How to use the get() HashMap method for structs


Hi all,

The get() method returns an Option<T> on a HashMap. When the stored object is a struct, one can’t use such constructs as:


if x is a field of a struct.


struct Number {
    french: String,
    spanish: String

fn main() {
    use std::collections::HashMap;
    let mut map: HashMap<i8,Number> = HashMap::new();
    let one = Number { french: "un".to_string(), spanish: "uno".to_string() };
    let two = Number { french: "deux".to_string(), spanish: "dos".to_string() };
    map.insert(1, one);
    map.insert(2, two);
    let n1 = map.get(&1).french;

doesn’t compile. I know that the key 1 exists, so why returning an > Option ? Always needs a match ?

Thanks for your help.


It’s because the definition of get(), where the HashMap is defined, doesn’t know that that key exists. get() can’t guarantee that every key you try to use with it will have a member in the hash table, so it must use the Option type so it can return something sensible when the key doesn’t have a matching entry. You can avoid using a match here by using unwrap() instead; unwrap() returns the object inside an Option and panics when it doesn’t exist:

let n1 = map.get(&1).unwrap().french;

Keep in mind that unwrap() is basically telling the compiler that you always expect the thing that you’re unwrapping exists; if it doesn’t your program will panic!


It shows my few knowledge of the language so far :wink:

Thanks a lot for the hint.


Look at the rest of Option’s API - there are additional methods that allow you to “unwrap” safely and functionally/ergonomically. Since the language doesn’t have the notion of null, absence of values is always signaled via Option (which is really nice IMO).