Merge two HashMaps adding the values in a function

---

examples/hashes/total-values-function/src/main.rs

use std::collections::HashMap;
fn main() {
    let a = HashMap::from([
        ("apple", 1),
        ("banana", 1),
    ]);
    let b = HashMap::from([
        ("apple", 2),
        ("peach", 2),
        ("grape", 2),
    ]);

    let mut total: HashMap<&str, i32> = HashMap::new();

    add(&mut total, &a);
    println!("{:#?}", total);

    add(&mut total, &b);
    println!("{:#?}", total);
}

fn add<'a>(total: &mut HashMap<&'a str, i32>, other : &HashMap<&'a str, i32>) {
    for (key, value) in other.iter() {
        *total.entry(key).or_insert(0) += value;
    }
}

// fn add(total: &mut HashMap<&str, i32>, other : &HashMap<&str, i32>) {
//     for (key, value) in other.iter() {
//         *total.entry(key).or_insert(0) += value;
//     }
// }

{
    "banana": 1,
    "apple": 1,
}
{
    "banana": 1,
    "peach": 2,
    "grape": 2,
    "apple": 3,
}