How does one create a HashMap with a default value

问题:

Being fairly new to Rust, I was wondering on how to create a HashMap with a default value for a key? For example, having a default value 0 for any key inserted in the HashMap.

In Rust, I know this creates an empty HashMap:

let mut mymap: HashMap<char, usize> = HashMap::new();

I am looking to maintain a counter for a set of keys, for which one way to go about it seems to be:

for ch in "AABCCDDD".chars() {
    mymap.insert(ch, 0)
}

Is there a way to do it in a much better way in Rust, maybe something equivalent to what Ruby provides:

mymap = Hash.new(0)
mymap["b"] = 1
mymap["a"] # 0

回答1:

Answering the problem you have...

I am looking to maintain a counter for a set of keys.

Then you want to look at How to lookup from and insert into a HashMap efficiently?. Hint: *map.entry(key).or_insert(0) += 1

---

Answering the question you asked...

How does one create a HashMap with a default value in Rust?

No, HashMaps do not have a place to store a default. Doing so would cause every user of that data structure to allocate space to store it, which would be a waste. You'd also have to handle the case where there is no appropriate default, or when a default cannot be easily created.

Instead, you can look up a value using HashMap::get and provide a default if it's missing using Option::unwrap_or:

use std::collections::HashMap;

fn main() {
    let mut map: HashMap<char, usize> = HashMap::new();
    map.insert('a', 42);

    let a = map.get(&'a').cloned().unwrap_or(0);
    let b = map.get(&'b').cloned().unwrap_or(0);

    println!("{}, {}", a, b); // 42, 0
}

Of course, you are welcome to wrap this in a function or a data structure to provide a nicer API.

---

ArtemGr brings up an interesting point:

in C++ there's a notion of a map inserting a default value when a key is accessed. That always seemed a bit leaky though: what if the type doesn't have a default? Rust is less demanding on the mapped types and more explicit about the presence (or absence) of a key.

Rust adds an additional wrinkle to this. Actually inserting a value would require that simply getting a value can also change the HashMap. This would invalidate any existing references to values in the HashMap, as a reallocation might be required. Thus you'd no longer be able to get references to two values at the same time! That would be very restrictive.

回答2:

What about using entry to get an element from the HashMap, and then modify it.

From the docs:

fn entry(&mut self, key: K) -> Entry<K, V>

Gets the given key's corresponding entry in the map for in-place manipulation.

example

use std::collections::HashMap;

let mut letters = HashMap::new();

for ch in "a short treatise on fungi".chars() {
    let counter = letters.entry(ch).or_insert(0);
    *counter += 1;
}

assert_eq!(letters[&'s'], 2);
assert_eq!(letters[&'t'], 3);
assert_eq!(letters[&'u'], 1);
assert_eq!(letters.get(&'y'), None);