Index

Storing Keys with Associated Values in Hash Maps

Storing Keys with Associated Values in Hash Maps

The type HashMap<K, V> stores a mapping of keys of type K to values of type V

It does this via a hashing function, which determines how it places these keys and values into memory

Creating a New Hash Map
You can create an empty hash map with new and add elements with insert.

fn main() {
    use std::collections::HashMap;

    let mut scores = HashMap::new();

    scores.insert(String::from("Blue"), 10);
    scores.insert(String::from("Yellow"), 50);
}

Note that we need to first use the HashMap from the collections portion of the standard library. Of our three common collections, this one is the least often used, so it’s not included in the features brought into scope automatically in the prelude.

Just like vectors, hash maps store their data on the heap. This HashMap has keys of type String and values of type i32

Like vectors, hash maps are homogeneous: all of the keys must have the same type, and all of the values must have the same type.

Another way of constructing a hash map is by using iterators and the collect method on a vector of tuples, where each tuple consists of a key and its value.

    use std::collections::HashMap;

    let teams = vec![String::from("Blue"), String::from("Yellow")];
    let initial_scores = vec![10, 50];

    let mut scores: HashMap<_, _> =
        teams.into_iter().zip(initial_scores.into_iter()).collect();

Hash Maps and Ownership

fn main() {
    use std::collections::HashMap;

    let field_name = String::from("Favorite color");
    let field_value = String::from("Blue");

    let mut map = HashMap::new();
    map.insert(field_name, field_value);
    // field_name and field_value are invalid at this point, try using them and
    // see what compiler error you get!
}

If we insert references to values into the hash map, the values won’t be moved into the hash map. The values that the references point to must be valid for at least as long as the hash map is valid.

Accessing Values in a Hash Map

    use std::collections::HashMap;

    let mut scores = HashMap::new();

    scores.insert(String::from("Blue"), 10);
    scores.insert(String::from("Yellow"), 50);

    let team_name = String::from("Blue");
    let score = scores.get(&team_name);

Here, score will have the value that’s associated with the Blue team, and the result will be Some(&10). The result is wrapped in Some because get returns an Option<&V>; if there’s no value for that key in the hash map, get will return None.

We can iterate over each key/value pair in a hash map in a similar manner as we do with vectors, using a for loop:

    use std::collections::HashMap;

    let mut scores = HashMap::new();

    scores.insert(String::from("Blue"), 10);
    scores.insert(String::from("Yellow"), 50);

    for (key, value) in &scores {
        println!("{}: {}", key, value);
    }

Updating a Hash Map

Overwriting a Value
If we insert a key and a value into a hash map and then insert that same key with a different value, the value associated with that key will be replaced.

    use std::collections::HashMap;

    let mut scores = HashMap::new();

    scores.insert(String::from("Blue"), 10);
    scores.insert(String::from("Blue"), 25);

    println!("{:?}", scores);

This code will print {"Blue": 25}. The original value of 10 has been overwritten.

Only Inserting a Value If the Key Has No Value

It’s common to check whether a particular key has a value and, if it doesn’t, insert a value for it. Hash maps have a special API for this called entry that takes the key you want to check as a parameter. The return value of the entry method is an enum called Entry that represents a value that might or might not exist. Let’s say we want to check whether the key for the Yellow team has a value associated with it. If it doesn’t, we want to insert the value 50, and the same for the Blue team

    use std::collections::HashMap;

    let mut scores = HashMap::new();
    scores.insert(String::from("Blue"), 10);

    scores.entry(String::from("Yellow")).or_insert(50);
    scores.entry(String::from("Blue")).or_insert(50);

    println!("{:?}", scores);

Running the code in Listing 8-25 will print {"Yellow": 50, "Blue": 10}

Updating a Value Based on the Old Value
Another common use case for hash maps is to look up a key’s value and then update it based on the old value. For instance, Listing shows code that counts how many times each word appears in some text. We use a hash map with the words as keys and increment the value to keep track of how many times we’ve seen that word. If it’s the first time we’ve seen a word, we’ll first insert the value 0.

fn main() {
    use std::collections::HashMap;

    let text = "hello world wonderful world";

    let mut map = HashMap::new();

    for word in text.split_whitespace() {
        let count = map.entry(word).or_insert(0);
        *count += 1;
    }

    println!("{:?}", map);
}

This code will print {"world": 2, "hello": 1, "wonderful": 1}.

The or_insert method actually returns a mutable reference (&mut V) to the value for this key. Here we store that mutable reference in the count variable, so in order to assign to that value, we must first dereference count using the asterisk (*).