use std::collections::HashMap;

fn main() {
    let mut map: HashMap<&str, i32> = HashMap::new();

    // insert if absent
    map.entry("a").or_insert(1);
    map.entry("a").or_insert(999); // won't overwrite

    // update based on existing value
    let text = "hello world hello";
    let mut word_count: HashMap<&str, i32> = HashMap::new();
    for word in text.split_whitespace() {
        let count = word_count.entry(word).or_insert(0);
        *count += 1;
    }
    println!("{:?}", word_count);
}