guessing-game.md

% Guessing Game

Let’s learn some Rust! For our first 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. Sounds good?

Along the way, we’ll learn a little bit about Rust. The next chapter, ‘Syntax and Semantics’, will dive deeper into each part.

Set up

Let’s set up a new project. Go to your projects directory. Remember how we had to create our directory structure and a Cargo.toml for hello_world? Cargo has a command that does that for us. Let’s give it a shot:

$ cd ~/projects
$ cargo new guessing_game --bin
$ cd guessing_game

We pass the name of our project to cargo new, and then the --bin flag, since we’re making a binary, rather than a library.

Check out the generated Cargo.toml:

[package]

name = "guessing_game"
version = "0.1.0"
authors = ["Your Name <[email protected]>"]

Cargo gets this information from your environment. If it’s not correct, go ahead and fix that.

Finally, Cargo generated a ‘Hello, world!’ for us. Check out src/main.rs:

fn main() {
    println!("Hello, world!");
}

Let’s try compiling what Cargo gave us:

$ cargo build
   Compiling guessing_game v0.1.0 (file:///home/you/projects/guessing_game)

Excellent! Open up your src/main.rs again. We’ll be writing all of our code in this file.

Before we move on, let me show you one more Cargo command: run. cargo run is kind of like cargo build, but it also then runs the produced executable. Try it out:

$ 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 need to quickly test each iteration before moving on to the next one.

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. There is also a second ‘prelude’, the io prelude, which serves a similar function: you import it, and it imports a number of useful, io-related things.

fn main() {

As you’ve seen before, the main() function is the entry point into your program. The fn syntax declares a new function, the ()s indicate that there are no arguments, and { starts the body of the function. Because we didn’t include a return type, it’s assumed to be (), an empty tuple.

    println!("Guess the number!");

    println!("Please input your guess.");

We previously learned that println!() is a macro that prints a string to the screen.

    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 ‘variable bindings’. They take this form:

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 doesn’t take a name on the left hand side of the assignment, it actually accepts a ‘pattern’. We’ll use patterns later. It’s easy enough to use for now:

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");

That’s a lot more! Let’s go 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. Methods are like associated functions, but are only available on a particular instance of a type, rather than the type itself. 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. Rust has a feature called ‘references’, which allows you to have multiple references to one piece of data, which can 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. 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, it 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:

        .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 done:

    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. 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 one, panic!s with a message you passed it. A panic! like this will cause our program to crash, displaying the message.

If we leave off calling 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.

There’s only one line of this first example left:

    println!("You guessed: {}", guess);
}

This prints out the string we saved our input in. The {}s are a placeholder, and so we pass it guess as an argument. If we had multiple {}s, we would pass multiple arguments:

let x = 5;
let y = 10;

println!("x and y: {} and {}", x, y);

Easy.

Anyway, that’s the tour. 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 these few lines at the bottom:

[dependencies]

rand="0.3.0"

The [dependencies] section of Cargo.toml is like the [package] section: everything that follows it is part of it, until the next section starts. Cargo uses the dependencies section to know what dependencies on external crates you have, and what versions you require. In this case, we’ve specified version 0.3.0, which Cargo understands to be any release that’s compatible with this specific version. Cargo understands Semantic Versioning, which is a standard for writing version numbers. A bare number like above is actually shorthand for ^0.3.0, meaning "anything compatible with 0.3.0". If we wanted to use only 0.3.0 exactly, we could say rand="=0.3.0" (note the two equal signs). And if we wanted to use the latest version we could use *. We could also use a range of versions. Cargo’s documentation contains more details.

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 v0.3.8
 Downloading libc v0.1.6
   Compiling libc v0.1.6
   Compiling rand v0.3.8
   Compiling guessing_game v0.1.0 (file:///home/you/projects/guessing_game)

(You may see different versions, of course.)

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, and that all of its dependencies are built, and so there’s no reason to do all that stuff. With nothing to do, it simply exits. If we open up src/main.rs again, make a trivial change, and then save it again, we’ll only see one line:

$ cargo build
   Compiling guessing_game v0.1.0 (file:///home/you/projects/guessing_game)

So, we told Cargo we wanted any 0.3.x version of rand, and so it fetched the latest version at the time this was written, v0.3.8. But what happens when next week, version v0.3.9 comes out, with an important bugfix? While getting bugfixes is important, what if 0.3.9 contains a regression that breaks our code?

The answer to this problem is the Cargo.lock file you’ll now find in your project directory. When you build your project for the first time, Cargo figures out all of the versions that fit your 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 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 0.3.8 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 v0.3.9? Cargo has another command, update, which says ‘ignore the lock, figure out all the latest versions that fit what we’ve specified. 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 the Cargo.toml directly. 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, but for now, that’s all we need to know. Cargo makes it really easy to re-use libraries, and so Rustaceans tend 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 use extern crate to let Rust know we’ll be making use of it. This also does the equivalent of a use rand; as well, so we can make use of 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.

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 use rand::Rng’d above, it has a gen_range() method available. This method takes two arguments, and generates a number between them. It’s inclusive on the lower bound, but exclusive on the upper bound, so we need 1 and 101 to get 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, so we can 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

Great! 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 our next step, though it doesn’t quite compile yet:

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, five new lines at the bottom that use it:

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. 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.

I 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:28:21: 28:35 error: mismatched types:
 expected `&collections::string::String`,
    found `&_`
(expected struct `collections::string::String`,
    found integral variable) [E0308]
src/main.rs:28     match guess.cmp(&secret_number) {
                                   ^~~~~~~~~~~~~~
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, and so it doesn’t make us write out the type. And 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, for comparison. 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, I thought we already had 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 an 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(). This means that 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.

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 I even added spaces before my guess, and it still figured out that I guessed 76. Run the program a few times, and verify that guessing the 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, we’ll panic! and 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!'

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 it’s the last thing in main(). We have only one more 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 returned by parse(), this 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: the successfully parsed integer, or an error type. In this case, we match on Ok(num), which sets the name num to the unwrapped Ok value (the integer), and then we return it on the right-hand side. 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. This catches everything that isn't Ok, and continue lets us move to the next iteration of the loop; in effect, this enables us to ignore all errors and continue with our program.

Now we should be good! Let’s try:

$ 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 have finished 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!

At this point, you have successfully built the Guessing Game! Congratulations!

This first project showed you a lot: let, match, methods, associated functions, using external crates, and more. Our next project will show off even more.