Let’s jump into Rust by working through a hands-on project together! This
chapter introduces you to a few common Rust concepts by showing you how to use
them in a real program. You’ll learn about let, match, methods, associated
functions, using external crates, and more! The following chapters will explore
these ideas in more detail. In this chapter, you’ll practice the fundamentals.
We’ll implement a classic beginner programming problem: a guessing game. Here’s how it works: the program will generate a random integer between 1 and 100. It will then prompt the player to enter a guess. After a guess is entered, the program will indicate whether the guess is too low or too high. If the guess is correct, the game will print a congratulatory message and exit.
Setting Up a New Project
To set up a new project, go to the projects directory that you created in Chapter 1 and make a new project using Cargo, like so:
$ cargo new guessing_game
$ cd guessing_game
The first command, cargo new, takes the name of the project (guessing_game)
as the first argument. The second command changes to the new project’s
directory.
Look at the generated Cargo.toml file:
Filename: Cargo.toml
[package]
name = "guessing_game"
version = "0.1.0"
edition = "2018"
# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
[dependencies]
As you saw in Chapter 1, cargo new generates a “Hello, world!” program for
you. Check out the src/main.rs file:
Filename: src/main.rs
#![allow(unused)] fn main() { fn main() { println!("Hello, world!"); } }
Now let’s compile this “Hello, world!” program and run it in the same step
using the cargo run command:
$ cargo run
Compiling guessing_game v0.1.0 (file:///projects/guessing_game)
Finished dev [unoptimized + debuginfo] target(s) in 1.50s
Running `target/debug/guessing_game`
Hello, world!
The run command comes in handy when you need to rapidly iterate on a project,
as we’ll do in this game, quickly testing each iteration before moving on to
the next one.
Reopen the src/main.rs file. You’ll be writing all the code in this file.
Processing a Guess
The first part of the guessing game program will ask for user input, process that input, and check that the input is in the expected form. To start, we’ll allow the player to input a guess. Enter the code in Listing 2-1 into src/main.rs.
Filename: src/main.rs
#![allow(unused)] fn main() { use std::io; fn main() { println!("Guess the number!"); println!("Please input your guess."); let mut guess = String::new(); io::stdin() .read_line(buf: &mut guess) .expect(msg: "Failed to read line"); println!("You guessed: {}", guess); } }
Listing 2-1: Code that gets a guess from the user and prints it
This code contains a lot of information, so let’s go over it line by line. To
obtain user input and then print the result as output, we need to bring the
io (input/output) library into scope. The io library comes from the
standard library (which is known as std):
use std::io; fn main() { println!("Guess the number!"); println!("Please input your guess."); let mut guess = String::new(); io::stdin() .read_line(&mut guess) .expect("Failed to read line"); println!("You guessed: {}", guess); }
By default, Rust brings only a few types into the scope of every program in
the prelude. If a type you want to use isn’t in the
prelude, you have to bring that type into scope explicitly with a use
statement. Using the std::io library provides you with a number of useful
features, including the ability to accept user input.
As you saw in Chapter 1, the main function is the entry point into the
program:
#![allow(unused)] fn main() { use std::io; fn main() { println!("Guess the number!"); println!("Please input your guess."); let mut guess = String::new(); io::stdin() .read_line(&mut guess) .expect("Failed to read line"); println!("You guessed: {}", guess); } }
The fn syntax declares a new function, the parentheses, (), indicate there
are no parameters, and the curly bracket, {, starts the body of the function.
As you also learned in Chapter 1, println! is a macro that prints a string to
the screen:
use std::io; fn main() { println!("Guess the number!"); println!("Please input your guess."); let mut guess = String::new(); io::stdin() .read_line(&mut guess) .expect("Failed to read line"); println!("You guessed: {}", guess); }
This code is printing a prompt stating what the game is and requesting input from the user.
Storing Values with Variables
Next, we’ll create a place to store the user input, like this:
use std::io; fn main() { println!("Guess the number!"); println!("Please input your guess."); let mut guess = String::new(); io::stdin() .read_line(&mut guess) .expect("Failed to read line"); println!("You guessed: {}", guess); }
Now the program is getting interesting! There’s a lot going on in this little
line. Notice that this is a let statement, which is used to create a
variable. Here’s another example:
#![allow(unused)] fn main() { let apples: i32 = 5; }
This line creates a new variable named apples and binds it to the value 5. In
Rust, variables are immutable by default. We’ll be discussing this concept in
detail in the “Variables and Mutability” section in Chapter 3. The following example shows how to use mut before
the variable name to make a variable mutable:
#![allow(unused)] fn main() { let apples: i32 = 5; // immutable let mut bananas: i32 = 5; // mutable }
Note: The
//syntax starts a comment that continues until the end of the line. Rust ignores everything in comments, which are discussed in more detail in Chapter 3.
Let’s return to the guessing game program. You now know that let mut guess
will introduce a mutable variable named guess. On the other side of the equal
sign (=) is the value that guess is bound to, which is the result of
calling String::new, a function that returns a new instance of a String.
String is a string type provided by the standard
library that is a growable, UTF-8 encoded bit of text.
The :: syntax in the ::new line indicates that new is an associated
function of the String type. An associated function is implemented on a
type, in this case String.
This new function creates a new, empty string. You’ll find a new function
on many types, because it’s a common name for a function that makes a new value
of some kind.
To summarize, the let mut guess = String::new(); line has created a mutable
variable that is currently bound to a new, empty instance of a String. Whew!
Recall that we included the input/output functionality from the standard
library with use std::io; on the first line of the program. Now we’ll call
the stdin function from the io module:
use std::io; fn main() { println!("Guess the number!"); println!("Please input your guess."); let mut guess = String::new(); io::stdin() .read_line(buf: &mut guess) .expect("Failed to read line"); println!("You guessed: {}", guess); }
If we hadn’t put the use std::io line at the beginning of the program, we
could have written this function call as std::io::stdin. The stdin function
returns an instance of std::io::Stdin, which is a
type that represents a handle to the standard input for your terminal.
The next part of the code, .read_line(&mut guess), calls the
read_line method on the standard input handle to
get input from the user. We’re also passing one argument to read_line: &mut guess.
The job of read_line is to take whatever the user types into standard input
and append that into a string (without overwriting its contents), so it takes
that string as an argument. The string argument needs to be mutable so the
method can change the string’s content by adding the user input.
The & indicates that this argument is a reference, which gives you a way to
let multiple parts of your code access one piece of data without needing to
copy that data into memory multiple times. References are a complex feature,
and one of Rust’s major advantages is how safe and easy it is to use
references. You don’t need to know a lot of those details to finish this
program. For now, all you need to know is that like variables, references are
immutable by default. Hence, you need to write &mut guess rather than
&guess to make it mutable. (Chapter 4 will explain references more
thoroughly.)
Handling Potential Failure with the Result Type
We’re still working on this line of code. Although we’re now discussing a third line of text, it’s still part of a single logical line of code. The next part is this method:
use std::io; fn main() { println!("Guess the number!"); println!("Please input your guess."); let mut guess = String::new(); io::stdin() .read_line(&mut guess) .expect(msg: "Failed to read line"); println!("You guessed: {}", guess); }
When you call a method with the .method_name() syntax, it’s often wise to
introduce a newline and other whitespace to help break up long lines. We could
have written this code as:
#![allow(unused)] fn main() { io::stdin().read_line(&mut guess).expect("Failed to read line"); }
However, one long line is difficult to read, so it’s best to divide it. Now let’s discuss what this line does.
As mentioned earlier, read_line puts what the user types into the string
we’re passing it, but it also returns a value—in this case, an
io::Result. Rust has a number of types named
Result in its standard library: a generic Result
as well as specific versions for submodules, such as io::Result.
The Result types are enumerations, often referred
to as enums. An enumeration is a type that can have a fixed set of values,
and those values are called the enum’s variants. Chapter 6 will cover enums
in more detail.
For Result, the variants are Ok or Err. The Ok variant indicates the
operation was successful, and inside Ok is the successfully generated value.
The Err variant means the operation failed, and Err contains information
about how or why the operation failed.
The purpose of these Result types is to encode error-handling information.
Values of the Result type, like values of any type, have methods defined on
them. An instance of io::Result has an expect method that you can call. If this instance of io::Result is an Err value,
expect will cause the program to crash and display the message that you
passed as an argument to expect. If the read_line method returns an Err,
it would likely be the result of an error coming from the underlying operating
system. If this instance of io::Result is an Ok value, expect will take
the return value that Ok is holding and return just that value to you so you
can use it. In this case, that value is the number of bytes in what the user
entered into standard input.
If you don’t call expect, the program will compile, but you’ll get a warning:
$ cargo build
Compiling guessing_game v0.1.0 (file:///projects/guessing_game)
warning: unused `Result` that must be used
--> src/main.rs:10:5
|
10 | io::stdin().read_line(&mut guess);
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
= note: `#[warn(unused_must_use)]` on by default
= note: this `Result` may be an `Err` variant, which should be handled
warning: `guessing_game` (bin "guessing_game") generated 1 warning
Finished dev [unoptimized + debuginfo] target(s) in 0.59s
Rust warns that you haven’t used the Result value returned from read_line,
indicating that the program hasn’t handled a possible error.
The right way to suppress the warning is to actually write error handling, but
because you just want to crash this program when a problem occurs, you can use
expect. You’ll learn about recovering from errors in Chapter 9.
Printing Values with println! Placeholders
Aside from the closing curly bracket, there’s only one more line to discuss in the code added so far, which is the following:
use std::io; fn main() { println!("Guess the number!"); println!("Please input your guess."); let mut guess = String::new(); io::stdin() .read_line(&mut guess) .expect("Failed to read line"); println!("You guessed: {}", guess); }
This line prints the string we saved the user’s input in. The set of curly
brackets, {}, is a placeholder: think of {} as little crab pincers that
hold a value in place. You can print more than one value using curly brackets:
the first set of curly brackets holds the first value listed after the format
string, the second set holds the second value, and so on. Printing multiple
values in one call to println! would look like this:
#![allow(unused)] fn main() { let x: i32 = 5; let y: i32 = 10; println!("x = {} and y = {}", x, y); }
This code would print x = 5 and y = 10.
Testing the First Part
Let’s test the first part of the guessing game. Run it using cargo run:
$ cargo run
Compiling guessing_game v0.1.0 (file:///projects/guessing_game)
Finished dev [unoptimized + debuginfo] target(s) in 6.44s
Running `target/debug/guessing_game`
Guess the number!
Please input your guess.
6
You guessed: 6
At this point, the first part of the game is done: we’re getting input from the keyboard and then printing it.
Generating a Secret Number
Next, we need to generate a secret number that the user will try to guess. The
secret number should be different every time so the game is fun to play more
than once. Let’s use a random number between 1 and 100 so the game isn’t too
difficult. Rust doesn’t yet include random number functionality in its standard
library. However, the Rust team does provide a rand crate.
Using a Crate to Get More Functionality
Remember that a crate is a collection of Rust source code files. The project
we’ve been building is a binary crate, which is an executable. The rand
crate is a library crate, which contains code intended to be used in other
programs.
Cargo’s coordination of external crates is where Cargo really shines. Before we
can write code that uses rand, we need to modify the Cargo.toml file to
include the rand crate as a dependency. Open that file now and add the
following line to the bottom beneath the [dependencies] section header that
Cargo created for you. Be sure to specify rand exactly as we have here, or
the code examples in this tutorial may not work.
Filename: Cargo.toml
rand = "0.8.3"
In the Cargo.toml file, everything that follows a header is part of a section
that continues until another section starts. The [dependencies] section is
where you tell Cargo which external crates your project depends on and which
versions of those crates you require. In this case, we’ll specify the rand
crate with the semantic version specifier 0.8.3. Cargo understands Semantic
Versioning (sometimes called SemVer), which is a
standard for writing version numbers. The number 0.8.3 is actually shorthand
for ^0.8.3, which means any version that is at least 0.8.3 but below
0.9.0. Cargo considers these versions to have public APIs compatible with
version 0.8.3, and this specification ensures you'll get the latest patch
release that will still compile with the code in this chapter. Any version
0.9.0 or greater is not guaranteed to have the same API as what the following
examples use.
Now, without changing any of the code, let’s build the project, as shown in Listing 2-2.
$ cargo build
Updating crates.io index
Downloaded rand v0.8.3
Downloaded libc v0.2.86
Downloaded getrandom v0.2.2
Downloaded cfg-if v1.0.0
Downloaded ppv-lite86 v0.2.10
Downloaded rand_chacha v0.3.0
Downloaded rand_core v0.6.2
Compiling rand_core v0.6.2
Compiling libc v0.2.86
Compiling getrandom v0.2.2
Compiling cfg-if v1.0.0
Compiling ppv-lite86 v0.2.10
Compiling rand_chacha v0.3.0
Compiling rand v0.8.3
Compiling guessing_game v0.1.0 (file:///projects/guessing_game)
Finished dev [unoptimized + debuginfo] target(s) in 2.53s
Listing 2-2: The output from running cargo build after
adding the rand crate as a dependency
You may see different version numbers (but they will all be compatible with the code, thanks to SemVer!), different lines (depending on the operating system), and the lines may be in a different order.
Now that we have an external dependency, Cargo fetches the latest versions of everything from the registry, which is a copy of data from Crates.io. Crates.io is where people in the Rust ecosystem post their open source Rust projects for others to use.
After updating the registry, Cargo checks the [dependencies] section and
downloads any crates you don’t have yet. In this case, although we only listed
rand as a dependency, Cargo also grabbed other crates that rand depends on
to work. After downloading the crates, Rust compiles them and then compiles the
project with the dependencies available.
If you immediately run cargo build again without making any changes, you
won’t get any output aside from the Finished line. Cargo knows it has already
downloaded and compiled the dependencies, and you haven’t changed anything
about them in your Cargo.toml file. Cargo also knows that you haven’t changed
anything about your code, so it doesn’t recompile that either. With nothing to
do, it simply exits.
If you open up the src/main.rs file, make a trivial change, and then save it and build again, you’ll only see two lines of output:
$ cargo build
Compiling guessing_game v0.1.0 (file:///projects/guessing_game)
Finished dev [unoptimized + debuginfo] target(s) in 2.53 secs
These lines show Cargo only updates the build with your tiny change to the src/main.rs file. Your dependencies haven’t changed, so Cargo knows it can reuse what it has already downloaded and compiled for those. It just rebuilds your part of the code.
Ensuring Reproducible Builds with the Cargo.lock File
Cargo has a mechanism that ensures you can rebuild the same artifact every time
you or anyone else builds your code: Cargo will use only the versions of the
dependencies you specified until you indicate otherwise. For example, what
happens if next week version 0.8.4 of the rand crate comes out, and that
version contains an important bug fix, but it also contains a regression that
will break your code?
The answer to this problem is the Cargo.lock file, which was created the
first time you ran cargo build and is now in your guessing_game directory.
When you build a project for the first time, Cargo figures out all the
versions of the dependencies that fit the criteria and then writes them to
the Cargo.lock file. When you build your project in the future, Cargo will
see that the Cargo.lock file exists and use the versions specified there
rather than doing all the work of figuring out versions again. This lets you
have a reproducible build automatically. In other words, your project will
remain at 0.8.3 until you explicitly upgrade, thanks to the Cargo.lock
file.
Updating a Crate to Get a New Version
When you do want to update a crate, Cargo provides another command, update,
which will ignore the Cargo.lock file and figure out all the latest versions
that fit your specifications in Cargo.toml. If that works, Cargo will write
those versions to the Cargo.lock file.
But by default, Cargo will only look for versions greater than 0.8.3 and less
than 0.9.0. If the rand crate has released two new versions, 0.8.4 and
0.9.0, you would see the following if you ran cargo update:
$ cargo update
Updating crates.io index
Updating rand v0.8.3 -> v0.8.4
At this point, you would also notice a change in your Cargo.lock file noting
that the version of the rand crate you are now using is 0.8.4.
If you wanted to use rand version 0.9.0 or any version in the 0.9.x
series, you’d have to update the Cargo.toml file to look like this instead:
[dependencies]
rand = "0.9.0"
The next time you run cargo build, Cargo will update the registry of crates
available and reevaluate your rand requirements according to the new version
you have specified.
There’s a lot more to say about Cargo and its ecosystem which we’ll discuss in Chapter 14, but for now, that’s all you need to know. Cargo makes it very easy to reuse libraries, so Rustaceans are able to write smaller projects that are assembled from a number of packages.
Generating a Random Number
Now that you’ve added the rand crate to Cargo.toml, let’s start using
rand. The next step is to update src/main.rs, as shown in Listing 2-3.
Filename: src/main.rs
#![allow(unused)] fn main() { use std::io; use rand::Rng; fn main() { println!("Guess the number!"); let secret_number = rand::thread_rng().gen_range(1..101); println!("The secret number is: {}", secret_number); println!("Please input your guess."); let mut guess = String::new(); io::stdin() .read_line(buf: &mut guess) .expect(msg: "Failed to read line"); println!("You guessed: {}", guess); } }
Listing 2-3: Adding code to generate a random number
First, we add a use line: use rand::Rng. The Rng trait defines
methods that random number generators implement, and this trait must be in
scope for us to use those methods. Chapter 10 will cover traits in detail.
Next, we’re adding two lines in the middle. The rand::thread_rng function
will give us the particular random number generator that we’re going to use:
one that is local to the current thread of execution and seeded by the
operating system. Then we call the gen_range method on the random number
generator. This method is defined by the Rng trait that we brought into scope
with the use rand::Rng statement. The gen_range method takes a range
expression as an argument and generates a random number in the range. The kind
of range expression we’re using here takes the form start..end. It’s
inclusive on the lower bound but exclusive on the upper bound, so we need to
specify 1..101 to request a number between 1 and 100. Alternatively, we could
pass the range 1..=100, which is equivalent.
Note: You won’t just know which traits to use and which methods and functions to call from a crate. Instructions for using a crate are in each crate’s documentation. Another neat feature of Cargo is that you can run the
cargo doc --opencommand, which will build documentation provided by all of your dependencies locally and open it in your browser. If you’re interested in other functionality in therandcrate, for example, runcargo doc --openand clickrandin the sidebar on the left.
The second line that we added to the middle of the code prints the secret number. This is useful while we’re developing the program to be able to test it, but we’ll delete it from the final version. It’s not much of a game if the program prints the answer as soon as it starts!
Try running the program a few times:
$ cargo run
Compiling guessing_game v0.1.0 (file:///projects/guessing_game)
Finished dev [unoptimized + debuginfo] target(s) in 2.53s
Running `target/debug/guessing_game`
Guess the number!
The secret number is: 7
Please input your guess.
4
You guessed: 4
$ cargo run
Finished dev [unoptimized + debuginfo] target(s) in 0.02s
Running `target/debug/guessing_game`
Guess the number!
The secret number is: 83
Please input your guess.
5
You guessed: 5
You should get different random numbers, and they should all be numbers between 1 and 100. Great job!
Comparing the Guess to the Secret Number
Now that we have user input and a random number, we can compare them. That step is shown in Listing 2-4. Note that this code won’t compile quite yet, as we will explain.
Filename: src/main.rs
#![allow(unused)] fn main() { use rand::Rng; use std::cmp::Ordering; use std::io; fn main() { // --snip-- println!("Guess the number!"); let secret_number = rand::thread_rng().gen_range(1..101); println!("The secret number is: {}", secret_number); println!("Please input your guess."); let mut guess = String::new(); io::stdin() .read_line(&mut guess) .expect("Failed to read line"); println!("You guessed: {}", guess); match guess.cmp(&secret_number) { Ordering::Less => println!("Too small!"), Ordering::Greater => println!("Too big!"), Ordering::Equal => println!("You win!"), } } }
Listing 2-4: Handling the possible return values of comparing two numbers
The first new bit here is another use statement, bringing a type called
std::cmp::Ordering into scope from the standard library. Like Result,
Ordering is another enum, but the variants for Ordering are Less,
Greater, and Equal. These are the three outcomes that are possible when you
compare two values.
Then we add five new lines at the bottom that use the Ordering type. The
cmp method compares two values and can be called on anything that can be
compared. It takes a reference to whatever you want to compare with: here it’s
comparing the guess to the secret_number. Then it returns a variant of the
Ordering enum we brought into scope with the use statement. We use a
match expression to decide what to do next based on
which variant of Ordering was returned from the call to cmp with the values
in guess and secret_number.
A match expression is made up of arms. An arm consists of a pattern and
the code that should be run if the value given to the beginning of the match
expression fits that arm’s pattern. Rust takes the value given to match and
looks through each arm’s pattern in turn. The match construct and patterns
are powerful features in Rust that let you express a variety of situations your
code might encounter and make sure that you handle them all. These features
will be covered in detail in Chapter 6 and Chapter 18, respectively.
Let’s walk through an example of what would happen with the match expression
used here. Say that the user has guessed 50 and the randomly generated secret
number this time is 38. When the code compares 50 to 38, the cmp method will
return Ordering::Greater, because 50 is greater than 38. The match
expression gets the Ordering::Greater value and starts checking each arm’s
pattern. It looks at the first arm’s pattern, Ordering::Less, and sees that
the value Ordering::Greater does not match Ordering::Less, so it ignores
the code in that arm and moves to the next arm. The next arm’s pattern,
Ordering::Greater, does match Ordering::Greater! The associated code in
that arm will execute and print Too big! to the screen. The match
expression ends because it has no need to look at the last arm in this scenario.
However, the code in Listing 2-4 won’t compile yet. Let’s try it:
$ cargo build
Compiling libc v0.2.86
Compiling getrandom v0.2.2
Compiling cfg-if v1.0.0
Compiling ppv-lite86 v0.2.10
Compiling rand_core v0.6.2
Compiling rand_chacha v0.3.0
Compiling rand v0.8.3
Compiling guessing_game v0.1.0 (file:///projects/guessing_game)
error[E0308]: mismatched types
--> src/main.rs:22:21
|
22 | match guess.cmp(&secret_number) {
| ^^^^^^^^^^^^^^ expected struct `String`, found integer
|
= note: expected reference `&String`
found reference `&{integer}`
For more information about this error, try `rustc --explain E0308`.
error: could not compile `guessing_game` due to previous error
The core of the error states that there are mismatched types. Rust has a
strong, static type system. However, it also has type inference. When we wrote
let mut guess = String::new(), Rust was able to infer that guess should be
a String and didn’t make us write the type. The secret_number, on the other
hand, is a number type. A few number types can have a value between 1 and 100:
i32, a 32-bit number; u32, an unsigned 32-bit number; i64, a 64-bit
number; as well as others. Rust defaults to an i32, which is the type of
secret_number unless you add type information elsewhere that would cause Rust
to infer a different numerical type. The reason for the error is that Rust
cannot compare a string and a number type.
Ultimately, we want to convert the String the program reads as input into a
real number type so we can compare it numerically to the secret number. We can
do that by adding another line to the main function body:
Filename: src/main.rs
use rand::Rng; use std::cmp::Ordering; use std::io; fn main() { println!("Guess the number!"); let secret_number = rand::thread_rng().gen_range(1..101); println!("The secret number is: {}", secret_number); println!("Please input your guess."); // --snip-- let mut guess = String::new(); io::stdin() .read_line(buf: &mut guess) .expect(msg: "Failed to read line"); let guess: u32 = guess.trim().parse().expect(msg: "Please type a number!"); println!("You guessed: {}", guess); match guess.cmp(&secret_number) { Ordering::Less => println!("Too small!"), Ordering::Greater => println!("Too big!"), Ordering::Equal => println!("You win!"), } }
The line is:
#![allow(unused)] fn main() { let guess: u32 = guess.trim().parse().expect("Please type a number!"); }
We create a variable named guess. But wait, doesn’t the program already have
a variable named guess? It does, but Rust allows us to shadow the previous
value of guess with a new one. This feature is often used in situations in
which you want to convert a value from one type to another type. Shadowing lets
us reuse the guess variable name rather than forcing us to create two unique
variables, such as guess_str and guess for example. (Chapter 3 covers
shadowing in more detail.)
We bind guess to the expression guess.trim().parse(). The guess in the
expression refers to the original guess that was a String with the input in
it. The trim method on a String instance will eliminate any whitespace at
the beginning and end. Although u32 can contain only numerical characters,
the user must press enter to satisfy
read_line. When the user presses enter, a
newline character is added to the string. For example, if the user types 5 and presses enter,
guess looks like this: 5\n. The \n represents “newline,” the result of
pressing enter (On Windows, pressing enter results in a carriage return and a newline,
\r\n). The trim method eliminates \n or \r\n, resulting in just 5.
The parse method on strings parses a string into some
kind of number. Because this method can parse a variety of number types, we
need to tell Rust the exact number type we want by using let guess: u32. The
colon (:) after guess tells Rust we’ll annotate the variable’s type. Rust
has a few built-in number types; the u32 seen here is an unsigned, 32-bit
integer. It’s a good default choice for a small positive number. You’ll learn
about other number types in Chapter 3. Additionally, the u32 annotation in
this example program and the comparison with secret_number means that Rust
will infer that secret_number should be a u32 as well. So now the
comparison will be between two values of the same type!
The call to parse could easily cause an error. If, for example, the string
contained A👍%, there would be no way to convert that to a number. Because it
might fail, the parse method returns a Result type, much as the read_line
method does (discussed earlier in “Handling Potential Failure with the
Result Type”). We’ll treat this Result the same way by using the expect method
again. If parse returns an Err Result variant because it couldn’t create
a number from the string, the expect call will crash the game and print the
message we give it. If parse can successfully convert the string to a number,
it will return the Ok variant of Result, and expect will return the
number that we want from the Ok value.
Let’s run the program now!
$ cargo run
Compiling guessing_game v0.1.0 (file:///projects/guessing_game)
Finished dev [unoptimized + debuginfo] target(s) in 0.43s
Running `target/debug/guessing_game`
Guess the number!
The secret number is: 58
Please input your guess.
76
You guessed: 76
Too big!
Nice! Even though spaces were added before the guess, the program still figured out that the user guessed 76. Run the program a few times to verify the different behavior with different kinds of input: guess the number correctly, guess a number that is too high, and guess a number that is too low.
We have most of the game working now, but the user can make only one guess. Let’s change that by adding a loop!
Allowing Multiple Guesses with Looping
The loop keyword creates an infinite loop. We’ll add that now to give users
more chances at guessing the number:
Filename: src/main.rs
use rand::Rng; use std::cmp::Ordering; use std::io; fn main() { println!("Guess the number!"); let secret_number = rand::thread_rng().gen_range(1..101); // --snip-- println!("The secret number is: {}", secret_number); loop { println!("Please input your guess."); // --snip-- let mut guess = String::new(); io::stdin() .read_line(&mut guess) .expect("Failed to read line"); let guess: u32 = guess.trim().parse().expect("Please type a number!"); println!("You guessed: {}", guess); match guess.cmp(&secret_number) { Ordering::Less => println!("Too small!"), Ordering::Greater => println!("Too big!"), Ordering::Equal => println!("You win!"), } } }
As you can see, we’ve moved everything into a loop from the guess input prompt onward. Be sure to indent the lines inside the loop another four spaces each and run the program again. Notice that there is a new problem because the program is doing exactly what we told it to do: ask for another guess forever! It doesn’t seem like the user can quit!
The user could always interrupt the program by using the keyboard shortcut ctrl-c. But there’s another way to escape this
insatiable monster, as mentioned in the parse discussion in “Comparing the
Guess to the Secret Number”: if the user enters a non-number answer, the program will crash. The
user can take advantage of that in order to quit, as shown here:
$ cargo run
Compiling guessing_game v0.1.0 (file:///projects/guessing_game)
Finished dev [unoptimized + debuginfo] target(s) in 1.50s
Running `target/debug/guessing_game`
Guess the number!
The secret number is: 59
Please input your guess.
45
You guessed: 45
Too small!
Please input your guess.
60
You guessed: 60
Too big!
Please input your guess.
59
You guessed: 59
You win!
Please input your guess.
quit
thread 'main' panicked at 'Please type a number!: ParseIntError { kind: InvalidDigit }', src/main.rs:28:47
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
Typing quit actually quits the game, but so will any other non-number input.
However, this is suboptimal to say the least. We want the game to automatically
stop when the correct number is guessed.
Quitting After a Correct Guess
Let’s program the game to quit when the user wins by adding a break statement:
Filename: src/main.rs
use rand::Rng; use std::cmp::Ordering; use std::io; fn main() { println!("Guess the number!"); let secret_number = rand::thread_rng().gen_range(1..101); println!("The secret number is: {}", secret_number); loop { println!("Please input your guess."); let mut guess = String::new(); io::stdin() .read_line(&mut guess) .expect("Failed to read line"); let guess: u32 = guess.trim().parse().expect("Please type a number!"); println!("You guessed: {}", guess); // --snip-- match guess.cmp(&secret_number) { Ordering::Less => println!("Too small!"), Ordering::Greater => println!("Too big!"), Ordering::Equal => { println!("You win!"); break; } } } }
Adding the break line after You win! makes the program exit the loop when
the user guesses the secret number correctly. Exiting the loop also means
exiting the program, because the loop is the last part of main.
Handling Invalid Input
To further refine the game’s behavior, rather than crashing the program when
the user inputs a non-number, let’s make the game ignore a non-number so the
user can continue guessing. We can do that by altering the line where guess
is converted from a String to a u32, as shown in Listing 2-5.
Filename: src/main.rs
use rand::Rng; use std::cmp::Ordering; use std::io; fn main() { println!("Guess the number!"); let secret_number = rand::thread_rng().gen_range(1..101); println!("The secret number is: {}", secret_number); loop { println!("Please input your guess."); let mut guess = String::new(); // --snip-- io::stdin() .read_line(buf: &mut guess) .expect(msg: "Failed to read line"); let guess: u32 = match guess.trim().parse() { Ok(num: u32) => num, Err(_) => continue, }; println!("You guessed: {}", guess); // --snip-- match guess.cmp(&secret_number) { Ordering::Less => println!("Too small!"), Ordering::Greater => println!("Too big!"), Ordering::Equal => { println!("You win!"); break; } } } }
Listing 2-5: Ignoring a non-number guess and asking for another guess instead of crashing the program
Switching from an expect call to a match expression is one way of moving
from crashing on an error to handling the error. Remember that parse returns
a Result type and Result is an enum that has the variants Ok or Err.
We’re using a match expression here, as we did with the Ordering result of
the cmp method.
If parse is able to successfully turn the string into a number, it will
return an Ok value that contains the resulting number. That Ok value will
match the first arm’s pattern, and the match expression will just return the
num value that parse produced and put inside the Ok value. That number
will end up right where we want it in the new guess variable we’re creating.
If parse is not able to turn the string into a number, it will return an
Err value that contains more information about the error. The Err value
does not match the Ok(num) pattern in the first match arm, but it does
match the Err(_) pattern in the second arm. The underscore, _, is a
catchall value; in this example, we’re saying we want to match all Err
values, no matter what information they have inside them. So the program will
execute the second arm’s code, continue, which tells the program to go to the
next iteration of the loop and ask for another guess. So, effectively, the
program ignores all errors that parse might encounter!
Now everything in the program should work as expected. Let’s try it:
$ cargo run
Compiling guessing_game v0.1.0 (file:///projects/guessing_game)
Finished dev [unoptimized + debuginfo] target(s) in 4.45s
Running `target/debug/guessing_game`
Guess the number!
The secret number is: 61
Please input your guess.
10
You guessed: 10
Too small!
Please input your guess.
99
You guessed: 99
Too big!
Please input your guess.
foo
Please input your guess.
61
You guessed: 61
You win!
Awesome! With one tiny final tweak, we will finish the guessing game. Recall
that the program is still printing the secret number. That worked well for
testing, but it ruins the game. Let’s delete the println! that outputs the
secret number. Listing 2-6 shows the final code.
Filename: src/main.rs
#![allow(unused)] fn main() { use rand::Rng; use std::cmp::Ordering; use std::io; fn main() { println!("Guess the number!"); let secret_number: u32 = rand::thread_rng().gen_range(1..101); loop { println!("Please input your guess."); let mut guess = String::new(); io::stdin() .read_line(buf: &mut guess) .expect(msg: "Failed to read line"); let guess: u32 = match guess.trim().parse() { Ok(num: u32) => num, Err(_) => continue, }; println!("You guessed: {}", guess); match guess.cmp(&secret_number) { Ordering::Less => println!("Too small!"), Ordering::Greater => println!("Too big!"), Ordering::Equal => { println!("You win!"); break; } } } } }
Listing 2-6: Complete guessing game code
Summary
At this point, you’ve successfully built the guessing game. Congratulations!
This project was a hands-on way to introduce you to many new Rust concepts:
let, match, functions, the use of external crates, and more. In the next
few chapters, you’ll learn about these concepts in more detail. Chapter 3
covers concepts that most programming languages have, such as variables, data
types, and functions, and shows how to use them in Rust. Chapter 4 explores
ownership, a feature that makes Rust different from other languages. Chapter 5
discusses structs and method syntax, and Chapter 6 explains how enums work.