How to create a Struct with Vec, Vec and HashMap<&str, &i32>?

Asked May 16 '22 at 13:27

Active May 17 '22 at 12:52

Viewed 62 times

-2

I would like to implement a struct as below. The struct instance, its fields and all the elements inside should be immutable and have the same lifetime. But I got stuck by the lifetime thing.

pub struct Foo {
    keys: Vec<String>,
    vals: Vec<i32>,
    mapping: HashMap<&str, &i32>,
}

And try to create a instance like this:

pub fn new(keys: &Vec<&str>, vals: &Vec<i32>) -> Foo {
    let result = Foo {
        keys: keys.iter().map(|x| x.to_string()).collect(),
        vals: vals.clone(),
        mapping: HashMap::new(),
    };
    let n = keys.len();
    for i in 0..n {
        result.mapping.insert(&result.keys[i], &result.vals[i]);
    }
    return result;
}

edited May 17 '22 at 12:52

asked May 16 '22 at 13:27

tushushu

The easiest way to solve this is to keep `mapping` away from `Foo`, or make `mapping` a `HashMap`. – E_net4 - Krabbe mit Hüten May 16 '22 at 13:37
Note that the question uses `i32` as keys. Assuming that is not a typo, you could have a `HashMap` mapping keys to indices in the `vals` and `keys` vectors. – Jmb May 16 '22 at 14:28
@Jmb It's a typo, the keys should be string type. – tushushu May 17 '22 at 12:47
@E_net4 The HashMap would be very huge, and it's required to avoid duplicating the heap string object. – tushushu May 17 '22 at 12:48
Should that seriously become a problem, consider interning the strings or reference counting them. – E_net4 - Krabbe mit Hüten May 17 '22 at 13:49
And by the way, [it is discouraged to make functions accept a `&Vec` or `&String`](https://stackoverflow.com/questions/40006219/why-is-it-discouraged-to-accept-a-reference-to-a-string-string-vec-vec-o). You can do `fn new(keys: &[&str], vals: &[i32])` instead. – E_net4 - Krabbe mit Hüten May 17 '22 at 13:51
I'm new to Rust, thanks so much @E_net4 These advices are very helpful. – tushushu May 19 '22 at 13:22

How to create a Struct with Vec, Vec and HashMap<&str, &i32>?

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