How to iterate over string, and look up each value in a HashMap in Rust?

Question

I would like to take an alphanumeric string, iterate character by character, and lookup each character in a HashMap.

Here is how I would do it in Python:

lookup: dict = {
    'J': 5,
    'H': 17,
    '4': 12
}

s: str = 'JH4'

looked_up: list[str] = [lookup[c] for c in s]
print(looked_up)
[5, 17, 12]

My attempt in Rust

use std::collections::HashMap;

fn main()
{

    let lookup: Hashmap<char, u32> = HashMap::from([
        ('J', 5),
        ('H', 17),
        ('4', 12)
    ]);

    let s = String::from("JH4");
    for c in s.chars()
    {
        // Stuck here
        // I would like to look up each character from s via the lookup key and return an array of
        // the values returned.
    }

}

PitaJ · Accepted Answer · 2022-09-09 17:36:29Z

2

chars() returns an iterator, so you can just map over each element, indexing the map:

use std::collections::HashMap;

fn main() {
    let lookup: HashMap<char, u32> = HashMap::from([
        ('J', 5),
        ('H', 17),
        ('4', 12)
    ]);

    let s = String::from("JH4");

    let looked_up: Vec<_> = s.chars().map(|c| lookup[&c]).collect();
    dbg!(looked_up);
}

This will panic if c is not a key of the map, so you may want to explicitly handle that case instead:

let looked_up: Vec<_> = s.chars().map(
    // return the value from the map, or if none exists,
    // return 0 by default
    |c| lookup.get(&c).unwrap_or(0)
).collect();

edited Sep 9, 2022 at 17:36

answered Sep 9, 2022 at 17:33

PitaJ

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2 Comments

Coldchain9 Over a year ago

Thanks. Just started playing around in Rust today. When typing Vec<_> is there a reason for the _ or should I type it to the values of my lookup, e.g. u32?

PitaJ Over a year ago

Vec<_> essentially tells the compiler "you already know what the inner type should be (because it's defined by the hashmap), so fill it in for me". You can, of course, just put Vec<u32> instead to be more explicit.

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