comprehensive-rust/src/std/hashmap.md

HashMap

Standard hash map with protection against HashDoS attacks:

use std::collections::HashMap;

fn main() {
    let mut page_counts = HashMap::new();
    page_counts.insert("Adventures of Huckleberry Finn".to_string(), 207);
    page_counts.insert("Grimms' Fairy Tales".to_string(), 751);
    page_counts.insert("Pride and Prejudice".to_string(), 303);

    if !page_counts.contains_key("Les Misérables") {
        println!("We know about {} books, but not Les Misérables.",
                 page_counts.len());
    }

    for book in ["Pride and Prejudice", "Alice's Adventure in Wonderland"] {
        match page_counts.get(book) {
            Some(count) => println!("{book}: {count} pages"),
            None => println!("{book} is unknown.")
        }
    }

    // Use the .entry() method to insert a value if nothing is found.
    for book in ["Pride and Prejudice", "Alice's Adventure in Wonderland"] {
        let page_count: &mut i32 = page_counts.entry(book.to_string()).or_insert(0);
        *page_count += 1;
    }

    println!("{page_counts:#?}");
}

• HashMap is not defined in the prelude and needs to be brought into scope.

• Try the following lines of code. The first line will see if a book is in the hashmap and if not return an alternative value. The second line will insert the alternative value in the hashmap if the book is not found.

  let pc1 = page_counts
      .get("Harry Potter and the Sorcerer's Stone ")
      .unwrap_or(&336);
  let pc2 = page_counts
      .entry("The Hunger Games".to_string())
      .or_insert(374);
  

• Unlike vec!, there is unfortunately no standard hashmap! macro.