New upstream version 1.55.0+dfsg1

[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:

```console
$ 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
{{#include ../listings/ch02-guessing-game-tutorial/no-listing-01-cargo-new/Cargo.toml}}
```

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
{{#rustdoc_include ../listings/ch02-guessing-game-tutorial/no-listing-01-cargo-new/src/main.rs}}
```

Now let’s compile this “Hello, world!” program and run it in the same step
using the `cargo run` command:

```console
{{#include ../listings/ch02-guessing-game-tutorial/no-listing-01-cargo-new/output.txt}}
```

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
{{#rustdoc_include ../listings/ch02-guessing-game-tutorial/listing-02-01/src/main.rs:all}}
```

<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
{{#rustdoc_include ../listings/ch02-guessing-game-tutorial/listing-02-01/src/main.rs: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
{{#rustdoc_include ../listings/ch02-guessing-game-tutorial/listing-02-01/src/main.rs: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
{{#rustdoc_include ../listings/ch02-guessing-game-tutorial/listing-02-01/src/main.rs:print}}
```

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
{{#rustdoc_include ../listings/ch02-guessing-game-tutorial/listing-02-01/src/main.rs:string}}
```

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 apples = 5;
```

This line creates a new variable named `apples` and binds it to the value 5. In
Rust, variables are immutable by default. We’ll be discussing this concept in
detail in the [“Variables and Mutability”][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 apples = 5; // immutable
let mut bananas = 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`.

This `new` function creates a new, empty string. You’ll find a `new` function
on many types, because it’s a common name for a function that makes a new value
of some kind.

To summarize, the `let mut guess = String::new();` line has created a mutable
variable that is currently bound to a new, empty instance of a `String`. Whew!

Recall that we included the input/output functionality from the standard
library with `use std::io;` on the first line of the program. Now we’ll call
the `stdin` function from the `io` module:

```rust,ignore
{{#rustdoc_include ../listings/ch02-guessing-game-tutorial/listing-02-01/src/main.rs:read}}
```

If we hadn’t put the `use std::io` line at the beginning of the program, we
could have written this function call as `std::io::stdin`. The `stdin` function
returns an instance of [`std::io::Stdin`][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 append that into a string (without overwriting its contents), so it takes
that string as an argument. The string argument needs to be mutable so the
method can change the string’s content by adding the user input.

The `&` indicates that this argument is a *reference*, which gives you a way to
let multiple parts of your code access one piece of data without needing to
copy that data into memory multiple times. References are a complex feature,
and one of Rust’s major advantages is how safe and easy it is to use
references. You don’t need to know a lot of those details to finish this
program. For now, all you need to know is that like variables, references are
immutable by default. Hence, you need to write `&mut guess` rather than
`&guess` to make it mutable. (Chapter 4 will explain references more
thoroughly.)

### Handling Potential Failure with the `Result` Type

We’re still working on this line of code. Although we’re now discussing a third
line of text, it’s still part of a single logical line of code. The next part
is this method:

```rust,ignore
{{#rustdoc_include ../listings/ch02-guessing-game-tutorial/listing-02-01/src/main.rs:expect}}
```

When you call a method with the `.method_name()` syntax, it’s often wise to
introduce a newline and other whitespace to help break up long lines. We could
have written this code as:

```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. 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:

```console
{{#include ../listings/ch02-guessing-game-tutorial/no-listing-02-without-expect/output.txt}}
```

Rust warns that you haven’t used the `Result` value returned from `read_line`,
indicating that the program hasn’t handled a possible error.

The right way to suppress the warning is to actually write error handling, but
because you just want to crash this program when a problem occurs, you can use
`expect`. You’ll learn about recovering from errors in Chapter 9.

### Printing Values with `println!` Placeholders

Aside from the closing curly bracket, there’s only one more line to discuss in
the code added so far, which is the following:

```rust,ignore
{{#rustdoc_include ../listings/ch02-guessing-game-tutorial/listing-02-01/src/main.rs:print_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`:

<!-- manual-regeneration
cd listings/ch02-guessing-game-tutorial/listing-02-01/
cargo clean
cargo run
input 6 -->

```console
$ cargo run
   Compiling guessing_game v0.1.0 (file:///projects/guessing_game)
    Finished dev [unoptimized + debuginfo] target(s) in 6.44s
     Running `target/debug/guessing_game`
Guess the number!
Please input your guess.
6
You guessed: 6
```

At this point, the first part of the game is done: we’re getting input from the
keyboard and then printing it.

## Generating a Secret Number

Next, we need to generate a secret number that the user will try to guess. The
secret number should be different every time so the game is fun to play more
than once. Let’s use a random number between 1 and 100 so the game isn’t too
difficult. Rust doesn’t yet include random number functionality in its standard
library. However, the Rust team does provide a [`rand` crate][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 coordination of external crates is where Cargo really shines. Before we
can write code that uses `rand`, we need to modify the *Cargo.toml* file to
include the `rand` crate as a dependency. Open that file now and add the
following line to the bottom beneath the `[dependencies]` section header that
Cargo created for you. Be sure to specify `rand` exactly as we have here, or
the code examples in this tutorial may not work.

<!-- When updating the version of `rand` used, also update the version of
`rand` used in these files so they all match:
* ch07-04-bringing-paths-into-scope-with-the-use-keyword.md
* ch14-03-cargo-workspaces.md
-->

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

```toml
{{#include ../listings/ch02-guessing-game-tutorial/listing-02-02/Cargo.toml:9:}}
```

In the *Cargo.toml* file, everything that follows a header is part of a section
that continues until another section starts. The `[dependencies]` section is
where you tell Cargo which external crates your project depends on and which
versions of those crates you require. In this case, we’ll specify the `rand`
crate with the semantic version specifier `0.8.3`. Cargo understands [Semantic
Versioning][semver]<!-- ignore --> (sometimes called *SemVer*), which is a
standard for writing version numbers. The number `0.8.3` is actually shorthand
for `^0.8.3`, which means any version that is at least `0.8.3` but below
`0.9.0`. Cargo considers these versions to have public APIs compatible with
version `0.8.3`, and this specification ensures you'll get the latest patch
release that will still compile with the code in this chapter. Any version
`0.9.0` or greater is not guaranteed to have the same API as what the following
examples use.

[semver]: http://semver.org

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

<!-- manual-regeneration
cd listings/ch02-guessing-game-tutorial/listing-02-02/
cargo clean
cargo build -->

```console
$ cargo build
    Updating crates.io index
  Downloaded rand v0.8.3
  Downloaded libc v0.2.86
  Downloaded getrandom v0.2.2
  Downloaded cfg-if v1.0.0
  Downloaded ppv-lite86 v0.2.10
  Downloaded rand_chacha v0.3.0
  Downloaded rand_core v0.6.2
   Compiling rand_core v0.6.2
   Compiling libc v0.2.86
   Compiling getrandom v0.2.2
   Compiling cfg-if v1.0.0
   Compiling ppv-lite86 v0.2.10
   Compiling rand_chacha v0.3.0
   Compiling rand v0.8.3
   Compiling guessing_game v0.1.0 (file:///projects/guessing_game)
    Finished dev [unoptimized + debuginfo] target(s) in 2.53s
```

<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!), different lines (depending on the operating
system), and the lines may be in a different order.

Now that we have an external dependency, Cargo fetches the latest versions of
everything from the *registry*, which is a copy of data from
[Crates.io][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 other crates that `rand` depends on
to work. After downloading the crates, Rust compiles them and then compiles the
project with the dependencies available.

If you immediately run `cargo build` again without making any changes, you
won’t get any output aside from the `Finished` line. Cargo knows it has already
downloaded and compiled the dependencies, and you haven’t changed anything
about them in your *Cargo.toml* file. Cargo also knows that you haven’t changed
anything about your code, so it doesn’t recompile that either. With nothing to
do, it simply exits.

If you open up the *src/main.rs* file, make a trivial change, and then save it
and build again, you’ll only see two lines of output:

<!-- manual-regeneration
cd listings/ch02-guessing-game-tutorial/listing-02-02/
touch src/main.rs
cargo build -->

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

These lines show Cargo only updates the build with your tiny change to the
*src/main.rs* file. Your dependencies haven’t changed, so Cargo knows it can
reuse what it has already downloaded and compiled for those. It just rebuilds
your part of the code.

#### Ensuring Reproducible Builds with the *Cargo.lock* File

Cargo has a mechanism that ensures you can rebuild the same artifact every time
you or anyone else builds your code: Cargo will use only the versions of the
dependencies you specified until you indicate otherwise. For example, what
happens if next week version 0.8.4 of the `rand` crate comes out, and that
version contains an important bug fix, but it also contains a regression that
will break your code?

The answer to this problem is the *Cargo.lock* file, which was created the
first time you ran `cargo build` and is now in your *guessing_game* directory.
When you build a project for the first time, Cargo figures out all the
versions of the dependencies that fit the criteria and then writes them to
the *Cargo.lock* file. When you build your project in the future, Cargo will
see that the *Cargo.lock* file exists and use the versions specified there
rather than doing all the work of figuring out versions again. This lets you
have a reproducible build automatically. In other words, your project will
remain at `0.8.3` until you explicitly upgrade, thanks to the *Cargo.lock*
file.

#### Updating a Crate to Get a New Version

When you *do* want to update a crate, Cargo provides another command, `update`,
which will ignore the *Cargo.lock* file and figure out all the latest versions
that fit your specifications in *Cargo.toml*. If that works, Cargo will write
those versions to the *Cargo.lock* file.

But by default, Cargo will only look for versions greater than `0.8.3` and less
than `0.9.0`. If the `rand` crate has released two new versions, `0.8.4` and
`0.9.0`, you would see the following if you ran `cargo update`:

<!-- manual-regeneration
cd listings/ch02-guessing-game-tutorial/listing-02-02/
cargo update
assuming there is a new 0.8.x version of rand; otherwise use another update
as a guide to creating the hypothetical output shown here -->

```console
$ cargo update
    Updating crates.io index
    Updating rand v0.8.3 -> v0.8.4
```

At this point, you would also notice a change in your *Cargo.lock* file noting
that the version of the `rand` crate you are now using is `0.8.4`.

If you wanted to use `rand` version `0.9.0` or any version in the `0.9.x`
series, you’d have to update the *Cargo.toml* file to look like this instead:

```toml
[dependencies]
rand = "0.9.0"
```

The next time you run `cargo build`, Cargo will update the registry of crates
available and reevaluate your `rand` requirements according to the new version
you have specified.

There’s a lot more to say about [Cargo][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
{{#rustdoc_include ../listings/ch02-guessing-game-tutorial/listing-02-03/src/main.rs:all}}
```

<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 a range
expression as an argument and generates a random number in the range. The kind
of range expression we’re using here takes the form `start..end`. It’s
inclusive on the lower bound but exclusive on the upper bound, so we need to
specify `1..101` to request a number between 1 and 100. Alternatively, we could
pass the range `1..=100`, which is equivalent.

> Note: You won’t just know which traits to use and which methods and functions
> to call from a crate. Instructions for using a crate are in each crate’s
> documentation. Another neat feature of Cargo is that you can run the `cargo
> doc --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:

<!-- manual-regeneration
cd listings/ch02-guessing-game-tutorial/listing-02-03/
cargo run
4
cargo run
5
-->

```console
$ cargo run
   Compiling guessing_game v0.1.0 (file:///projects/guessing_game)
    Finished dev [unoptimized + debuginfo] target(s) in 2.53s
     Running `target/debug/guessing_game`
Guess the number!
The secret number is: 7
Please input your guess.
4
You guessed: 4

$ cargo run
    Finished dev [unoptimized + debuginfo] target(s) in 0.02s
     Running `target/debug/guessing_game`
Guess the number!
The secret number is: 83
Please input your guess.
5
You guessed: 5
```

You should get different random numbers, and they should all be numbers between
1 and 100. Great job!

## Comparing the Guess to the Secret Number

Now that we have user input and a random number, we can compare them. That step
is shown in Listing 2-4. Note that this code won’t compile quite yet, as we
will explain.

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

```rust,ignore,does_not_compile
{{#rustdoc_include ../listings/ch02-guessing-game-tutorial/listing-02-04/src/main.rs:here}}
```

<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:

```console
{{#include ../listings/ch02-guessing-game-tutorial/listing-02-04/output.txt}}
```

The core of the error states that there are *mismatched types*. Rust has a
strong, static type system. However, it also has type inference. When we wrote
`let mut guess = String::new()`, Rust was able to infer that `guess` should be
a `String` and didn’t make us write the type. The `secret_number`, on the other
hand, is a number type. A few number types can have a value between 1 and 100:
`i32`, a 32-bit number; `u32`, an unsigned 32-bit number; `i64`, a 64-bit
number; as well as others. Rust defaults to an `i32`, which is the type of
`secret_number` unless you add type information elsewhere that would cause Rust
to infer a different numerical type. The reason for the error is that Rust
cannot compare a string and a number type.

Ultimately, we want to convert the `String` the program reads as input into a
real number type so we can compare it numerically to the secret number. We can
do that by adding another line to the `main` function body:

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

```rust,ignore
{{#rustdoc_include ../listings/ch02-guessing-game-tutorial/no-listing-03-convert-string-to-number/src/main.rs:here}}
```

The line is:

```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> (On Windows, pressing <span
class="keystroke">enter</span> results in a carriage return and a newline,
`\r\n`). The `trim` method eliminates `\n` or `\r\n`, resulting in just `5`.

The [`parse` method on strings][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!

<!-- manual-regeneration
cd listings/ch02-guessing-game-tutorial/no-listing-03-convert-string-to-number/
cargo run
  76
-->

```console
$ cargo run
   Compiling guessing_game v0.1.0 (file:///projects/guessing_game)
    Finished dev [unoptimized + debuginfo] target(s) in 0.43s
     Running `target/debug/guessing_game`
Guess the number!
The secret number is: 58
Please input your guess.
  76
You guessed: 76
Too big!
```

Nice! Even though spaces were added before the guess, the program still figured
out that the user guessed 76. Run the program a few times to verify the
different behavior with different kinds of input: guess the number correctly,
guess a number that is too high, and guess a number that is too low.

We have most of the game working now, but the user can make only one guess.
Let’s change that by adding a loop!

## Allowing Multiple Guesses with Looping

The `loop` keyword creates an infinite loop. We’ll add that now to give users
more chances at guessing the number:

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

```rust,ignore
{{#rustdoc_include ../listings/ch02-guessing-game-tutorial/no-listing-04-looping/src/main.rs:here}}
```

As you can see, we’ve moved everything into a loop from the guess input prompt
onward. Be sure to indent the lines inside the loop another four spaces each
and run the program again. Notice that there is a new problem because the
program is doing exactly what we told it to do: ask for another guess forever!
It doesn’t seem like the user can quit!

The user could always interrupt the program by using the keyboard shortcut <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:

<!-- manual-regeneration
cd listings/ch02-guessing-game-tutorial/no-listing-04-looping/
cargo run
(too small guess)
(too big guess)
(correct guess)
quit
-->

```console
$ cargo run
   Compiling guessing_game v0.1.0 (file:///projects/guessing_game)
    Finished dev [unoptimized + debuginfo] target(s) in 1.50s
     Running `target/debug/guessing_game`
Guess the number!
The secret number is: 59
Please input your guess.
45
You guessed: 45
Too small!
Please input your guess.
60
You guessed: 60
Too big!
Please input your guess.
59
You guessed: 59
You win!
Please input your guess.
quit
thread 'main' panicked at 'Please type a number!: ParseIntError { kind: InvalidDigit }', src/main.rs:28:47
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
```

Typing `quit` actually quits the game, but so will any other non-number input.
However, this is suboptimal to say the least. We want the game to automatically
stop when the correct number is guessed.

### Quitting After a Correct Guess

Let’s program the game to quit when the user wins by adding a `break` statement:

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

```rust,ignore
{{#rustdoc_include ../listings/ch02-guessing-game-tutorial/no-listing-05-quitting/src/main.rs:here}}
```

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
{{#rustdoc_include ../listings/ch02-guessing-game-tutorial/listing-02-05/src/main.rs:here}}
```

<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 one way of moving
from crashing on an error to handling the error. Remember that `parse` returns
a `Result` type and `Result` is an enum that has the variants `Ok` or `Err`.
We’re using a `match` expression here, as we did with the `Ordering` result of
the `cmp` method.

If `parse` is able to successfully turn the string into a number, it will
return an `Ok` value that contains the resulting number. That `Ok` value will
match the first arm’s pattern, and the `match` expression will just return the
`num` value that `parse` produced and put inside the `Ok` value. That number
will end up right where we want it in the new `guess` variable we’re creating.

If `parse` is *not* able to turn the string into a number, it will return an
`Err` value that contains more information about the error. The `Err` value
does not match the `Ok(num)` pattern in the first `match` arm, but it does
match the `Err(_)` pattern in the second arm. The underscore, `_`, is a
catchall value; in this example, we’re saying we want to match all `Err`
values, no matter what information they have inside them. So the program will
execute the second arm’s code, `continue`, which tells the program to go to the
next iteration of the `loop` and ask for another guess. So, effectively, the
program ignores all errors that `parse` might encounter!

Now everything in the program should work as expected. Let’s try it:

<!-- manual-regeneration
cd listings/ch02-guessing-game-tutorial/listing-02-05/
cargo run
(too small guess)
(too big guess)
foo
(correct guess)
-->

```console
$ cargo run
   Compiling guessing_game v0.1.0 (file:///projects/guessing_game)
    Finished dev [unoptimized + debuginfo] target(s) in 4.45s
     Running `target/debug/guessing_game`
Guess the number!
The secret number is: 61
Please input your guess.
10
You guessed: 10
Too small!
Please input your guess.
99
You guessed: 99
Too big!
Please input your guess.
foo
Please input your guess.
61
You guessed: 61
You win!
```

Awesome! With one tiny final tweak, we will finish the guessing game. Recall
that the program is still printing the secret number. That worked well for
testing, but it ruins the game. Let’s delete the `println!` that outputs the
secret number. Listing 2-6 shows the final code.

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

```rust,ignore
{{#rustdoc_include ../listings/ch02-guessing-game-tutorial/listing-02-06/src/main.rs}}
```

<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`, 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