mirror of https://github.com/rust-lang/book
1007 lines
37 KiB
Markdown
1007 lines
37 KiB
Markdown
<!-- DO NOT EDIT THIS FILE.
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This file is periodically generated from the content in the `/src/`
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directory, so all fixes need to be made in `/src/`.
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-->
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[TOC]
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# Programming a Guessing Game
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Let’s jump into Rust by working through a hands-on project together! This
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chapter introduces you to a few common Rust concepts by showing you how to use
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them in a real program. You’ll learn about `let`, `match`, methods, associated
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functions, external crates, and more! In the following chapters, we’ll explore
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these ideas in more detail. In this chapter, you’ll just practice the
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fundamentals.
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We’ll implement a classic beginner programming problem: a guessing game. Here’s
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how it works: the program will generate a random integer between 1 and 100. It
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will then prompt the player to enter a guess. After a guess is entered, the
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program will indicate whether the guess is too low or too high. If the guess is
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correct, the game will print a congratulatory message and exit.
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## Setting Up a New Project
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To set up a new project, go to the *projects* directory that you created in
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Chapter 1 and make a new project using Cargo, like so:
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```
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$ cargo new guessing_game
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$ cd guessing_game
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```
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The first command, `cargo new`, takes the name of the project (`guessing_game`)
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as the first argument. The second command changes to the new project’s
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directory.
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Look at the generated *Cargo.toml* file:
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Filename: Cargo.toml
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```
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[package]
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name = "guessing_game"
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version = "0.1.0"
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edition = "2021"
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# See more keys and their definitions at
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https://doc.rust-lang.org/cargo/reference/manifest.html
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[dependencies]
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```
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As you saw in Chapter 1, `cargo new` generates a “Hello, world!” program for
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you. Check out the *src/main.rs* file:
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Filename: src/main.rs
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```
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fn main() {
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println!("Hello, world!");
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}
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```
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Now let’s compile this “Hello, world!” program and run it in the same step
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using the `cargo run` command:
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```
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$ cargo run
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Compiling guessing_game v0.1.0 (file:///projects/guessing_game)
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Finished dev [unoptimized + debuginfo] target(s) in 1.50s
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Running `target/debug/guessing_game`
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Hello, world!
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```
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The `run` command comes in handy when you need to rapidly iterate on a project,
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as we’ll do in this game, quickly testing each iteration before moving on to
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the next one.
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Reopen the *src/main.rs* file. You’ll be writing all the code in this file.
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## Processing a Guess
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The first part of the guessing game program will ask for user input, process
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that input, and check that the input is in the expected form. To start, we’ll
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allow the player to input a guess. Enter the code in Listing 2-1 into
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*src/main.rs*.
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Filename: src/main.rs
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```
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use std::io;
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fn main() {
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println!("Guess the number!");
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println!("Please input your guess.");
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let mut guess = String::new();
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io::stdin()
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.read_line(&mut guess)
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.expect("Failed to read line");
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println!("You guessed: {guess}");
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}
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```
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Listing 2-1: Code that gets a guess from the user and prints it
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This code contains a lot of information, so let’s go over it line by line. To
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obtain user input and then print the result as output, we need to bring the
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`io` input/output library into scope. The `io` library comes from the standard
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library, known as `std`:
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```
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use std::io;
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```
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By default, Rust has a set of items defined in the standard library that it
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brings into the scope of every program. This set is called the *prelude*, and
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you can see everything in it at
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*https://doc.rust-lang.org/std/prelude/index.html*.
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If a type you want to use isn’t in the prelude, you have to bring that type
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into scope explicitly with a `use` statement. Using the `std::io` library
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provides you with a number of useful features, including the ability to accept
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user input.
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As you saw in Chapter 1, the `main` function is the entry point into the
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program:
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```
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fn main() {
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```
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The `fn` syntax declares a new function; the parentheses, `()`, indicate there
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are no parameters; and the curly bracket, `{`, starts the body of the function.
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As you also learned in Chapter 1, `println!` is a macro that prints a string to
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the screen:
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```
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println!("Guess the number!");
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println!("Please input your guess.");
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```
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This code is printing a prompt stating what the game is and requesting input
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from the user.
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### Storing Values with Variables
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Next, we’ll create a *variable* to store the user input, like this:
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```
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let mut guess = String::new();
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```
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Now the program is getting interesting! There’s a lot going on in this little
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line. We use the `let` statement to create the variable. Here’s another example:
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```
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let apples = 5;
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```
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This line creates a new variable named `apples` and binds it to the value 5. In
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Rust, variables are immutable by default, meaning once we give the variable a
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value, the value won’t change. We’ll be discussing this concept in detail in
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“Variables and Mutability” on page XX. To make a variable mutable, we add `mut`
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before the variable name:
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```
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let apples = 5; // immutable
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let mut bananas = 5; // mutable
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```
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> Note: The `//` syntax starts a comment that continues until the end of the
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line. Rust ignores everything in comments. We’ll discuss comments in more
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detail in Chapter 3.
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Returning to the guessing game program, you now know that `let mut guess` will
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introduce a mutable variable named `guess`. The equal sign (`=`) tells Rust we
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want to bind something to the variable now. On the right of the equal sign is
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the value that `guess` is bound to, which is the result of calling
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`String::new`, a function that returns a new instance of a `String`. `String`
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is a string type provided by the standard library that is a growable, UTF-8
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encoded bit of text.
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The `::` syntax in the `::new` line indicates that `new` is an associated
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function of the `String` type. An *associated function* is a function that’s
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implemented on a type, in this case `String`. This `new` function creates a
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new, empty string. You’ll find a `new` function on many types because it’s a
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common name for a function that makes a new value of some kind.
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In full, the `let mut guess = String::new();` line has created a mutable
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variable that is currently bound to a new, empty instance of a `String`. Whew!
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### Receiving User Input
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Recall that we included the input/output functionality from the standard
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library with `use std::io;` on the first line of the program. Now we’ll call
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the `stdin` function from the `io` module, which will allow us to handle user
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input:
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```
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io::stdin()
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.read_line(&mut guess)
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```
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If we hadn’t imported the `io` library with `use std::io;` at the beginning of
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the program, we could still use the function by writing this function call as
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`std::io::stdin`. The `stdin` function returns an instance of `std::io::Stdin`,
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which is a type that represents a handle to the standard input for your
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terminal.
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Next, the line `.read_line(&mut guess)` calls the `read_line` method on the
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standard input handle to get input from the user. We’re also passing `&mut
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guess` as the argument to `read_line` to tell it what string to store the user
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input in. The full job of `read_line` is to take whatever the user types into
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standard input and append that into a string (without overwriting its
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contents), so we therefore pass that string as an argument. The string argument
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needs to be mutable so the method can change the string’s content.
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The `&` indicates that this argument is a *reference*, which gives you a way to
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let multiple parts of your code access one piece of data without needing to
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copy that data into memory multiple times. References are a complex feature,
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and one of Rust’s major advantages is how safe and easy it is to use
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references. You don’t need to know a lot of those details to finish this
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program. For now, all you need to know is that, like variables, references are
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immutable by default. Hence, you need to write `&mut guess` rather than
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`&guess` to make it mutable. (Chapter 4 will explain references more
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thoroughly.)
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### Handling Potential Failure with Result
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We’re still working on this line of code. We’re now discussing a third line of
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text, but note that it’s still part of a single logical line of code. The next
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part is this method:
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```
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.expect("Failed to read line");
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```
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We could have written this code as:
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```
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io::stdin().read_line(&mut guess).expect("Failed to read line");
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```
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However, one long line is difficult to read, so it’s best to divide it. It’s
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often wise to introduce a newline and other whitespace to help break up long
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lines when you call a method with the `.method_name()` syntax. Now let’s
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discuss what this line does.
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As mentioned earlier, `read_line` puts whatever the user enters into the string
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we pass to it, but it also returns a `Result` value. `Result` is an
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*enumeration*, often called an *enum*, which is a type that can be in one of
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multiple possible states. We call each possible state a *variant*.
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Chapter 6 will cover enums in more detail. The purpose of these `Result` types
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is to encode error-handling information.
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`Result`’s variants are `Ok` and `Err`. The `Ok` variant indicates the
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operation was successful, and inside `Ok` is the successfully generated value.
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The `Err` variant means the operation failed, and `Err` contains information
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about how or why the operation failed.
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Values of the `Result` type, like values of any type, have methods defined on
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them. An instance of `Result` has an `expect` method that you can call. If this
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instance of `Result` is an `Err` value, `expect` will cause the program to
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crash and display the message that you passed as an argument to `expect`. If
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the `read_line` method returns an `Err`, it would likely be the result of an
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error coming from the underlying operating system. If this instance of `Result`
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is an `Ok` value, `expect` will take the return value that `Ok` is holding and
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return just that value to you so you can use it. In this case, that value is
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the number of bytes in the user’s input.
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If you don’t call `expect`, the program will compile, but you’ll get a warning:
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```
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$ cargo build
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Compiling guessing_game v0.1.0 (file:///projects/guessing_game)
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warning: unused `Result` that must be used
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--> src/main.rs:10:5
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|
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10 | io::stdin().read_line(&mut guess);
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| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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|
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= note: `#[warn(unused_must_use)]` on by default
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= note: this `Result` may be an `Err` variant, which should be handled
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warning: `guessing_game` (bin "guessing_game") generated 1 warning
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Finished dev [unoptimized + debuginfo] target(s) in 0.59s
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```
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Rust warns that you haven’t used the `Result` value returned from `read_line`,
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indicating that the program hasn’t handled a possible error.
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The right way to suppress the warning is to actually write error-handling code,
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but in our case we just want to crash this program when a problem occurs, so we
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can use `expect`. You’ll learn about recovering from errors in Chapter 9.
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### Printing Values with println! Placeholders
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Aside from the closing curly bracket, there’s only one more line to discuss in
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the code so far:
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```
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println!("You guessed: {guess}");
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```
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This line prints the string that now contains the user’s input. The `{}` set of
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curly brackets is a placeholder: think of `{}` as little crab pincers that hold
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a value in place. When printing the value of a variable, the variable name can
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go inside the curly brackets. When printing the result of evaluating an
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expression, place empty curly brackets in the format string, then follow the
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format string with a comma-separated list of expressions to print in each empty
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curly bracket placeholder in the same order. Printing a variable and the result
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of an expression in one call to `println!` would look like this:
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```
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let x = 5;
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let y = 10;
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println!("x = {x} and y + 2 = {}", y + 2);
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```
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This code would print `x = 5 and y = 12`.
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### Testing the First Part
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Let’s test the first part of the guessing game. Run it using `cargo run`:
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```
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$ cargo run
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Compiling guessing_game v0.1.0 (file:///projects/guessing_game)
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Finished dev [unoptimized + debuginfo] target(s) in 6.44s
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Running `target/debug/guessing_game`
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Guess the number!
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Please input your guess.
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6
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You guessed: 6
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```
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At this point, the first part of the game is done: we’re getting input from the
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keyboard and then printing it.
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## Generating a Secret Number
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Next, we need to generate a secret number that the user will try to guess. The
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secret number should be different every time so the game is fun to play more
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than once. We’ll use a random number between 1 and 100 so the game isn’t too
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difficult. Rust doesn’t yet include random number functionality in its standard
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library. However, the Rust team does provide a `rand` crate at
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*https://crates.io/crates/rand* with said functionality.
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### Using a Crate to Get More Functionality
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Remember that a crate is a collection of Rust source code files. The project
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we’ve been building is a *binary crate*, which is an executable. The `rand`
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crate is a *library crate*, which contains code that is intended to be used in
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other programs and can’t be executed on its own.
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Cargo’s coordination of external crates is where Cargo really shines. Before we
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can write code that uses `rand`, we need to modify the *Cargo.toml* file to
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include the `rand` crate as a dependency. Open that file now and add the
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following line to the bottom, beneath the `[dependencies]` section header that
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Cargo created for you. Be sure to specify `rand` exactly as we have here, with
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this version number, or the code examples in this tutorial may not work:
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Filename: Cargo.toml
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```
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[dependencies]
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rand = "0.8.5"
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```
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In the *Cargo.toml* file, everything that follows a header is part of that
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section that continues until another section starts. In `[dependencies]` you
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tell Cargo which external crates your project depends on and which versions of
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those crates you require. In this case, we specify the `rand` crate with the
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semantic version specifier `0.8.5`. Cargo understands Semantic Versioning
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(sometimes called *SemVer*), which is a standard for writing version numbers.
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The specifier `0.8.5` is actually shorthand for `^0.8.5`, which means any
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version that is at least 0.8.5 but below 0.9.0.
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Cargo considers these versions to have public APIs compatible with version
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0.8.5, and this specification ensures you’ll get the latest patch release that
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will still compile with the code in this chapter. Any version 0.9.0 or greater
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is not guaranteed to have the same API as what the following examples use.
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Now, without changing any of the code, let’s build the project, as shown in
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Listing 2-2.
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```
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$ cargo build
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Updating crates.io index
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Downloaded rand v0.8.5
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Downloaded libc v0.2.127
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Downloaded getrandom v0.2.7
|
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Downloaded cfg-if v1.0.0
|
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Downloaded ppv-lite86 v0.2.16
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Downloaded rand_chacha v0.3.1
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Downloaded rand_core v0.6.3
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Compiling rand_core v0.6.3
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Compiling libc v0.2.127
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Compiling getrandom v0.2.7
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Compiling cfg-if v1.0.0
|
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Compiling ppv-lite86 v0.2.16
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Compiling rand_chacha v0.3.1
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Compiling rand v0.8.5
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Compiling guessing_game v0.1.0 (file:///projects/guessing_game)
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Finished dev [unoptimized + debuginfo] target(s) in 2.53s
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```
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Listing 2-2: The output from running `cargo build` after adding the `rand`
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crate as a dependency
|
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|
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You may see different version numbers (but they will all be compatible with the
|
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code, thanks to SemVer!) and different lines (depending on the operating
|
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system), and the lines may be in a different order.
|
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|
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When we include an external dependency, Cargo fetches the latest versions of
|
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everything that dependency needs from the *registry*, which is a copy of data
|
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from Crates.io at *https://crates.io*. Crates.io is where people in the Rust
|
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ecosystem post their open source Rust projects for others to use.
|
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|
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After updating the registry, Cargo checks the `[dependencies]` section and
|
||
downloads any crates listed that aren’t already downloaded. In this case,
|
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although we only listed `rand` as a dependency, Cargo also grabbed other crates
|
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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
|
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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
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do, it simply exits.
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||
|
||
If you open the *src/main.rs* file, make a trivial change, and then save it and
|
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build again, you’ll only see two lines of output:
|
||
|
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```
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$ cargo build
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||
Compiling guessing_game v0.1.0 (file:///projects/guessing_game)
|
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Finished dev [unoptimized + debuginfo] target(s) in 2.53 secs
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```
|
||
|
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These lines show that Cargo only updates the build with your tiny change to the
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*src/main.rs* file. Your dependencies haven’t changed, so Cargo knows it can
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reuse what it has already downloaded and compiled for those.
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||
#### 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, say that
|
||
next week version 0.8.6 of the `rand` crate comes out, and that version
|
||
contains an important bug fix, but it also contains a regression that will
|
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break your code. To handle this, Rust creates the *Cargo.lock* file the first
|
||
time you run `cargo build`, so we now have this in the *guessing_game*
|
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directory.
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|
||
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 will 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 0.8.5 until you explicitly upgrade, thanks to the *Cargo.lock* file.
|
||
Because the *Cargo.lock* file is important for reproducible builds, it’s often
|
||
checked into source control with the rest of the code in your project.
|
||
|
||
#### Updating a Crate to Get a New Version
|
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|
||
When you *do* want to update a crate, Cargo provides the command `update`,
|
||
which will ignore the *Cargo.lock* file and figure out all the latest versions
|
||
that fit your specifications in *Cargo.toml*. Cargo will then write those
|
||
versions to the *Cargo.lock* file. Otherwise, by default, Cargo will only look
|
||
for versions greater than 0.8.5 and less than 0.9.0. If the `rand` crate has
|
||
released the two new versions 0.8.6 and 0.9.0, you would see the following if
|
||
you ran `cargo update`:
|
||
|
||
```
|
||
$ cargo update
|
||
Updating crates.io index
|
||
Updating rand v0.8.5 -> v0.8.6
|
||
```
|
||
|
||
Cargo ignores the 0.9.0 release. 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 0.8.6. To use `rand` version 0.9.0 or any version in the 0.9.*x*
|
||
series, you’d have to update the *Cargo.toml* file to look like this instead:
|
||
|
||
```
|
||
[dependencies]
|
||
rand = "0.9.0"
|
||
```
|
||
|
||
The next time you run `cargo build`, Cargo will update the registry of crates
|
||
available and reevaluate your `rand` requirements according to the new version
|
||
you have specified.
|
||
|
||
There’s a lot more to say about Cargo and its ecosystem, which we’ll discuss in
|
||
Chapter 14, but for now, that’s all you need to know. Cargo makes it very easy
|
||
to reuse libraries, so Rustaceans are able to write smaller projects that are
|
||
assembled from a number of packages.
|
||
|
||
### Generating a Random Number
|
||
|
||
Let’s start using `rand` to generate a number to guess. The next step is to
|
||
update *src/main.rs*, as shown in Listing 2-3.
|
||
|
||
Filename: src/main.rs
|
||
|
||
```
|
||
use std::io;
|
||
1 use rand::Rng;
|
||
|
||
fn main() {
|
||
println!("Guess the number!");
|
||
|
||
2 let secret_number = rand::thread_rng().gen_range(1..=100);
|
||
|
||
3 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}");
|
||
}
|
||
```
|
||
|
||
Listing 2-3: Adding code to generate a random number
|
||
|
||
First we add the line `use rand::Rng;` [1]. 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. In the first line [2], we call the
|
||
`rand::thread_rng` function that gives us the particular random number
|
||
generator we’re going to use: one that is local to the current thread of
|
||
execution and is 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` and is inclusive on the lower and upper bounds, so we
|
||
need to specify `1..=100` 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, so each crate has documentation with instructions for
|
||
using it. Another neat feature of Cargo is that running the `cargo doc --open`
|
||
command will build documentation provided by all 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 new line [3] prints the secret number. This is useful while we’re
|
||
developing the program to be able to test it, but we’ll delete it from the
|
||
final version. It’s not much of a game if the program prints the answer as soon
|
||
as it starts!
|
||
|
||
Try running the program a few times:
|
||
|
||
```
|
||
$ cargo run
|
||
Compiling guessing_game v0.1.0 (file:///projects/guessing_game)
|
||
Finished dev [unoptimized + debuginfo] target(s) in 2.53s
|
||
Running `target/debug/guessing_game`
|
||
Guess the number!
|
||
The secret number is: 7
|
||
Please input your guess.
|
||
4
|
||
You guessed: 4
|
||
|
||
$ cargo run
|
||
Finished dev [unoptimized + debuginfo] target(s) in 0.02s
|
||
Running `target/debug/guessing_game`
|
||
Guess the number!
|
||
The secret number is: 83
|
||
Please input your guess.
|
||
5
|
||
You guessed: 5
|
||
```
|
||
|
||
You should get different random numbers, and they should all be numbers between
|
||
1 and 100. Great job!
|
||
|
||
## Comparing the Guess to the Secret Number
|
||
|
||
Now that we have user input and a random number, we can compare them. That step
|
||
is shown in Listing 2-4. Note that this code won’t compile just yet, as we will
|
||
explain.
|
||
|
||
Filename: src/main.rs
|
||
|
||
```
|
||
use rand::Rng;
|
||
1 use std::cmp::Ordering;
|
||
use std::io;
|
||
|
||
fn main() {
|
||
--snip--
|
||
|
||
println!("You guessed: {guess}");
|
||
|
||
2 match guess.3 cmp(&secret_number) {
|
||
Ordering::Less => println!("Too small!"),
|
||
Ordering::Greater => println!("Too big!"),
|
||
Ordering::Equal => println!("You win!"),
|
||
}
|
||
}
|
||
```
|
||
|
||
Listing 2-4: Handling the possible return values of comparing two numbers
|
||
|
||
First we add another `use` statement [1], bringing a type called
|
||
`std::cmp::Ordering` into scope from the standard library. The `Ordering` type
|
||
is another enum and has the variants `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 [3] 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 `guess` to `secret_number`. Then it returns a variant of the
|
||
`Ordering` enum we brought into scope with the `use` statement. We use a
|
||
`match` expression [2] to decide what to do next based on which variant of
|
||
`Ordering` was returned from the call to `cmp` with the values in `guess` and
|
||
`secret_number`.
|
||
|
||
A `match` expression is made up of *arms*. An arm consists of a *pattern* to
|
||
match against, and the code that should be run if the value given to `match`
|
||
fits that arm’s pattern. Rust takes the value given to `match` and looks
|
||
through each arm’s pattern in turn. Patterns and the `match` construct are
|
||
powerful Rust features: they let you express a variety of situations your code
|
||
might encounter and they make sure you handle them all. These features will be
|
||
covered in detail in Chapter 6 and Chapter 18, respectively.
|
||
|
||
Let’s walk through an example with the `match` expression we use 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 is
|
||
`Ordering::Greater`, which *does* match `Ordering::Greater`! The associated
|
||
code in that arm will execute and print `Too big!` to the screen. The `match`
|
||
expression ends after the first successful match, so it won’t look at the last
|
||
arm in this scenario.
|
||
|
||
However, the code in Listing 2-4 won’t compile yet. Let’s try it:
|
||
|
||
```
|
||
$ cargo build
|
||
Compiling guessing_game v0.1.0 (file:///projects/guessing_game)
|
||
error[E0308]: mismatched types
|
||
--> src/main.rs:22:21
|
||
|
|
||
22 | match guess.cmp(&secret_number) {
|
||
| ^^^^^^^^^^^^^^ expected struct `String`, found integer
|
||
|
|
||
= note: expected reference `&String`
|
||
found reference `&{integer}`
|
||
```
|
||
|
||
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 of Rust’s 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. Unless otherwise specified, 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 do
|
||
so by adding this line to the `main` function body:
|
||
|
||
Filename: src/main.rs
|
||
|
||
```
|
||
--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!"),
|
||
}
|
||
```
|
||
|
||
We create a variable named `guess`. But wait, doesn’t the program already have
|
||
a variable named `guess`? It does, but helpfully Rust allows us to shadow the
|
||
previous value of `guess` with a new one. *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. We’ll cover this in more detail in
|
||
Chapter 3, but for now, know that this feature is often used when you want to
|
||
convert a value from one type to another type.
|
||
|
||
We bind this new variable to the expression `guess.trim().parse()`. The `guess`
|
||
in the expression refers to the original `guess` variable that contained the
|
||
input as a string. The `trim` method on a `String` instance will eliminate any
|
||
whitespace at the beginning and end, which we must do to be able to compare the
|
||
string to the `u32`, which can only contain numerical data. The user must press
|
||
enter to satisfy `read_line` and input their guess, which adds a newline
|
||
character to the string. For example, if the user types `5` and presses enter,
|
||
`guess` looks like this: `5\n`. The `\n` represents “newline.” (On Windows,
|
||
pressing enter results in a carriage return and a newline, `\r\n`.) The `trim`
|
||
method eliminates `\n` or `\r\n`, resulting in just `5`.
|
||
|
||
The `parse` method on strings converts a string to another type. Here, we use
|
||
it to convert from a string to a number. 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 Rust will infer that `secret_number` should be a
|
||
`u32` as well. So now the comparison will be between two values of the same
|
||
type!
|
||
|
||
The `parse` method will only work on characters that can logically be converted
|
||
into numbers and so can easily cause errors. 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
|
||
Result” on page XX). We’ll treat this `Result` the same way by using the
|
||
`expect` method again. If `parse` returns an `Err` `Result` variant because it
|
||
couldn’t create a number from the string, the `expect` call will crash the game
|
||
and print the message we give it. If `parse` can successfully convert the
|
||
string to a number, it will return the `Ok` variant of `Result`, and `expect`
|
||
will return the number that we want from the `Ok` value.
|
||
|
||
Let’s run the program now:
|
||
|
||
```
|
||
$ cargo run
|
||
Compiling guessing_game v0.1.0 (file:///projects/guessing_game)
|
||
Finished dev [unoptimized + debuginfo] target(s) in 0.43s
|
||
Running `target/debug/guessing_game`
|
||
Guess the number!
|
||
The secret number is: 58
|
||
Please input your guess.
|
||
76
|
||
You guessed: 76
|
||
Too big!
|
||
```
|
||
|
||
Nice! Even though spaces were added before the guess, the program still figured
|
||
out that the user guessed 76. Run the program a few times to verify the
|
||
different behavior with different kinds of input: guess the number correctly,
|
||
guess a number that is too high, and guess a number that is too low.
|
||
|
||
We have most of the game working now, but the user can make only one guess.
|
||
Let’s change that by adding a loop!
|
||
|
||
## Allowing Multiple Guesses with Looping
|
||
|
||
The `loop` keyword creates an infinite loop. We’ll add a loop to give users
|
||
more chances at guessing the number:
|
||
|
||
Filename: src/main.rs
|
||
|
||
```
|
||
--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 from the guess input prompt onward into
|
||
a loop. Be sure to indent the lines inside the loop another four spaces each
|
||
and run the program again. The program will now ask for another guess forever,
|
||
which actually introduces a new problem. It doesn’t seem like the user can quit!
|
||
|
||
The user could always interrupt the program by using the keyboard shortcut
|
||
ctrl-C. But there’s another way to escape this insatiable monster, as mentioned
|
||
in the `parse` discussion in “Comparing the Guess to the Secret Number” on page
|
||
XX: if the user enters a non-number answer, the program will crash. We can take
|
||
advantage of that to allow the user to quit, as shown here:
|
||
|
||
```
|
||
$ cargo run
|
||
Compiling guessing_game v0.1.0 (file:///projects/guessing_game)
|
||
Finished dev [unoptimized + debuginfo] target(s) in 1.50s
|
||
Running `target/debug/guessing_game`
|
||
Guess the number!
|
||
The secret number is: 59
|
||
Please input your guess.
|
||
45
|
||
You guessed: 45
|
||
Too small!
|
||
Please input your guess.
|
||
60
|
||
You guessed: 60
|
||
Too big!
|
||
Please input your guess.
|
||
59
|
||
You guessed: 59
|
||
You win!
|
||
Please input your guess.
|
||
quit
|
||
thread 'main' panicked at 'Please type a number!: ParseIntError
|
||
{ kind: InvalidDigit }', src/main.rs:28:47
|
||
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
|
||
```
|
||
|
||
Typing `quit` will quit the game, but as you’ll notice, so will entering any
|
||
other non-number input. This is suboptimal, to say the least; we want the game
|
||
to also stop when the correct number is guessed.
|
||
|
||
### Quitting After a Correct Guess
|
||
|
||
Let’s program the game to quit when the user wins by adding a `break` statement:
|
||
|
||
Filename: src/main.rs
|
||
|
||
```
|
||
--snip--
|
||
|
||
match guess.cmp(&secret_number) {
|
||
Ordering::Less => println!("Too small!"),
|
||
Ordering::Greater => println!("Too big!"),
|
||
Ordering::Equal => {
|
||
println!("You win!");
|
||
break;
|
||
}
|
||
}
|
||
```
|
||
|
||
Adding the `break` line after `You win!` makes the program exit the loop when
|
||
the user guesses the secret number correctly. Exiting the loop also means
|
||
exiting the program, because the loop is the last part of `main`.
|
||
|
||
### Handling Invalid Input
|
||
|
||
To further refine the game’s behavior, rather than crashing the program when
|
||
the user inputs a non-number, let’s make the game ignore a non-number so the
|
||
user can continue guessing. We can do that by altering the line where `guess`
|
||
is converted from a `String` to a `u32`, as shown in Listing 2-5.
|
||
|
||
Filename: src/main.rs
|
||
|
||
```
|
||
--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--
|
||
```
|
||
|
||
Listing 2-5: Ignoring a non-number guess and asking for another guess instead
|
||
of crashing the program
|
||
|
||
We switch from an `expect` call to a `match` expression to 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` and `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 resultant number. That `Ok` value will
|
||
match the first arm’s pattern, and the `match` expression will just return the
|
||
`num` value that `parse` produced and put inside the `Ok` value. That number
|
||
will end up right where we want it in the new `guess` variable we’re creating.
|
||
|
||
If `parse` is *not* able to turn the string into a number, it will return an
|
||
`Err` value that contains more information about the error. The `Err` value
|
||
does not match the `Ok(num)` pattern in the first `match` arm, but it does
|
||
match the `Err(_)` pattern in the second arm. The underscore, `_`, is a
|
||
catchall value; in this example, we’re saying we want to match all `Err`
|
||
values, no matter what information they have inside them. So the program will
|
||
execute the second arm’s code, `continue`, which tells the program to go to the
|
||
next iteration of the `loop` and ask for another guess. So, effectively, the
|
||
program ignores all errors that `parse` might encounter!
|
||
|
||
Now everything in the program should work as expected. Let’s try it:
|
||
|
||
```
|
||
$ cargo run
|
||
Compiling guessing_game v0.1.0 (file:///projects/guessing_game)
|
||
Finished dev [unoptimized + debuginfo] target(s) in 4.45s
|
||
Running `target/debug/guessing_game`
|
||
Guess the number!
|
||
The secret number is: 61
|
||
Please input your guess.
|
||
10
|
||
You guessed: 10
|
||
Too small!
|
||
Please input your guess.
|
||
99
|
||
You guessed: 99
|
||
Too big!
|
||
Please input your guess.
|
||
foo
|
||
Please input your guess.
|
||
61
|
||
You guessed: 61
|
||
You win!
|
||
```
|
||
|
||
Awesome! With one tiny final tweak, we will finish the guessing game. Recall
|
||
that the program is still printing the secret number. That worked well for
|
||
testing, but it ruins the game. Let’s delete the `println!` that outputs the
|
||
secret number. Listing 2-6 shows the final code.
|
||
|
||
Filename: src/main.rs
|
||
|
||
```
|
||
use rand::Rng;
|
||
use std::cmp::Ordering;
|
||
use std::io;
|
||
|
||
fn main() {
|
||
println!("Guess the number!");
|
||
|
||
let secret_number = rand::thread_rng().gen_range(1..=100);
|
||
|
||
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;
|
||
}
|
||
}
|
||
}
|
||
}
|
||
```
|
||
|
||
Listing 2-6: Complete guessing game code
|
||
|
||
At this point, you’ve successfully built the guessing game. Congratulations!
|
||
|
||
## Summary
|
||
|
||
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.
|
||
|