Create empty HashMap, insert key-value pairs

---

• std::collections
• HashMap

• When we create a HashMap we don't necessarily have to define the types of the keys and the values as those can be deducted from the later assignments.
• If we'd like to add new key-value pairs to the hash, we need to declare it as mutable.
• The insert method allows us to add a new key-value pair.
• len will tell us the number of keys.

examples/hashes/create-empty-hash/src/main.rs

use std::collections::HashMap;

fn main() {
    let mut counter = HashMap::new();
    println!("{}", counter.len());
    println!("{:?}", counter);

    counter.insert("foo", 1);
    counter.insert("bar", 2);
    println!("{:?}", counter.len());
    println!("{:?}", counter);
}

0
{}
2
{"foo": 1, "bar": 2}