Hash Maps
The last of our common collections is the hash map. 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. Many different programming languages support this kind of data
structure, but often with a different name: hash, map, object, hash table, or
associative array, just to name a few.
Hash maps are useful for when you want to be able to look up data not by an index, as you can with vectors, but by using a key that can be of any type. For example, in a game, you could keep track of each team’s score in a hash map where each key is a team’s name and the values are each team’s score. Given a team name, you can retrieve their score.
We’ll go over the basic API of hash maps in this chapter, but there are many
more goodies hiding in the functions defined on HashMap by the standard
library. As always, check the standard library documentation for more
information.
Creating a New Hash Map
We can create an empty HashMap with new, and add elements with insert.
Here we’re keeping track of the scores of two teams whose names are Blue and
Yellow. The Blue team will start with 10 points and the Yellow team starts with
50:
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 imported automatically in the
prelude. Hash maps also have less support from the standard library; there’s no
built-in macro to construct them, for example.
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 the collect method on a
vector of tuples, where each tuple consists of a key and its value. The
collect method gathers up data into a number of collection types, including
HashMap. For example, if we had the team names and initial scores in two
separate vectors, we can use the zip method to create a vector of tuples
where “Blue” is paired with 10, and so forth. Then we can use the collect
method to turn that vector of tuples into a HashMap:
use std::collections::HashMap;
let teams = vec![String::from("Blue"), String::from("Yellow")];
let initial_scores = vec![10, 50];
let scores: HashMap<_, _> = teams.iter().zip(initial_scores.iter()).collect();
The type annotation HashMap<_, _> is needed here because it’s possible to
collect into many different data structures, and Rust doesn’t know which you
want unless you specify. For the type parameters for the key and value types,
however, we use underscores and Rust can infer the types that the hash map
contains based on the types of the data in the vector.
Hash Maps and Ownership
For types that implement the Copy trait, like i32, the values are copied
into the hash map. For owned values like String, the values will be moved and
the hash map will be the owner of those values:
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
We would not be able to use the bindings field_name and field_value after
they have been moved into the hash map with the call to insert.
If we insert references to values into the hash map, the values themselves will not 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, though. We will talk more about these issues in the Lifetimes section of Chapter 10.
Accessing Values in a Hash Map
We can get a value out of the hash map by providing its key to the get method:
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. The program will need to handle the Option in one of the
ways that we covered in Chapter 6.
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);
}
This will print each pair, in an arbitrary order:
Yellow: 50
Blue: 10
Updating a Hash Map
While the number of keys and values is growable, each individual key can only have one value associated with it at a time. When we want to change the data in a hash map, we have to decide how to handle the case when a key already has a value assigned. We could choose to replace the old value with the new value, completely disregarding the old value. We could choose to keep the old value and ignore the new value, and only add the new value if the key doesn’t already have a value. Or we could combine the old value and the new value. Let’s look at how to do each of these!
Overwriting a Value
If we insert a key and a value into a hash map, then insert that same key with
a different value, the value associated with that key will be replaced. Even
though this following code calls insert twice, the hash map will only contain
one key/value pair because we’re inserting the value for the Blue team’s key
both times:
use std::collections::HashMap;
let mut scores = HashMap::new();
scores.insert(String::from("Blue"), 10);
scores.insert(String::from("Blue"), 25);
println!("{:?}", scores);
This will print {"Blue": 25}. The original value of 10 has been overwritten.
Only Insert If the Key Has No Value
It’s common to want to check if a particular key has a value and, if it does
not, insert a value for it. Hash maps have a special API for this, called
entry, that takes the key we want to check as an argument. The return value
of the entry function is an enum, Entry, that represents a value that might
or might not exist. Let’s say that we want to check if 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. With the entry API, the code for this looks
like:
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);
The or_insert method on Entry returns the value for the corresponding
Entry key if it exists, and if not, inserts its argument as the new value for
this key and returns the modified Entry. This is much cleaner than writing
the logic ourselves, and in addition, plays more nicely with the borrow checker.
This code will print {"Yellow": 50, "Blue": 10}. The first call to entry
will insert the key for the Yellow team with the value 50, since the Yellow
team doesn’t have a value already. The second call to entry will not change
the hash map since the Blue team already has the value 10.
Update a Value Based on the Old Value
Another common use case for hash maps is to look up a key’s value then update
it, based on the old value. For instance, if we wanted to count how many times
each word appeared in some text, we could 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 this is the first time we’ve seen a word, we’ll first insert the value 0.
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 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
(*). The mutable reference goes out of scope at the end of the for loop, so
all of these changes are safe and allowed by the borrowing rules.
Hashing Function
By default, HashMap uses a cryptographically secure hashing function that can
provide resistance to Denial of Service (DoS) attacks. This is not the fastest
hashing algorithm out there, but the tradeoff for better security that comes
with the drop in performance is worth it. If you profile your code and find
that the default hash function is too slow for your purposes, you can switch to
another function by specifying a different hasher. A hasher is a type that
implements the BuildHasher trait. We’ll be talking about traits and how to
implement them in Chapter 10. You don't necessarily have to implement your own
hasher from scratch; crates.io has libraries that others have shared that
provide hashers implementing many common hashing algorithms.
Summary
Vectors, strings, and hash maps will take you far in programs where you need to store, access, and modify data. Here are some exercises you should now be equipped to solve:
- Given a list of integers, use a vector and return the mean (average), median (when sorted, the value in the middle position), and mode (the value that occurs most often; a hash map will be helpful here) of the list.
- Convert strings to Pig Latin, where the first consonant of each word is moved to the end of the word with an added “ay”, so “first” becomes “irst-fay”. Words that start with a vowel get “hay” added to the end instead (“apple” becomes “apple-hay”). Remember about UTF-8 encoding!
- Using a hash map and vectors, create a text interface to allow a user to add employee names to a department in the company. For example, “Add Sally to Engineering” or “Add Amir to Sales”. Then let the user retrieve a list of all people in a department or all people in the company by department, sorted alphabetically.
The standard library API documentation describes methods these types have that will be helpful for these exercises!
We’re getting into more complex programs where operations can fail, which means it’s a perfect time to go over error handling next!