HashMap

Standard hash map with protection against HashDoS attacks:

# // Copyright 2023 Google LLC
# // SPDX-License-Identifier: Apache-2.0
#
use std::collections::HashMap;

fn main() {
    let mut page_counts = HashMap::new();
    page_counts.insert("Adventures of Huckleberry Finn", 207);
    page_counts.insert("Grimms' Fairy Tales", 751);
    page_counts.insert("Pride and Prejudice", 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).or_insert(0);
        *page_count += 1;
    }

    dbg!(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.

  rust,ignoreCopy

  # // Copyright 2023 Google LLC
  # // SPDX-License-Identifier: Apache-2.0
  #
  let pc1 = page_counts
      .get("Harry Potter and the Sorcerer's Stone")
      .unwrap_or(&336);
  let pc2 = page_counts
      .entry("The Hunger Games")
      .or_insert(374);
  

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

  • Although, since Rust 1.56, HashMap implements From<[(K, V); N]>, which allows us to easily initialize a hash map from a literal array:

    rust,ignoreCopy

    # // Copyright 2023 Google LLC
    # // SPDX-License-Identifier: Apache-2.0
    #
    let page_counts = HashMap::from([
      ("Harry Potter and the Sorcerer's Stone".to_string(), 336),
      ("The Hunger Games".to_string(), 374),
    ]);
    

• Alternatively HashMap can be built from any Iterator that yields key-value tuples.

• This type has several "method-specific" return types, such as std::collections::hash_map::Keys. These types often appear in searches of the Rust docs. Show students the docs for this type, and the helpful link back to the keys method.