to write some toy code that stores the number of times it sees a word in a HashMap. If the key exists, it increments a counter by one, if the key doesn't exist, it adds it with the value 1. I instinctively want to do this with a pattern match, but I hit a borrow mutable more than once error:

fn read_file(name: &str) -> io::Result<HashMap<String, i32>> {
    let b = BufReader::new(File::open(name)?);
    let mut c = HashMap::new();

    for line in b.lines() {
        let line = line?;
        for word in line.split(" ") {
            match c.get_mut(word) {
                Some(i) => {
                    *i += 1;
                },
                None => {
                    c.insert(word.to_string(), 1);
                }
            }
        }
    }

    Ok(c)
}

The error I get is:

error[E0499]: cannot borrow `c` as mutable more than once at a time
  --> <anon>:21:21
   |
16 |             match c.get_mut(word) {
   |                   - first mutable borrow occurs here
...
21 |                     c.insert(word.to_string(), 1);
   |                     ^ second mutable borrow occurs here
22 |                 }
23 |             }
   |             - first borrow ends here

I understand why the compiler is grumpy: I've told it I'm going to mutate the value keyed on word, but then the insert isn't on that value. However, the insert is on a None, so I would have thought the compiler might have realized there was no chance of mutating c[s] now.

I feel like this method should work, but I am missing a trick. What am I doing wrong?

EDIT: I realize I can do this using

        if c.contains_key(word) {
            if let Some(i) = c.get_mut(s) {
                *i += 1;
            }
        } else {
            c.insert(word.to_string(), 1);
        }

but this seems horribly ugly code vs the pattern match (particularly having to do the contains_key() check as an if, and then essentially doing that check again using Some.

Answer 1

You have to use the Entry "pattern":

use std::collections::HashMap;
use std::collections::hash_map::Entry::{Occupied, Vacant};

fn main() {
    let mut words = vec!["word1".to_string(), "word2".to_string(), "word1".to_string(), "word3".to_string()];
    let mut wordCount = HashMap::<String, u32>::new();

    for w in words {
        let val = match wordCount.entry(w) {
           Vacant(entry) => entry.insert(0),
           Occupied(entry) => entry.into_mut(),
        };

        // do stuff with the value
        *val += 1;
    }

    for k in wordCount.iter() {
        println!("{:?}", k);
    }
}

The Entry object allows you to insert a value if its missing, or to modify it if it already exists.

https://doc.rust-lang.org/stable/std/collections/hash_map/enum.Entry.html

Answer 2

HashMap::entry() is the method to use here. In most cases you want to use with Entry::or_insert() to insert a value:

for word in line.split(" ") {
    *c.entry(word).or_insert(0) += 1;
}

In case the value to be inserted need to be expensively calculated, you can use Entry::or_insert_with() to make sure the computation is only executed when it needs to. Both or_insert methods will probably cover all of your needs. But if you, for whatever reason, want to do something else, you can still simply match on the Entry enum.

Answer 3

This is basically not an issue anymore. With non-lexical lifetimes (NLL), your code compiles without problems. Your example on the Playground.

NLL is a new way the compiler reasons about borrows. NLL has been enabled in Rust 2018 (≥ 1.31). It is planned to be enabled in Rust 2015 eventually as well. You can read more about NLL and editions in this official blog post.

In this particular case, I still think A.B.'s answer (entry(word).or_insert(0)) is the best solution, simply because it is very concise.