31 KiB
Guessing Game
Let's jump into Rust with a hands-on project! We’ll implement a classic beginner programming problem: the guessing game. Here’s how it works: Our program will generate a random integer between one and a hundred. It will then prompt us to enter a guess. Upon entering our guess, it will tell us if we’re too low or too high. Once we guess correctly, it will congratulate us.
Set up
Let’s set up a new project. Go to your projects directory, and create a new project using Cargo.
$ cd ~/projects
$ cargo new guessing_game --bin
$ cd guessing_game
We pass the name of our project to cargo new, then the --bin flag, since
we’re going to be making another binary like in Chapter 1.
Take a look at the generated Cargo.toml:
[package]
name = "guessing_game"
version = "0.1.0"
authors = ["Your Name <you@example.com>"]
[dependencies]
If the authors information that Cargo got from your environment is not correct, go ahead and fix that.
And as we saw in the last chapter, cargo new generates a ‘Hello, world!’ for
us. Check out src/main.rs:
fn main() {
println!("Hello, world!");
}
Let’s try compiling what Cargo gave us and running it in the same step, using the cargo run command:
$ cargo run
Compiling guessing_game v0.1.0 (file:///home/you/projects/guessing_game)
Running `target/debug/guessing_game`
Hello, world!
Great! The run command comes in handy when you need to rapidly iterate on a
project. Our game is such a project: we want to quickly test each
iteration before moving on to the next one.
Now open up your src/main.rs again. We’ll be writing all of our code in this
file.
Processing a Guess
Let’s get to it! The first thing we need to do for our guessing game is
allow our player to input a guess. Put this in your src/main.rs:
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);
}
There’s a lot here! Let’s go over it, bit by bit.
use std::io;
We’ll need to take user input and then print the result as output. As such, we
need the io library from the standard library. Rust only imports a few things
by default into every program, the ‘prelude’. If it’s not in the
prelude, you’ll have to use it directly. Using the std::io library gets
you a number of useful io-related things, so that's what we've done here.
fn main() {
As you’ve seen in Chapter 1, the main() function is the entry point into the
program. The fn syntax declares a new function, the ()s indicate that
there are no arguments, and { starts the body of the function.
println!("Guess the number!");
println!("Please input your guess.");
We previously learned in Chapter 1 that println!() is a macro that
prints a string to the screen.
Variable Bindings
let mut guess = String::new();
Now we’re getting interesting! There’s a lot going on in this little line. The first thing to notice is that this is a let statement, which is used to create what are called ‘variable bindings’. Here's an example:
let foo = bar;
This will create a new binding named foo, and bind it to the value bar. In
many languages, this is called a ‘variable’, but Rust’s variable bindings have
a few tricks up their sleeves.
For example, they’re immutable by default. That’s why our example
uses mut: it makes a binding mutable, rather than immutable.
let foo = 5; // immutable.
let mut bar = 5; // mutable
Oh, and // will start a comment, until the end of the line. Rust ignores
everything in comments.
So now we know that let mut guess will introduce a mutable binding named
guess, but we have to look at the other side of the = for what it’s
bound to: String::new().
String is a string type, provided by the standard library. A
String is a growable, UTF-8 encoded bit of text.
The ::new() syntax uses :: because this is an ‘associated function’ of
a particular type. That is to say, it’s associated with String itself,
rather than a particular instance of a String. Some languages call this a
‘static method’.
This function is named new(), because it creates a new, empty String.
You’ll find a new() function on many types, as it’s a common name for making
a new value of some kind.
Let’s move forward:
io::stdin().read_line(&mut guess)
.expect("Failed to read line");
Let’s go through this together bit-by-bit. The first line has two parts. Here’s the first:
io::stdin()
Remember how we used std::io on the first line of the program? We’re now
calling an associated function on it. If we didn’t use std::io, we could
have written this line as std::io::stdin().
This particular function returns a handle to the standard input for your terminal. More specifically, a std::io::Stdin.
The next part will use this handle to get input from the user:
.read_line(&mut guess)
Here, we call the read_line() method on our handle. We’re also
passing one argument to read_line(): &mut guess.
Remember how we bound guess above? We said it was mutable. However,
read_line doesn’t take a String as an argument: it takes a &mut String.
The & is the feature of Rust called a ‘reference’, which allows you to have
multiple ways to access one piece of data in order to reduce copying.
References are a complex feature, as one of Rust’s major selling points is how
safe and easy it is to use references. We don’t need to know a lot of those
details to finish our program right now, though; Chapter XX will cover them in
more detail. For now, all we need to know is that like let bindings,
references are immutable by default. Hence, we need to write &mut guess,
rather than &guess.
Why does read_line() take a mutable reference to a string? Its job is
to take what the user types into standard input and place that into a
string. So it takes that string as an argument, and in order to add
the input, that string needs to be mutable.
But we’re not quite done with this line of code, though. While it’s a single line of text, it’s only the first part of the single logical line of code. This is the second part of the line:
.expect("Failed to read line");
When you call a method with the .foo() syntax, you may introduce a newline
and other whitespace. This helps you split up long lines. We could have
written this code as:
io::stdin().read_line(&mut guess).expect("failed to read line");
But that gets hard to read. So we’ve split it up, two lines for two method calls.
The Result Type
We already talked about read_line(), but what about expect()? Well,
we already mentioned that read_line() puts what the user types into the &mut String we pass 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, and then specific versions for
sub-libraries, like io::Result.
The purpose of these Result types is to encode error handling information.
Values of the Result type, like any type, have methods defined on them. In
this case, io::Result has an expect() method that takes a value
it’s called on, and if it isn’t a successful result, will cause our program to
crash and display the message that we passed as an argument to expect().
If we don't call this method, our program will compile, but we’ll get a warning:
$ cargo build
Compiling guessing_game v0.1.0 (file:///home/you/projects/guessing_game)
src/main.rs:10:5: 10:39 warning: unused result which must be used,
#[warn(unused_must_use)] on by default
src/main.rs:10 io::stdin().read_line(&mut guess);
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Rust warns us that we haven’t used the Result value. This warning comes from
a special annotation that io::Result has. Rust is trying to tell you that you
haven’t handled a possible error. The right way to suppress the error is to
actually write error handling. Luckily, if we want to crash if there’s a
problem, we can use expect(). If we can recover from the error somehow, we’d
do something else, but we’ll save that for a future project.
println!() Placeholders
There’s only one line of this first example left, aside from the closing curly brace:
println!("You guessed: {}", guess);
}
This prints out the string we saved our input in. The {}s are a placeholder:
think of {} as little crab pincers, holding a value in place. The first {}
holds the first value after the format string, the second set holds the second
value, and so on. Printing out multiple values in one call to println!() would then look like this:
let x = 5;
let y = 10;
println!("x and y: {} and {}", x, y);
Which would print out "x and y: 5 and 10".
Anyway, back to our guessing game. We can run what we have with cargo run:
$ cargo run
Compiling guessing_game v0.1.0 (file:///home/you/projects/guessing_game)
Running `target/debug/guessing_game`
Guess the number!
Please input your guess.
6
You guessed: 6
All right! Our first part is done: we can get input from the keyboard and then print it back out.
Generating a secret number
Next, we need to generate a secret number. Rust does not yet include random
number functionality in its standard library. The Rust team does, however,
provide a rand crate. A ‘crate’ is a package of Rust code.
We’ve been building a ‘binary crate’, which is an executable. rand is a
‘library crate’, which contains code that’s intended to be used with other
programs.
Using external crates is where Cargo really shines. Before we can write
the code using rand, we need to modify our Cargo.toml. Open it up, and
add this line at the bottom beneath the [dependencies] section header that
Cargo created for you:
[dependencies]
rand = "0.3.14"
The [dependencies] section of Cargo.toml is like the [package] section:
everything that follows the section heading is part of that section, until
another section starts. Cargo uses the dependencies section to know what
dependencies on external crates you have and what versions of those crates you
require. In this case, we’ve specified the rand crate with the semantic
version specifier 0.3.14. Cargo understands Semantic Versioning, a
standard for writing version numbers. A bare number like above is actually
shorthand for ^0.3.14, which means "any version that has a public API
compatible with version 0.3.14".
Now, without changing any of our code, let’s build our project:
$ cargo build
Updating registry `https://github.com/rust-lang/crates.io-index`
Downloading rand v0.3.14
Downloading libc v0.2.14
Compiling libc v0.2.14
Compiling rand v0.3.14
Compiling guessing_game v0.1.0 (file:///home/you/projects/guessing_game)
You may see different versions (but they will be compatible, thanks to semver!) and the lines may be in a different order.
Lots of new output! 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 our [dependencies] and downloads
any we don’t have yet. In this case, while we only said we wanted to depend on
rand, we’ve also grabbed a copy of libc. This is because rand depends on
libc to work. After downloading them, it compiles them and then compiles
our project.
If we run cargo build again, we’ll get different output:
$ cargo build
That’s right, no output! Cargo knows that our project has been built, that
all of its dependencies are built, and that no changes have been made. There’s
no reason to do all that stuff again. With nothing to do, it simply
exits. If we open up src/main.rs, make a trivial change, then save it again,
we’ll only see one line:
$ cargo build
Compiling guessing_game v0.1.0 (file:///home/you/projects/guessing_game)
What happens when next week version v0.3.15 of the rand crate comes out,
with an important bugfix? While getting bugfixes is important, what if 0.3.15
contains a regression that breaks our code?
The answer to this problem is the Cargo.lock file created the first time we
ran cargo build that is now in your project directory. When you build your
project for the first time, Cargo figures out all of the versions that fit your
criteria 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 then use
that specific version rather than do all the work of figuring out versions
again. This lets you have a repeatable build automatically. In other words,
we’ll stay at 0.3.14 until we explicitly upgrade, and so will anyone who we
share our code with, thanks to the lock file.
What about when we do want to use v0.3.15? Cargo has another command,
update, which says ‘ignore the Cargo.lock file and figure out all the
latest versions that fit what we’ve specified in Cargo.toml. If that works,
write those versions out to the lock file’. But by default, Cargo will only
look for versions larger than 0.3.0 and smaller than 0.4.0. If we want to
move to 0.4.x, we’d have to update what is in the Cargo.toml file. When we
do, the next time we cargo build, Cargo will update the index and re-evaluate
our rand requirements.
There’s a lot more to say about Cargo and its ecosystem that we will get into in Chapter XX, but for now, that’s all we need to know. Cargo makes it really easy to re-use libraries, so Rustaceans are able to write smaller projects which are assembled out of a number of sub-packages.
Let’s get on to actually using rand. Here’s our next step:
extern crate rand;
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(&mut guess)
.expect("failed to read line");
println!("You guessed: {}", guess);
}
The first thing we’ve done is change the first line. It now says extern crate rand. Because we declared rand in our [dependencies], we can now put
extern crate in our code to let Rust know we’ll be making use of that
dependency. This also does the equivalent of a use rand; as well, so we can
call anything in the rand crate by prefixing it with rand::.
Next, we added another use line: use rand::Rng. We’re going to use a
method in a moment, and it requires that Rng be in scope to work. The basic
idea is this: methods are defined on something called ‘traits’, and for the
method to work, it needs the trait to be in scope. For more about the
details, read the traits section in Chapter XX.
There are two other lines we added, in the middle:
let secret_number = rand::thread_rng().gen_range(1, 101);
println!("The secret number is: {}", secret_number);
We use the rand::thread_rng() function to get a copy of the random number
generator, which is local to the particular thread of execution
we’re in. Because we put use rand::Rng above, the random number generator has
a gen_range() method available. This method takes two numbers as arguments
and generates a random number between them. It’s inclusive on the lower bound
but exclusive on the upper bound, so we need 1 and 101 to ask for a number
ranging from one to a hundred.
The second line prints out the secret number. This is useful while we’re developing our program to let us easily test it out, but we’ll be deleting it for the final version. It’s not much of a game if it prints out the answer when you start it up!
Try running our new program a few times:
$ cargo run
Compiling guessing_game v0.1.0 (file:///home/you/projects/guessing_game)
Running `target/debug/guessing_game`
Guess the number!
The secret number is: 7
Please input your guess.
4
You guessed: 4
$ cargo run
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 between 1 and 100. Great job! Next up: comparing our guess to the secret number.
Comparing guesses
Now that we’ve got user input, let’s compare our guess to the secret number. Here’s part of our next step. It won't quite compile yet though:
extern crate rand;
use std::io;
use std::cmp::Ordering;
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(&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!"),
}
}
A few new bits here. The first is another use. We bring a type called
std::cmp::Ordering into scope. Then we add five new lines at the bottom that
use that type:
match guess.cmp(&secret_number) {
Ordering::Less => println!("Too small!"),
Ordering::Greater => println!("Too big!"),
Ordering::Equal => println!("You win!"),
}
The cmp() method can be called on anything that can be compared, and it
takes a reference to the thing you want to compare it to. It returns the
Ordering type we used earlier. We use a match statement to
determine exactly what kind of Ordering it is. Ordering is an
enum, short for ‘enumeration’, which looks like this:
enum Foo {
Bar,
Baz,
}
With this definition, anything of type Foo can be either a
Foo::Bar or a Foo::Baz. We use the :: to indicate the
namespace for a particular enum variant.
The Ordering enum has three possible variants: Less, Equal,
and Greater. The match statement takes a value of a type and lets you
create an ‘arm’ for each possible value. An arm is made up of a pattern and the
code that we should execute if the pattern matches the value of the type. Since
we have three types of Ordering, we have three arms:
match guess.cmp(&secret_number) {
Ordering::Less => println!("Too small!"),
Ordering::Greater => println!("Too big!"),
Ordering::Equal => println!("You win!"),
}
If it’s Less, we print Too small!, if it’s Greater, Too big!, and if
Equal, You win!. match is really useful and is used often in Rust.
We did mention that this won’t quite compile yet, though. Let’s try it:
$ cargo build
Compiling guessing_game v0.1.0 (file:///home/you/projects/guessing_game)
src/main.rs:23:21: 23:35 error: mismatched types [E0308]
src/main.rs:23 match guess.cmp(&secret_number) {
^~~~~~~~~~~~~~
src/main.rs:23:21: 23:35 help: run `rustc --explain E0308` to see a detailed explanation
src/main.rs:23:21: 23:35 note: expected type `&std::string::String`
src/main.rs:23:21: 23:35 note: found type `&_`
error: aborting due to previous error
Could not compile `guessing_game`.
Whew! This is a big error. The core of it is that we have ‘mismatched types’.
Rust has a strong, static type system. However, it also has type inference.
When we wrote let guess = String::new(), Rust was able to infer that guess
should be a String, so it doesn’t make us write out the type. With our
secret_number, there are a number of types which can have a value between one
and a hundred: i32, a thirty-two-bit number, or u32, an unsigned
thirty-two-bit number, or i64, a sixty-four-bit number or others. So far,
that hasn’t mattered, and so Rust defaults to an i32. However, here, Rust
doesn’t know how to compare the guess and the secret_number. They need to
be the same type.
Ultimately, we want to convert the String we read as input
into a real number type so that we can compare it to the guess numerically. We
can do that with two more lines. Here’s our new program:
extern crate rand;
use std::io;
use std::cmp::Ordering;
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(&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!"),
}
}
The new two lines:
let guess: u32 = guess.trim().parse()
.expect("Please type a number!");
Wait a minute, didn't we already have a guess? We do, but Rust allows us
to ‘shadow’ the previous guess with a new one. This is often used in this
exact situation, where guess starts as a String, but we want to convert it
to a u32. Shadowing lets us re-use the guess name rather than forcing us
to come up with two unique names like guess_str and guess or something
else.
We bind guess to an expression that looks like something we wrote earlier:
guess.trim().parse()
Here, guess refers to the old guess, the one that was a String with our
input in it. The trim() method on Strings will eliminate any white space at
the beginning and end of our string. This is important, as we had to press the
‘return’ key to satisfy read_line(). If we type 5 and hit return, guess
looks like this: 5\n. The \n represents ‘newline’, the enter key. trim()
gets rid of this, leaving our string with only the 5.
The parse() method on strings parses a string into some kind of
number. Since it can parse a variety of numbers, we need to give Rust a hint as
to the exact type of number we want. Hence, let guess: u32. The colon (:)
after guess tells Rust we’re going to annotate its type. u32 is an
unsigned, thirty-two bit integer. Rust has a number of built-in number
types, but we’ve chosen u32. It’s a good default choice for a small
positive number. You'll see the other number types in Chapter XX.
Just like read_line(), our call to parse() could cause an error. What if
our string contained A👍%? There’d be no way to convert that to a number. As
such, we’ll do the same thing we did with read_line(): use the expect()
method to crash if there’s an error.
Let’s try our program out!
$ cargo run
Compiling guessing_game v0.1.0 (file:///home/you/projects/guessing_game)
Running `target/guessing_game`
Guess the number!
The secret number is: 58
Please input your guess.
76
You guessed: 76
Too big!
Nice! You can see we even added spaces before our guess, and it still figured out that we guessed 76. Run the program a few times. Verify that guessing the secret number works, as well as guessing a number too small.
Now we’ve got most of the game working, but we can only make one guess. Let’s change that by adding loops!
Looping
The loop keyword gives us an infinite loop. Let’s add that in:
extern crate rand;
use std::io;
use std::cmp::Ordering;
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);
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);
match guess.cmp(&secret_number) {
Ordering::Less => println!("Too small!"),
Ordering::Greater => println!("Too big!"),
Ordering::Equal => println!("You win!"),
}
}
}
And try it out. But wait, didn’t we just add an infinite loop? Yup. Remember
our discussion about parse()? If we give a non-number answer, the program
will crash and, therefore, quit. Observe:
$ cargo run
Compiling guessing_game v0.1.0 (file:///home/you/projects/guessing_game)
Running `target/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/libcore/result.rs:785
note: Run with `RUST_BACKTRACE=1` for a backtrace.
error: Process didn't exit successfully: `target/debug/guess` (exit code: 101)
Ha! quit actually quits. As does any other non-number input. Well, this is
suboptimal to say the least. First, let’s actually quit when you win the game:
extern crate rand;
use std::io;
use std::cmp::Ordering;
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);
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);
match guess.cmp(&secret_number) {
Ordering::Less => println!("Too small!"),
Ordering::Greater => println!("Too big!"),
Ordering::Equal => {
println!("You win!");
break;
}
}
}
}
By adding the break line after the You win!, we’ll exit the loop when we
win. Exiting the loop also means exiting the program, since the loop is the last
thing in main(). We have another tweak to make: when someone inputs a
non-number, we don’t want to quit, we want to ignore it. We can do that
like this:
extern crate rand;
use std::io;
use std::cmp::Ordering;
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);
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 = match guess.trim().parse() {
Ok(num) => 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;
}
}
}
}
These are the lines that changed:
let guess: u32 = match guess.trim().parse() {
Ok(num) => num,
Err(_) => continue,
};
This is how you generally move from ‘crash on error’ to ‘actually handle the
error’: by switching from expect() to a match statement. A Result is the
return type of parse(). Result is an enum like Ordering, but in this
case, each variant has some data associated with it. Ok is a success, and
Err is a failure. Each contains more information: in this case, the
successfully parsed integer or an error type, respectively. When we match an
Ok(num), that pattern sets the name num to the value inside the Ok (the
integer), and the code we run just returns that integer. In the Err case, we
don’t care what kind of error it is, so we just use the catch-all _ instead
of a name. So for all errors, we run the code continue, which lets us move to
the next iteration of the loop, effectively ignoring the errors.
Now we should be good! Let’s try it:
$ cargo run
Compiling guessing_game v0.1.0 (file:///home/you/projects/guessing_game)
Running `target/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 last tweak, we can finish the guessing game. Can you think of what it is? That’s right, we don’t want to print out the secret number. It was good for testing, but it kind of ruins the game. Here’s our final source:
extern crate rand;
use std::io;
use std::cmp::Ordering;
use rand::Rng;
fn main() {
println!("Guess the number!");
let secret_number = rand::thread_rng().gen_range(1, 101);
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 = match guess.trim().parse() {
Ok(num) => 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;
}
}
}
}
Complete!
This project showed you a lot: let, match, methods, associated
functions, using external crates, and more.
At this point, you have successfully built the Guessing Game! Congratulations!