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[rustc.git] / src / doc / book / src / ch02-00-guessing-game-tutorial.md

# Programming a Guessing Game

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:

```text
$ 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:

<span class="filename">Filename: Cargo.toml</span>

```toml
[package]
name = "guessing_game"
version = "0.1.0"
authors = ["Your Name <you@example.com>"]
edition = "2018"

[dependencies]
```

If the author information that Cargo obtained from your environment is not
correct, fix that in the file and save it again.

As you saw in Chapter 1, `cargo new` generates a “Hello, world!” program for
you. Check out the *src/main.rs* file:

<span class="filename">Filename: src/main.rs</span>

```rust
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:

```text
$ cargo run
   Compiling guessing_game v0.1.0 (file:///projects/guessing_game)
    Finished dev [unoptimized + debuginfo] target(s) in 1.50 secs
     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*.

<span class="filename">Filename: src/main.rs</span>

```rust,ignore
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);
}
```

<span class="caption">Listing 2-1: Code that gets a guess from the user and
prints it</span>

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`):

```rust,ignore
use std::io;
```

By default, Rust brings only a few types into the scope of every program in
[the *prelude*][prelude]<!-- ignore -->. 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.

[prelude]: ../std/prelude/index.html

As you saw in Chapter 1, the `main` function is the entry point into the
program:

```rust,ignore
fn main() {
```

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:

```rust,ignore
println!("Guess the number!");

println!("Please input your 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:

```rust,ignore
let mut guess = String::new();
```

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:

```rust,ignore
let foo = bar;
```

This line creates a new variable named `foo` and binds it to the value of the
`bar` variable. In Rust, variables are immutable by default. We’ll be
discussing this concept in detail in the [“Variables and Mutability”]
[variables-and-mutability]<!-- ignore --> section in Chapter 3. The following
example shows how to use `mut` before the variable name to make a variable
mutable:

```rust,ignore
let foo = 5; // immutable
let mut bar = 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`][string]<!-- ignore --> is a string type provided by the standard
library that is a growable, UTF-8 encoded bit of text.

[string]: ../std/string/struct.String.html

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`, rather than on a particular instance of a `String`. Some
languages call this a *static method*.

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 an
associated function, `stdin`, on `io`:

```rust,ignore
io::stdin().read_line(&mut guess)
    .expect("Failed to read line");
```

If we hadn’t listed 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`][iostdin]<!-- ignore -->, which is a
type that represents a handle to the standard input for your terminal.

[iostdin]: ../std/io/struct.Stdin.html

The next part of the code, `.read_line(&mut guess)`, calls the
[`read_line`][read_line]<!-- ignore --> method on the standard input handle to
get input from the user. We’re also passing one argument to `read_line`: `&mut
guess`.

[read_line]: ../std/io/struct.Stdin.html#method.read_line

The job of `read_line` is to take whatever the user types into standard input
and place that into a string, 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 not quite done with this line of code. Although what we’ve discussed so
far is a single line of text, it’s only the first part of the single logical
line of code. The second part is this method:

```rust,ignore
.expect("Failed to read line");
```

When you call a method with the `.foo()` 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:

```rust,ignore
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: two
lines for two method calls. 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`][ioresult]<!-- ignore -->. Rust has a number of types named
`Result` in its standard library: a generic [`Result`][result]<!-- ignore -->
as well as specific versions for submodules, such as `io::Result`.

[ioresult]: ../std/io/type.Result.html
[result]: ../std/result/enum.Result.html

The `Result` types are [*enumerations*][enums]<!-- ignore -->, 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.

[enums]: ch06-00-enums.html

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][expect]<!-- ignore
--> 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.

[expect]: ../std/result/enum.Result.html#method.expect

If you don’t call `expect`, the program will compile, but you’ll get a warning:

```text
$ cargo build
   Compiling guessing_game v0.1.0 (file:///projects/guessing_game)
warning: unused `std::result::Result` which must be used
  --> src/main.rs:10:5
   |
10 |     io::stdin().read_line(&mut guess);
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: #[warn(unused_must_use)] on by default
```

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 brackets, there’s only one more line to discuss in
the code added so far, which is the following:

```rust,ignore
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:

```rust
let x = 5;
let y = 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`:

```text
$ cargo run
   Compiling guessing_game v0.1.0 (file:///projects/guessing_game)
    Finished dev [unoptimized + debuginfo] target(s) in 2.53 secs
     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][randcrate].

[randcrate]: https://crates.io/crates/rand

### 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 use of external crates is where it 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:

<span class="filename">Filename: Cargo.toml</span>

```toml
[dependencies]

rand = "0.3.14"
```

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 `0.3.14`. Cargo understands [Semantic
Versioning][semver]<!-- ignore --> (sometimes called *SemVer*), which is a
standard for writing version numbers. The number `0.3.14` is actually shorthand
for `^0.3.14`, which means “any version that has a public API compatible with
version 0.3.14.”

[semver]: http://semver.org

Now, without changing any of the code, let’s build the project, as shown in
Listing 2-2.

```text
$ 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:///projects/guessing_game)
    Finished dev [unoptimized + debuginfo] target(s) in 2.53 secs
```

<span class="caption">Listing 2-2: The output from running `cargo build` after
adding the rand crate as a dependency</span>

You may see different version numbers (but they will all be compatible with
the code, thanks to SemVer!), 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][cratesio]. Crates.io is where people in the Rust ecosystem post
their open source Rust projects for others to use.

[cratesio]: https://crates.io

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 a copy of `libc`, because `rand`
depends on `libc` 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:

```text
$ 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 0.3.15 of the `rand` crate comes out and
contains an important bug fix but 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 `0.3.14` 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 `0.3.0` and less
than `0.4.0`. If the `rand` crate has released two new versions, `0.3.15` and
`0.4.0`, you would see the following if you ran `cargo update`:

```text
$ cargo update
    Updating registry `https://github.com/rust-lang/crates.io-index`
    Updating rand v0.3.14 -> v0.3.15
```

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 `0.3.15`.

If you wanted to use `rand` version `0.4.0` or any version in the `0.4.x`
series, you’d have to update the *Cargo.toml* file to look like this instead:

```toml
[dependencies]

rand = "0.4.0"
```

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][doccargo]<!-- ignore --> and [its
ecosystem][doccratesio]<!-- ignore --> 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.

[doccargo]: http://doc.crates.io
[doccratesio]: http://doc.crates.io/crates-io.html

### 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.

<span class="filename">Filename: src/main.rs</span>

```rust,ignore
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);
}
```

<span class="caption">Listing 2-3: Adding code to generate a random
number</span>

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 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 to specify `1`
and `101` to request a number between 1 and 100.

> 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 --open` command, 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 the `rand` crate, for example, run `cargo doc --open`
> and click `rand` in 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:

```text
$ cargo run
   Compiling guessing_game v0.1.0 (file:///projects/guessing_game)
    Finished dev [unoptimized + debuginfo] target(s) in 2.53 secs
     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 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.

<span class="filename">Filename: src/main.rs</span>

```rust,ignore,does_not_compile
use std::io;
use std::cmp::Ordering;
use rand::Rng;

fn main() {

    // ---snip---

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

    match guess.cmp(&secret_number) {
        Ordering::Less => println!("Too small!"),
        Ordering::Greater => println!("Too big!"),
        Ordering::Equal => println!("You win!"),
    }
}
```

<span class="caption">Listing 2-4: Handling the possible return values of
comparing two numbers</span>

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`][match]<!-- ignore --> 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`.

[match]: ch06-02-match.html

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:

```text
$ cargo build
   Compiling guessing_game v0.1.0 (file:///projects/guessing_game)
error[E0308]: mismatched types
  --> src/main.rs:23:21
   |
23 |     match guess.cmp(&secret_number) {
   |                     ^^^^^^^^^^^^^^ expected struct `std::string::String`, found integral variable
   |
   = note: expected type `&std::string::String`
   = note:    found type `&{integer}`

error: aborting due to previous error
Could not compile `guessing_game`.
```

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 the following two lines to the `main` function body:

<span class="filename">Filename: src/main.rs</span>

```rust,ignore
// --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!"),
    }
}
```

The two new lines are:

```rust,ignore
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 <span class="keystroke">enter</span> to satisfy
`read_line`. When the user presses <span class="keystroke">enter</span>, a
newline character is added to the string. For example, if the user types <span
class="keystroke">5</span> and presses <span class="keystroke">enter</span>,
`guess` looks like this: `5\n`. The `\n` represents “newline,” the result of
pressing <span class="keystroke">enter</span>. The `trim` method eliminates
`\n`, resulting in just `5`.

The [`parse` method on strings][parse]<!-- ignore --> 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!

[parse]: ../std/primitive.str.html#method.parse

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”](#handling-potential-failure-with-the-result-type)<!-- ignore
-->). 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!

```text
$ cargo run
   Compiling guessing_game v0.1.0 (file:///projects/guessing_game)
    Finished dev [unoptimized + debuginfo] target(s) in 0.43 secs
     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:

<span class="filename">Filename: src/main.rs</span>

```rust,ignore
// --snip--

    println!("The secret number is: {}", secret_number);

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

        // --snip--

        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 halt the program by using the keyboard shortcut <span
class="keystroke">ctrl-c</span>. But there’s another way to escape this
insatiable monster, as mentioned in the `parse` discussion in [“Comparing the
Guess to the Secret Number”](#comparing-the-guess-to-the-secret-number)<!--
ignore -->: 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:

```text
$ cargo run
   Compiling guessing_game v0.1.0 (file:///projects/guessing_game)
    Finished dev [unoptimized + debuginfo] target(s) in 1.50 secs
     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/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)
```

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:

<span class="filename">Filename: src/main.rs</span>

```rust,ignore
// --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.

<span class="filename">Filename: src/main.rs</span>

```rust,ignore
// --snip--

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);

// --snip--
```

<span class="caption">Listing 2-5: Ignoring a non-number guess and asking for
another guess instead of crashing the program</span>

Switching from an `expect` call to a `match` expression is how you generally
move 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:

```text
$ cargo run
   Compiling guessing_game v0.1.0 (file:///projects/guessing_game)
     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.

<span class="filename">Filename: src/main.rs</span>

```rust,ignore
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;
            }
        }
    }
}
```

<span class="caption">Listing 2-6: Complete guessing game code</span>

## 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`, methods, associated 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.

[variables-and-mutability]:
ch03-01-variables-and-mutability.html#variables-and-mutability