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Common Collections
Rust’s standard library includes a number of very useful data structures called collections. Most other data types represent one specific value, but collections can contain multiple values. Unlike the built-in array and tuple types, the data these collections point to is stored on the heap, which means the amount of data does not need to be known at compile time and can grow or shrink as the program runs. Each kind of collection has different capabilities and costs, and choosing an appropriate one for your current situation is a skill you’ll develop over time. In this chapter, we’ll discuss three collections that are used very often in Rust programs:
- A vector allows you to store a variable number of values next to each other.
- A string is a collection of characters. We’ve mentioned the
Stringtype previously, but in this chapter we’ll talk about it in depth. - A hash map allows you to associate a value with a specific key. It’s a particular implementation of the more general data structure called a map.
To learn about the other kinds of collections provided by the standard library, see the documentation at https://doc.rust-lang.org/std/collections/index.html.
We’ll discuss how to create and update vectors, strings, and hash maps, as well as what makes each special.
Storing Lists of Values with Vectors
The first collection type we’ll look at is Vec<T>, also known as a vector.
Vectors allow you to store more than one value in a single data structure that
puts all the values next to each other in memory. Vectors can only store values
of the same type. They are useful when you have a list of items, such as the
lines of text in a file or the prices of items in a shopping cart.
Creating a New Vector
To create a new empty vector, we call the Vec::new function, as shown in
Listing 8-1.
let v: Vec<i32> = Vec::new();
Listing 8-1: Creating a new, empty vector to hold values of type i32
Note that we added a type annotation here. Because we aren’t inserting any
values into this vector, Rust doesn’t know what kind of elements we intend to
store. This is an important point. Vectors are implemented using generics;
we’ll cover how to use generics with your own types in Chapter 10. For now,
know that the Vec<T> type provided by the standard library can hold any type.
When we create a vector to hold a specific type, we can specify the type within
angle brackets. In Listing 8-1, we’ve told Rust that the Vec<T> in v will
hold elements of the i32 type.
More often, you’ll create a Vec<T> with initial values and Rust will infer
the type of value you want to store, so you rarely need to do this type
annotation. Rust conveniently provides the vec! macro, which will create a
new vector that holds the values you give it. Listing 8-2 creates a new
Vec<i32> that holds the values 1, 2, and 3. The integer type is i32
because that’s the default integer type, as we discussed in “Data Types” on
page XX.
let v = vec![1, 2, 3];
Listing 8-2: Creating a new vector containing values
Because we’ve given initial i32 values, Rust can infer that the type of v
is Vec<i32>, and the type annotation isn’t necessary. Next, we’ll look at how
to modify a vector.
Updating a Vector
To create a vector and then add elements to it, we can use the push method,
as shown in Listing 8-3.
let mut v = Vec::new();
v.push(5);
v.push(6);
v.push(7);
v.push(8);
Listing 8-3: Using the push method to add values to a vector
As with any variable, if we want to be able to change its value, we need to
make it mutable using the mut keyword, as discussed in Chapter 3. The numbers
we place inside are all of type i32, and Rust infers this from the data, so
we don’t need the Vec<i32> annotation.
Reading Elements of Vectors
There are two ways to reference a value stored in a vector: via indexing or by
using the get method. In the following examples, we’ve annotated the types of
the values that are returned from these functions for extra clarity.
Listing 8-4 shows both methods of accessing a value in a vector, with indexing
syntax and the get method.
let v = vec![1, 2, 3, 4, 5];
1 let third: &i32 = &v[2];
println!("The third element is {third}");
2 let third: Option<&i32> = v.get(2);
match third {
Some(third) => println!("The third element is {third}"),
None => println!("There is no third element."),
}
Listing 8-4: Using indexing syntax and using the get method to access an item
in a vector
Note a few details here. We use the index value of 2 to get the third element
[1] because vectors are indexed by number, starting at zero. Using & and []
gives us a reference to the element at the index value. When we use the get
method with the index passed as an argument [2], we get an Option<&T> that we
can use with match.
Rust provides these two ways to reference an element so you can choose how the program behaves when you try to use an index value outside the range of existing elements. As an example, let’s see what happens when we have a vector of five elements and then we try to access an element at index 100 with each technique, as shown in Listing 8-5.
let v = vec![1, 2, 3, 4, 5];
let does_not_exist = &v[100];
let does_not_exist = v.get(100);
Listing 8-5: Attempting to access the element at index 100 in a vector containing five elements
When we run this code, the first [] method will cause the program to panic
because it references a nonexistent element. This method is best used when you
want your program to crash if there’s an attempt to access an element past the
end of the vector.
When the get method is passed an index that is outside the vector, it returns
None without panicking. You would use this method if accessing an element
beyond the range of the vector may happen occasionally under normal
circumstances. Your code will then have logic to handle having either
Some(&element) or None, as discussed in Chapter 6. For example, the index
could be coming from a person entering a number. If they accidentally enter a
number that’s too large and the program gets a None value, you could tell the
user how many items are in the current vector and give them another chance to
enter a valid value. That would be more user-friendly than crashing the program
due to a typo!
When the program has a valid reference, the borrow checker enforces the ownership and borrowing rules (covered in Chapter 4) to ensure this reference and any other references to the contents of the vector remain valid. Recall the rule that states you can’t have mutable and immutable references in the same scope. That rule applies in Listing 8-6, where we hold an immutable reference to the first element in a vector and try to add an element to the end. This program won’t work if we also try to refer to that element later in the function.
let mut v = vec![1, 2, 3, 4, 5];
let first = &v[0];
v.push(6);
println!("The first element is: {first}");
Listing 8-6: Attempting to add an element to a vector while holding a reference to an item
Compiling this code will result in this error:
error[E0502]: cannot borrow `v` as mutable because it is also borrowed as
immutable
--> src/main.rs:6:5
|
4 | let first = &v[0];
| - immutable borrow occurs here
5 |
6 | v.push(6);
| ^^^^^^^^^ mutable borrow occurs here
7 |
8 | println!("The first element is: {first}");
| ----- immutable borrow later used here
The code in Listing 8-6 might look like it should work: why should a reference to the first element care about changes at the end of the vector? This error is due to the way vectors work: because vectors put the values next to each other in memory, adding a new element onto the end of the vector might require allocating new memory and copying the old elements to the new space, if there isn’t enough room to put all the elements next to each other where the vector is currently stored. In that case, the reference to the first element would be pointing to deallocated memory. The borrowing rules prevent programs from ending up in that situation.
Note: For more on the implementation details of the
Vec<T>type, see “The Rustonomicon” at https://doc.rust-lang.org/nomicon/vec/vec.html.
Iterating Over the Values in a Vector
To access each element in a vector in turn, we would iterate through all of the
elements rather than use indices to access one at a time. Listing 8-7 shows how
to use a for loop to get immutable references to each element in a vector of
i32 values and print them.
let v = vec![100, 32, 57];
for i in &v {
println!("{i}");
}
Listing 8-7: Printing each element in a vector by iterating over the elements
using a for loop
We can also iterate over mutable references to each element in a mutable vector
in order to make changes to all the elements. The for loop in Listing 8-8
will add 50 to each element.
let mut v = vec![100, 32, 57];
for i in &mut v {
*i += 50;
}
Listing 8-8: Iterating over mutable references to elements in a vector
To change the value that the mutable reference refers to, we have to use the
* dereference operator to get to the value in i before we can use the +=
operator. We’ll talk more about the dereference operator in “Following the
Pointer to the Value” on page XX.
Iterating over a vector, whether immutably or mutably, is safe because of the
borrow checker’s rules. If we attempted to insert or remove items in the for
loop bodies in Listing 8-7 and Listing 8-8, we would get a compiler error
similar to the one we got with the code in Listing 8-6. The reference to the
vector that the for loop holds prevents simultaneous modification of the
whole vector.
Using an Enum to Store Multiple Types
Vectors can only store values that are of the same type. This can be inconvenient; there are definitely use cases for needing to store a list of items of different types. Fortunately, the variants of an enum are defined under the same enum type, so when we need one type to represent elements of different types, we can define and use an enum!
For example, say we want to get values from a row in a spreadsheet in which some of the columns in the row contain integers, some floating-point numbers, and some strings. We can define an enum whose variants will hold the different value types, and all the enum variants will be considered the same type: that of the enum. Then we can create a vector to hold that enum and so, ultimately, holds different types. We’ve demonstrated this in Listing 8-9.
enum SpreadsheetCell {
Int(i32),
Float(f64),
Text(String),
}
let row = vec![
SpreadsheetCell::Int(3),
SpreadsheetCell::Text(String::from("blue")),
SpreadsheetCell::Float(10.12),
];
Listing 8-9: Defining an enum to store values of different types in one vector
Rust needs to know what types will be in the vector at compile time so it knows
exactly how much memory on the heap will be needed to store each element. We
must also be explicit about what types are allowed in this vector. If Rust
allowed a vector to hold any type, there would be a chance that one or more of
the types would cause errors with the operations performed on the elements of
the vector. Using an enum plus a match expression means that Rust will ensure
at compile time that every possible case is handled, as discussed in Chapter 6.
If you don’t know the exhaustive set of types a program will get at runtime to store in a vector, the enum technique won’t work. Instead, you can use a trait object, which we’ll cover in Chapter 17.
Now that we’ve discussed some of the most common ways to use vectors, be sure
to review the API documentation for all of the many useful methods defined on
Vec<T> by the standard library. For example, in addition to push, a pop
method removes and returns the last element.
Dropping a Vector Drops Its Elements
Like any other struct, a vector is freed when it goes out of scope, as
annotated in Listing 8-10.
{
let v = vec![1, 2, 3, 4];
// do stuff with v
} // <- v goes out of scope and is freed here
Listing 8-10: Showing where the vector and its elements are dropped
When the vector gets dropped, all of its contents are also dropped, meaning the integers it holds will be cleaned up. The borrow checker ensures that any references to contents of a vector are only used while the vector itself is valid.
Let’s move on to the next collection type: String!
Storing UTF-8 Encoded Text with Strings
We talked about strings in Chapter 4, but we’ll look at them in more depth now. New Rustaceans commonly get stuck on strings for a combination of three reasons: Rust’s propensity for exposing possible errors, strings being a more complicated data structure than many programmers give them credit for, and UTF-8. These factors combine in a way that can seem difficult when you’re coming from other programming languages.
We discuss strings in the context of collections because strings are
implemented as a collection of bytes, plus some methods to provide useful
functionality when those bytes are interpreted as text. In this section, we’ll
talk about the operations on String that every collection type has, such as
creating, updating, and reading. We’ll also discuss the ways in which String
is different from the other collections, namely how indexing into a String is
complicated by the differences between how people and computers interpret
String data.
What Is a String?
We’ll first define what we mean by the term string. Rust has only one string
type in the core language, which is the string slice str that is usually seen
in its borrowed form &str. In Chapter 4, we talked about string slices,
which are references to some UTF-8 encoded string data stored elsewhere. String
literals, for example, are stored in the program’s binary and are therefore
string slices.
The String type, which is provided by Rust’s standard library rather than
coded into the core language, is a growable, mutable, owned, UTF-8 encoded
string type. When Rustaceans refer to “strings” in Rust, they might be
referring to either the String or the string slice &str types, not just one
of those types. Although this section is largely about String, both types are
used heavily in Rust’s standard library, and both String and string slices
are UTF-8 encoded.
Creating a New String
Many of the same operations available with Vec<T> are available with String
as well because String is actually implemented as a wrapper around a vector
of bytes with some extra guarantees, restrictions, and capabilities. An example
of a function that works the same way with Vec<T> and String is the new
function to create an instance, shown in Listing 8-11.
let mut s = String::new();
Listing 8-11: Creating a new, empty String
This line creates a new, empty string called s, into which we can then load
data. Often, we’ll have some initial data with which we want to start the
string. For that, we use the to_string method, which is available on any type
that implements the Display trait, as string literals do. Listing 8-12 shows
two examples.
let data = "initial contents";
let s = data.to_string();
// the method also works on a literal directly:
let s = "initial contents".to_string();
Listing 8-12: Using the to_string method to create a String from a string
literal
This code creates a string containing initial contents.
We can also use the function String::from to create a String from a string
literal. The code in Listing 8-13 is equivalent to the code in Listing 8-12
that uses to_string.
let s = String::from("initial contents");
Listing 8-13: Using the String::from function to create a String from a
string literal
Because strings are used for so many things, we can use many different generic
APIs for strings, providing us with a lot of options. Some of them can seem
redundant, but they all have their place! In this case, String::from and
to_string do the same thing, so which one you choose is a matter of style and
readability.
Remember that strings are UTF-8 encoded, so we can include any properly encoded data in them, as shown in Listing 8-14.
let hello = String::from("السلام عليكم");
let hello = String::from("Dobrý den");
let hello = String::from("Hello");
let hello = String::from("שָׁלוֹם");
let hello = String::from("नमस्ते");
let hello = String::from("こんにちは");
let hello = String::from("안녕하세요");
let hello = String::from("你好");
let hello = String::from("Olá");
let hello = String::from("Здравствуйте");
let hello = String::from("Hola");
Listing 8-14: Storing greetings in different languages in strings
All of these are valid String values.
Updating a String
A String can grow in size and its contents can change, just like the contents
of a Vec<T>, if you push more data into it. In addition, you can conveniently
use the + operator or the format! macro to concatenate String values.
Appending to a String with push_str and push
We can grow a String by using the push_str method to append a string slice,
as shown in Listing 8-15.
let mut s = String::from("foo");
s.push_str("bar");
Listing 8-15: Appending a string slice to a String using the push_str method
After these two lines, s will contain foobar. The push_str method takes a
string slice because we don’t necessarily want to take ownership of the
parameter. For example, in the code in Listing 8-16, we want to be able to use
s2 after appending its contents to s1.
let mut s1 = String::from("foo");
let s2 = "bar";
s1.push_str(s2);
println!("s2 is {s2}");
Listing 8-16: Using a string slice after appending its contents to a String
If the push_str method took ownership of s2, we wouldn’t be able to print
its value on the last line. However, this code works as we’d expect!
The push method takes a single character as a parameter and adds it to the
String. Listing 8-17 adds the letter l to a String using the push
method.
let mut s = String::from("lo");
s.push('l');
Listing 8-17: Adding one character to a String value using push
As a result, s will contain lol.
Concatenation with the + Operator or the format! Macro
Often, you’ll want to combine two existing strings. One way to do so is to use
the + operator, as shown in Listing 8-18.
let s1 = String::from("Hello, ");
let s2 = String::from("world!");
let s3 = s1 + &s2; // note s1 has been moved here and can no longer be used
Listing 8-18: Using the + operator to combine two String values into a new
String value
The string s3 will contain Hello, world!. The reason s1 is no longer
valid after the addition, and the reason we used a reference to s2, has to do
with the signature of the method that’s called when we use the + operator.
The + operator uses the add method, whose signature looks something like
this:
fn add(self, s: &str) -> String {
In the standard library, you’ll see add defined using generics and associated
types. Here, we’ve substituted in concrete types, which is what happens when we
call this method with String values. We’ll discuss generics in Chapter 10.
This signature gives us the clues we need in order to understand the tricky
bits of the + operator.
First, s2 has an &, meaning that we’re adding a reference of the second
string to the first string. This is because of the s parameter in the add
function: we can only add a &str to a String; we can’t add two String
values together. But wait—the type of &s2 is &String, not &str, as
specified in the second parameter to add. So why does Listing 8-18 compile?
The reason we’re able to use &s2 in the call to add is that the compiler
can coerce the &String argument into a &str. When we call the add
method, Rust uses a deref coercion, which here turns &s2 into &s2[..].
We’ll discuss deref coercion in more depth in Chapter 15. Because add does
not take ownership of the s parameter, s2 will still be a valid String
after this operation.
Second, we can see in the signature that add takes ownership of self
because self does not have an &. This means s1 in Listing 8-18 will be
moved into the add call and will no longer be valid after that. So, although
let s3 = s1 + &s2; looks like it will copy both strings and create a new one,
this statement actually takes ownership of s1, appends a copy of the contents
of s2, and then returns ownership of the result. In other words, it looks
like it’s making a lot of copies, but it isn’t; the implementation is more
efficient than copying.
If we need to concatenate multiple strings, the behavior of the + operator
gets unwieldy:
let s1 = String::from("tic");
let s2 = String::from("tac");
let s3 = String::from("toe");
let s = s1 + "-" + &s2 + "-" + &s3;
At this point, s will be tic-tac-toe. With all of the + and "
characters, it’s difficult to see what’s going on. For combining strings in
more complicated ways, we can instead use the format! macro:
let s1 = String::from("tic");
let s2 = String::from("tac");
let s3 = String::from("toe");
let s = format!("{s1}-{s2}-{s3}");
This code also sets s to tic-tac-toe. The format! macro works like
println!, but instead of printing the output to the screen, it returns a
String with the contents. The version of the code using format! is much
easier to read, and the code generated by the format! macro uses references
so that this call doesn’t take ownership of any of its parameters.
Indexing into Strings
In many other programming languages, accessing individual characters in a
string by referencing them by index is a valid and common operation. However,
if you try to access parts of a String using indexing syntax in Rust, you’ll
get an error. Consider the invalid code in Listing 8-19.
let s1 = String::from("hello");
let h = s1[0];
Listing 8-19: Attempting to use indexing syntax with a String
This code will result in the following error:
error[E0277]: the type `String` cannot be indexed by `{integer}`
--> src/main.rs:3:13
|
3 | let h = s1[0];
| ^^^^^ `String` cannot be indexed by `{integer}`
|
= help: the trait `Index<{integer}>` is not implemented for
`String`
The error and the note tell the story: Rust strings don’t support indexing. But why not? To answer that question, we need to discuss how Rust stores strings in memory.
Internal Representation
A String is a wrapper over a Vec<u8>. Let’s look at some of our properly
encoded UTF-8 example strings from Listing 8-14. First, this one:
let hello = String::from("Hola");
In this case, len will be 4, which means the vector storing the string
"Hola" is 4 bytes long. Each of these letters takes one byte when encoded in
UTF-8. The following line, however, may surprise you (note that this string
begins with the capital Cyrillic letter Ze, not the Arabic number 3):
let hello = String::from("Здравствуйте");
If you were asked how long the string is, you might say 12. In fact, Rust’s answer is 24: that’s the number of bytes it takes to encode “Здравствуйте” in UTF-8, because each Unicode scalar value in that string takes 2 bytes of storage. Therefore, an index into the string’s bytes will not always correlate to a valid Unicode scalar value. To demonstrate, consider this invalid Rust code:
let hello = "Здравствуйте";
let answer = &hello[0];
You already know that answer will not be З, the first letter. When encoded
in UTF-8, the first byte of З is 208 and the second is 151, so it would
seem that answer should in fact be 208, but 208 is not a valid character
on its own. Returning 208 is likely not what a user would want if they asked
for the first letter of this string; however, that’s the only data that Rust
has at byte index 0. Users generally don’t want the byte value returned, even
if the string contains only Latin letters: if &"hello"[0] were valid code
that returned the byte value, it would return 104, not h.
The answer, then, is that to avoid returning an unexpected value and causing bugs that might not be discovered immediately, Rust doesn’t compile this code at all and prevents misunderstandings early in the development process.
Bytes and Scalar Values and Grapheme Clusters! Oh My!
Another point about UTF-8 is that there are actually three relevant ways to look at strings from Rust’s perspective: as bytes, scalar values, and grapheme clusters (the closest thing to what we would call letters).
If we look at the Hindi word “नमस्ते” written in the Devanagari script, it is
stored as a vector of u8 values that looks like this:
[224, 164, 168, 224, 164, 174, 224, 164, 184, 224, 165, 141, 224,
164, 164, 224, 165, 135]
That’s 18 bytes and is how computers ultimately store this data. If we look at
them as Unicode scalar values, which are what Rust’s char type is, those
bytes look like this:
['न', 'म', 'स', '्', 'त', 'े']
There are six char values here, but the fourth and sixth are not letters:
they’re diacritics that don’t make sense on their own. Finally, if we look at
them as grapheme clusters, we’d get what a person would call the four letters
that make up the Hindi word:
["न", "म", "स्", "ते"]
Rust provides different ways of interpreting the raw string data that computers store so that each program can choose the interpretation it needs, no matter what human language the data is in.
A final reason Rust doesn’t allow us to index into a String to get a
character is that indexing operations are expected to always take constant time
(O(1)). But it isn’t possible to guarantee that performance with a String,
because Rust would have to walk through the contents from the beginning to the
index to determine how many valid characters there were.
Slicing Strings
Indexing into a string is often a bad idea because it’s not clear what the return type of the string-indexing operation should be: a byte value, a character, a grapheme cluster, or a string slice. If you really need to use indices to create string slices, therefore, Rust asks you to be more specific.
Rather than indexing using [] with a single number, you can use [] with a
range to create a string slice containing particular bytes:
let hello = "Здравствуйте";
let s = &hello[0..4];
Here, s will be a &str that contains the first four bytes of the string.
Earlier, we mentioned that each of these characters was two bytes, which means
s will be Зд.
If we were to try to slice only part of a character’s bytes with something like
&hello[0..1], Rust would panic at runtime in the same way as if an invalid
index were accessed in a vector:
thread 'main' panicked at 'byte index 1 is not a char boundary;
it is inside 'З' (bytes 0..2) of `Здравствуйте`', src/main.rs:4:14
You should use caution when creating string slices with ranges, because doing so can crash your program.
Methods for Iterating Over Strings
The best way to operate on pieces of strings is to be explicit about whether
you want characters or bytes. For individual Unicode scalar values, use the
chars method. Calling chars on “Зд” separates out and returns two values of
type char, and you can iterate over the result to access each element:
for c in "Зд".chars() {
println!("{c}");
}
This code will print the following:
З
д
Alternatively, the bytes method returns each raw byte, which might be
appropriate for your domain:
for b in "Зд".bytes() {
println!("{b}");
}
This code will print the four bytes that make up this string:
208
151
208
180
But be sure to remember that valid Unicode scalar values may be made up of more than one byte.
Getting grapheme clusters from strings, as with the Devanagari script, is complex, so this functionality is not provided by the standard library. Crates are available at https://crates.io if this is the functionality you need.
Strings Are Not So Simple
To summarize, strings are complicated. Different programming languages make
different choices about how to present this complexity to the programmer. Rust
has chosen to make the correct handling of String data the default behavior
for all Rust programs, which means programmers have to put more thought into
handling UTF-8 data up front. This trade-off exposes more of the complexity of
strings than is apparent in other programming languages, but it prevents you
from having to handle errors involving non-ASCII characters later in your
development life cycle.
The good news is that the standard library offers a lot of functionality built
off the String and &str types to help handle these complex situations
correctly. Be sure to check out the documentation for useful methods like
contains for searching in a string and replace for substituting parts of a
string with another string.
Let’s switch to something a bit less complex: hash maps!
Storing Keys with Associated Values in 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 using a hashing
function, which determines how it places these keys and values into memory.
Many programming languages support this kind of data structure, but they often
use a different name, such as hash, map, object, hash table,
dictionary, or associative array, just to name a few.
Hash maps are useful when you want to look up data not by using 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 in which each key is a team’s name and the values are each team’s score. Given a team name, you can retrieve its score.
We’ll go over the basic API of hash maps in this section, but many more goodies
are hiding in the functions defined on HashMap<K, V> by the standard library.
As always, check the standard library documentation for more information.
Creating a New Hash Map
One way to create an empty hash map is to use new and to add elements with
insert. In Listing 8-20, we’re keeping track of the scores of two teams whose
names are Blue and Yellow. The Blue team starts 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);
Listing 8-20: Creating a new hash map and inserting some keys and values
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. 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.
Accessing Values in a Hash Map
We can get a value out of the hash map by providing its key to the get
method, as shown in Listing 8-21.
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).copied().unwrap_or(0);
Listing 8-21: Accessing the score for the Blue team stored in the hash map
Here, score will have the value that’s associated with the Blue team, and the
result will be 10. The get method returns an Option<&V>; if there’s no
value for that key in the hash map, get will return None. This program
handles the Option by calling copied to get an Option<i32> rather than an
Option<&i32>, then unwrap_or to set score to zero if scores doesn’t
have an entry for the key.
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 code will print each pair in an arbitrary order:
Yellow: 50
Blue: 10
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, as demonstrated in Listing 8-22.
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!
Listing 8-22: Showing that keys and values are owned by the hash map once they’re inserted
We aren’t able to use the variables field_name and field_value after
they’ve been moved into the hash map with the call to insert.
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. We’ll talk more about these issues in “Validating References with Lifetimes” on page XX.
Updating a Hash Map
Although the number of key and value pairs is growable, each unique key can
only have one value associated with it at a time (but not vice versa: for
example, both the Blue team and the Yellow team could have the value 10
stored in the scores hash map).
When you want to change the data in a hash map, you have to decide how to handle the case when a key already has a value assigned. You could replace the old value with the new value, completely disregarding the old value. You could keep the old value and ignore the new value, only adding the new value if the key doesn’t already have a value. Or you 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 and then insert that same key
with a different value, the value associated with that key will be replaced.
Even though the code in Listing 8-23 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);
Listing 8-23: Replacing a value stored with a particular key
This code will print {"Blue": 25}. The original value of 10 has been
overwritten.
Adding a Key and Value Only If a Key Isn’t Present
It’s common to check whether a particular key already exists in the hash map with a value and then to take the following actions: if the key does exist in the hash map, the existing value should remain the way it is; if the key doesn’t exist, insert it and 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. Using the entry API, the code looks like Listing 8-24.
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);
Listing 8-24: Using the entry method to only insert if the key does not
already have a value
The or_insert method on Entry is defined to return a mutable reference to
the value for the corresponding Entry key if that key exists, and if not, it
inserts the parameter as the new value for this key and returns a mutable
reference to the new value. This technique is much cleaner than writing the
logic ourselves and, in addition, plays more nicely with the borrow checker.
Running the code in Listing 8-24 will print {"Yellow": 50, "Blue": 10}. The
first call to entry will insert the key for the Yellow team with the value
50 because the Yellow team doesn’t have a value already. The second call to
entry will not change the hash map because the Blue team already has the
value 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 8-25 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.
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);
Listing 8-25: Counting occurrences of words using a hash map that stores words and counts
This code will print {"world": 2, "hello": 1, "wonderful": 1}. You might see
the same key–value pairs printed in a different order: recall from “Accessing
Values in a Hash Map” on page XX that iterating over a hash map happens in an
arbitrary order.
The split_whitespace method returns an iterator over subslices, separated by
whitespace, of the value in text. The or_insert method returns a mutable
reference (&mut V) to the value for the specified 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 Functions
By default, HashMap uses a hashing function called SipHash that can provide
resistance to denial-of-service (DoS) attacks involving hash tables. This is
not the fastest hashing algorithm available, but the trade-off 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 talk about
traits and how to implement them in Chapter 10. You don’t necessarily have to
implement your own hasher from scratch; https://crates.io has libraries
shared by other Rust users that provide hashers implementing many common
hashing algorithms.
Summary
Vectors, strings, and hash maps will provide a large amount of functionality necessary in programs when 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 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. The first consonant of each word is moved to the end of the word and ay is added, so first becomes irst-fay. Words that start with a vowel have hay added to the end instead (apple becomes apple-hay). Keep in mind the details 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 a 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 that vectors, strings, and hash maps have that will be helpful for these exercises!
We’re getting into more complex programs in which operations can fail, so it’s a perfect time to discuss error handling. We’ll do that next!