mirror of https://github.com/rust-lang/book
1660 lines
56 KiB
Markdown
1660 lines
56 KiB
Markdown
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[TOC]
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# Common Programming Concepts
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This chapter covers concepts that appear in almost every programming language
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and how they work in Rust. Many programming languages have much in common at
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their core. None of the concepts presented in this chapter are unique to Rust,
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but we’ll discuss them in the context of Rust and explain the conventions
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around using these concepts.
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Specifically, you’ll learn about variables, basic types, functions, comments,
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and control flow. These foundations will be in every Rust program, and learning
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them early will give you a strong core to start from.
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> #### Keywords
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>
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> The Rust language has a set of *keywords* that are reserved for use by
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> the language only, much as in other languages. Keep in mind that you cannot
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> use these words as names of variables or functions. Most of the keywords have
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> special meanings, and you’ll be using them to do various tasks in your Rust
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> programs; a few have no current functionality associated with them but have
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> been reserved for functionality that might be added to Rust in the future. You
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> can find a list of the keywords in Appendix A.
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## Variables and Mutability
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As mentioned in the “Storing Values with Variables” section, by default
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variables are immutable. This is one of many nudges Rust gives you to write
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your code in a way that takes advantage of the safety and easy concurrency that
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Rust offers. However, you still have the option to make your variables mutable.
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Let’s explore how and why Rust encourages you to favor immutability and why
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sometimes you might want to opt out.
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When a variable is immutable, once a value is bound to a name, you can’t change
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that value. To illustrate this, let’s generate a new project called *variables*
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in your *projects* directory by using `cargo new variables`.
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Then, in your new *variables* directory, open *src/main.rs* and replace its
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code with the following code that won’t compile just yet:
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Filename: src/main.rs
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```
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fn main() {
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let x = 5;
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println!("The value of x is: {}", x);
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x = 6;
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println!("The value of x is: {}", x);
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}
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```
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Save and run the program using `cargo run`. You should receive an error
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message, as shown in this output:
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```
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$ cargo run
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Compiling variables v0.1.0 (file:///projects/variables)
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error[E0384]: cannot assign twice to immutable variable `x`
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--> src/main.rs:4:5
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2 | let x = 5;
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| -
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| |
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| first assignment to `x`
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| help: consider making this binding mutable: `mut x`
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3 | println!("The value of x is: {}", x);
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4 | x = 6;
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| ^^^^^ cannot assign twice to immutable variable
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```
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This example shows how the compiler helps you find errors in your programs.
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Even though compiler errors can be frustrating, they only mean your program
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isn’t safely doing what you want it to do yet; they do *not* mean that you’re
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not a good programmer! Experienced Rustaceans still get compiler errors.
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The error message indicates that the cause of the error is that you `` cannot
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assign twice to immutable variable `x` ``, because you tried to assign a second
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value to the immutable `x` variable.
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It’s important that we get compile-time errors when we attempt to change a
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value that we previously designated as immutable because this very situation
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can lead to bugs. If one part of our code operates on the assumption that a
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value will never change and another part of our code changes that value, it’s
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possible that the first part of the code won’t do what it was designed to do.
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The cause of this kind of bug can be difficult to track down after the fact,
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especially when the second piece of code changes the value only *sometimes*.
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In Rust, the compiler guarantees that when you state that a value won’t change,
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it really won’t change. That means that when you’re reading and writing code,
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you don’t have to keep track of how and where a value might change. Your code
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is thus easier to reason through.
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But mutability can be very useful. Variables are immutable only by default; as
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you did in Chapter 2, you can make them mutable by adding `mut` in front of the
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variable name. In addition to allowing this value to change, `mut` conveys
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intent to future readers of the code by indicating that other parts of the code
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will be changing this variable’s value.
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For example, let’s change *src/main.rs* to the following:
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Filename: src/main.rs
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```
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fn main() {
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let mut x = 5;
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println!("The value of x is: {}", x);
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x = 6;
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println!("The value of x is: {}", x);
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}
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```
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When we run the program now, we get this:
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```
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$ cargo run
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Compiling variables v0.1.0 (file:///projects/variables)
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Finished dev [unoptimized + debuginfo] target(s) in 0.30s
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Running `target/debug/variables`
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The value of x is: 5
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The value of x is: 6
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```
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We’re allowed to change the value that `x` binds to from `5` to `6` when `mut`
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is used. In some cases, you’ll want to make a variable mutable because it makes
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the code more convenient to write than if it had only immutable variables.
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There are multiple trade-offs to consider in addition to the prevention of
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bugs. For example, in cases where you’re using large data structures, mutating
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an instance in place may be faster than copying and returning newly allocated
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instances. With smaller data structures, creating new instances and writing in
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a more functional programming style may be easier to think through, so lower
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performance might be a worthwhile penalty for gaining that clarity.
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### Differences Between Variables and Constants
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Being unable to change the value of a variable might have reminded you of
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another programming concept that most other languages have: *constants*. Like
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immutable variables, constants are values that are bound to a name and are not
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allowed to change, but there are a few differences between constants and
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variables.
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First, you aren’t allowed to use `mut` with constants. Constants aren’t just
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immutable by default—they’re always immutable.
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You declare constants using the `const` keyword instead of the `let` keyword,
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and the type of the value *must* be annotated. We’re about to cover types and
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type annotations in the next section, “Data Types,” so don’t worry about the
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details right now. Just know that you must always annotate the type.
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Constants can be declared in any scope, including the global scope, which makes
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them useful for values that many parts of code need to know about.
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The last difference is that constants may be set only to a constant expression,
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not the result of a value that could only be computed at runtime.
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Here’s an example of a constant declaration where the constant’s name is
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`THREE_HOURS_IN_SECONDS` and its value is set to the result of multiplying 60
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(the number of seconds in a minute) by 60 (the number of minutes in an hour) by
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3 (the number of hours we want to count in this program):
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```
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const THREE_HOURS_IN_SECONDS: u32 = 60 * 60 * 3;
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```
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Rust’s naming convention for constants is to use all uppercase with underscores
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between words. The compiler is able to evaluate a limited set of operations at
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compile time, which lets us choose to write out this value in a way that’s
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easier to understand and verify, rather than setting this constant to the value
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10,800. See the Rust Reference’s section on constant evaluation at
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*https://doc.rust-lang.org/reference/const_eval.html* for more information on
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what operations can be used when declaring constants.
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Constants are valid for the entire time a program runs, within the scope they
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were declared in. This property makes constants useful for values in your
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application domain that multiple parts of the program might need to know about,
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such as the maximum number of points any player of a game is allowed to earn or
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the speed of light.
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Naming hardcoded values used throughout your program as constants is useful in
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conveying the meaning of that value to future maintainers of the code. It also
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helps to have only one place in your code you would need to change if the
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hardcoded value needed to be updated in the future.
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### Shadowing
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As you saw in the guessing game tutorial in the “Comparing the Guess to the
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Secret Number” section in Chapter 2, you can declare a new variable with the
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same name as a previous variable. Rustaceans say that the first variable is
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*shadowed* by the second, which means that the second variable’s value is what
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the program sees when the variable is used. We can shadow a variable by using
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the same variable’s name and repeating the use of the `let` keyword as follows:
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Filename: src/main.rs
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```
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fn main() {
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let x = 5;
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let x = x + 1;
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{
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let x = x * 2;
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println!("The value of x in the inner scope is: {}", x);
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}
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println!("The value of x is: {}", x);
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}
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```
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This program first binds `x` to a value of `5`. Then it shadows `x` by
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repeating `let x =`, taking the original value and adding `1` so the value of
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`x` is then `6`. Then, within an inner scope, the third `let` statement also
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shadows `x`, multiplying the previous value by `2` to give `x` a value of `12`.
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When that scope is over, the inner shadowing ends and `x` returns to being `6`.
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When we run this program, it will output the following:
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```
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$ cargo run
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Compiling variables v0.1.0 (file:///projects/variables)
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Finished dev [unoptimized + debuginfo] target(s) in 0.31s
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Running `target/debug/variables`
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The value of x in the inner scope is: 12
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The value of x is: 6
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```
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Shadowing is different from marking a variable as `mut`, because we’ll get a
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compile-time error if we accidentally try to reassign to this variable without
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using the `let` keyword. By using `let`, we can perform a few transformations
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on a value but have the variable be immutable after those transformations have
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been completed.
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The other difference between `mut` and shadowing is that because we’re
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effectively creating a new variable when we use the `let` keyword again, we can
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change the type of the value but reuse the same name. For example, say our
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program asks a user to show how many spaces they want between some text by
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inputting space characters, but we really want to store that input as a number:
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```
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let spaces = " ";
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let spaces = spaces.len();
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```
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This construct is allowed because the first `spaces` variable is a string type
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and the second `spaces` variable, which is a brand-new variable that happens to
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have the same name as the first one, is a number type. Shadowing thus spares us
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from having to come up with different names, such as `spaces_str` and
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`spaces_num`; instead, we can reuse the simpler `spaces` name. However, if we
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try to use `mut` for this, as shown here, we’ll get a compile-time error:
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```
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let mut spaces = " ";
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spaces = spaces.len();
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```
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The error says we’re not allowed to mutate a variable’s type:
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```
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$ cargo run
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Compiling variables v0.1.0 (file:///projects/variables)
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error[E0308]: mismatched types
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--> src/main.rs:3:14
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3 | spaces = spaces.len();
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| ^^^^^^^^^^^^ expected `&str`, found `usize`
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```
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Now that we’ve explored how variables work, let’s look at more data types they
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can have.
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## Data Types
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Every value in Rust is of a certain *data type*, which tells Rust what kind of
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data is being specified so it knows how to work with that data. We’ll look at
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two data type subsets: scalar and compound.
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Keep in mind that Rust is a *statically typed* language, which means that it
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must know the types of all variables at compile time. The compiler can usually
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infer what type we want to use based on the value and how we use it. In cases
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when many types are possible, such as when we converted a `String` to a numeric
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type using `parse` in the “Comparing the Guess to the Secret Number” section in
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Chapter 2, we must add a type annotation, like this:
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```
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let guess: u32 = "42".parse().expect("Not a number!");
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```
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If we don’t add the type annotation here, Rust will display the following
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error, which means the compiler needs more information from us to know which
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type we want to use:
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```
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$ cargo build
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Compiling no_type_annotations v0.1.0 (file:///projects/no_type_annotations)
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error[E0282]: type annotations needed
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--> src/main.rs:2:9
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2 | let guess = "42".parse().expect("Not a number!");
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| ^^^^^ consider giving `guess` a type
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```
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You’ll see different type annotations for other data types.
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### Scalar Types
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A *scalar* type represents a single value. Rust has four primary scalar types:
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integers, floating-point numbers, Booleans, and characters. You may recognize
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these from other programming languages. Let’s jump into how they work in Rust.
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#### Integer Types
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An *integer* is a number without a fractional component. We used one integer
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type in Chapter 2, the `u32` type. This type declaration indicates that the
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value it’s associated with should be an unsigned integer (signed integer types
|
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start with `i`, instead of `u`) that takes up 32 bits of space. Table 3-1 shows
|
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the built-in integer types in Rust. Each variant in the Signed and Unsigned
|
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columns (for example, `i16`) can be used to declare the type of an integer
|
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value.
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Table 3-1: Integer Types in Rust
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| Length | Signed | Unsigned |
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|---------|---------|----------|
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| 8-bit | `i8` | `u8` |
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| 16-bit | `i16` | `u16` |
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| 32-bit | `i32` | `u32` |
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| 64-bit | `i64` | `u64` |
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| 128-bit | `i128` | `u128` |
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| arch | `isize` | `usize` |
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Each variant can be either signed or unsigned and has an explicit size.
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*Signed* and *unsigned* refer to whether it’s possible for the number to be
|
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negative—in other words, whether the number needs to have a sign with it
|
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(signed) or whether it will only ever be positive and can therefore be
|
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represented without a sign (unsigned). It’s like writing numbers on paper: when
|
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the sign matters, a number is shown with a plus sign or a minus sign; however,
|
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when it’s safe to assume the number is positive, it’s shown with no sign.
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|
Signed numbers are stored using two’s complement representation.
|
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|
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|
Each signed variant can store numbers from -(2<sup>n - 1</sup>) to 2<sup>n -
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1</sup> - 1 inclusive, where *n* is the number of bits that variant uses. So an
|
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`i8` can store numbers from -(2<sup>7</sup>) to 2<sup>7</sup> - 1, which equals
|
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-128 to 127. Unsigned variants can store numbers from 0 to 2<sup>n</sup> - 1,
|
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so a `u8` can store numbers from 0 to 2<sup>8</sup> - 1, which equals 0 to 255.
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|
Additionally, the `isize` and `usize` types depend on the kind of computer your
|
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program is running on: 64 bits if you’re on a 64-bit architecture and 32 bits
|
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|
if you’re on a 32-bit architecture.
|
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|
|
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|
You can write integer literals in any of the forms shown in Table 3-2. Note
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that number literals that can be multiple numeric types allow a type suffix,
|
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|
such as `57u8`, to designate the type. Number literals can also use `_` as a
|
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visual separator to make the number easier to read, such as `1_000`, which will
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have the same value as if you had specified `1000`.
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Table 3-2: Integer Literals in Rust
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| Number literals | Example |
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|------------------|---------------|
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| Decimal | `98_222` |
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| Hex | `0xff` |
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| Octal | `0o77` |
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| Binary | `0b1111_0000` |
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| Byte (`u8` only) | `b'A'` |
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|
|
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|
So how do you know which type of integer to use? If you’re unsure, Rust’s
|
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|
defaults are generally good places to start: integer types default to `i32`.
|
|||
|
The primary situation in which you’d use `isize` or `usize` is when indexing
|
|||
|
some sort of collection.
|
|||
|
|
|||
|
> ##### Integer Overflow
|
|||
|
>
|
|||
|
> Let’s say you have a variable of type `u8` that can hold values between 0 and 255.
|
|||
|
> If you try to change the variable to a value outside of that range, such
|
|||
|
> as 256, *integer overflow* will occur. Rust has some interesting rules
|
|||
|
> involving this behavior. When you’re compiling in debug mode, Rust includes
|
|||
|
> checks for integer overflow that cause your program to *panic* at runtime if
|
|||
|
> this behavior occurs. Rust uses the term panicking when a program exits with
|
|||
|
> an error; we’ll discuss panics in more depth in the “Unrecoverable Errors
|
|||
|
> with `panic!`” section in
|
|||
|
> Chapter 9.
|
|||
|
>
|
|||
|
> When you’re compiling in release mode with the `--release` flag, Rust does
|
|||
|
> *not* include checks for integer overflow that cause panics. Instead, if
|
|||
|
> overflow occurs, Rust performs *two’s complement wrapping*. In short, values
|
|||
|
> greater than the maximum value the type can hold “wrap around” to the minimum
|
|||
|
> of the values the type can hold. In the case of a `u8`, the value 256 becomes
|
|||
|
> 0, the value 257 becomes 1, and so on. The program won’t panic, but the
|
|||
|
> variable will have a value that probably isn’t what you were expecting it to
|
|||
|
> have. Relying on integer overflow’s wrapping behavior is considered an error.
|
|||
|
>
|
|||
|
> To explicitly handle the possibility of overflow, you can use these families
|
|||
|
> of methods that the standard library provides on primitive numeric types:
|
|||
|
>
|
|||
|
> - Wrap in all modes with the `wrapping_*` methods, such as `wrapping_add`
|
|||
|
> - Return the `None` value if there is overflow with the `checked_*` methods
|
|||
|
> - Return the value and a boolean indicating whether there was overflow with
|
|||
|
> the `overflowing_*` methods
|
|||
|
> - Saturate at the value’s minimum or maximum values with `saturating_*`
|
|||
|
> methods
|
|||
|
|
|||
|
#### Floating-Point Types
|
|||
|
|
|||
|
Rust also has two primitive types for *floating-point numbers*, which are
|
|||
|
numbers with decimal points. Rust’s floating-point types are `f32` and `f64`,
|
|||
|
which are 32 bits and 64 bits in size, respectively. The default type is `f64`
|
|||
|
because on modern CPUs it’s roughly the same speed as `f32` but is capable of
|
|||
|
more precision.
|
|||
|
|
|||
|
Here’s an example that shows floating-point numbers in action:
|
|||
|
|
|||
|
Filename: src/main.rs
|
|||
|
|
|||
|
```
|
|||
|
fn main() {
|
|||
|
let x = 2.0; // f64
|
|||
|
|
|||
|
let y: f32 = 3.0; // f32
|
|||
|
}
|
|||
|
```
|
|||
|
|
|||
|
Floating-point numbers are represented according to the IEEE-754 standard. The
|
|||
|
`f32` type is a single-precision float, and `f64` has double precision.
|
|||
|
|
|||
|
#### Numeric Operations
|
|||
|
|
|||
|
Rust supports the basic mathematical operations you’d expect for all of the
|
|||
|
number types: addition, subtraction, multiplication, division, and remainder.
|
|||
|
Integer division rounds down to the nearest integer. The following code shows
|
|||
|
how you’d use each numeric operation in a `let` statement:
|
|||
|
|
|||
|
Filename: src/main.rs
|
|||
|
|
|||
|
```
|
|||
|
fn main() {
|
|||
|
// addition
|
|||
|
let sum = 5 + 10;
|
|||
|
|
|||
|
// subtraction
|
|||
|
let difference = 95.5 - 4.3;
|
|||
|
|
|||
|
// multiplication
|
|||
|
let product = 4 * 30;
|
|||
|
|
|||
|
// division
|
|||
|
let quotient = 56.7 / 32.2;
|
|||
|
let floored = 2 / 3; // Results in 0
|
|||
|
|
|||
|
// remainder
|
|||
|
let remainder = 43 % 5;
|
|||
|
}
|
|||
|
```
|
|||
|
|
|||
|
Each expression in these statements uses a mathematical operator and evaluates
|
|||
|
to a single value, which is then bound to a variable. Appendix B contains a
|
|||
|
list of all operators that Rust provides.
|
|||
|
|
|||
|
#### The Boolean Type
|
|||
|
|
|||
|
As in most other programming languages, a Boolean type in Rust has two possible
|
|||
|
values: `true` and `false`. Booleans are one byte in size. The Boolean type in
|
|||
|
Rust is specified using `bool`. For example:
|
|||
|
|
|||
|
Filename: src/main.rs
|
|||
|
|
|||
|
```
|
|||
|
fn main() {
|
|||
|
let t = true;
|
|||
|
|
|||
|
let f: bool = false; // with explicit type annotation
|
|||
|
}
|
|||
|
```
|
|||
|
|
|||
|
The main way to use Boolean values is through conditionals, such as an `if`
|
|||
|
expression. We’ll cover how `if` expressions work in Rust in the “Control
|
|||
|
Flow” section.
|
|||
|
|
|||
|
#### The Character Type
|
|||
|
|
|||
|
So far we’ve worked only with numbers, but Rust supports letters too. Rust’s
|
|||
|
`char` type is the language’s most primitive alphabetic type, and the following
|
|||
|
code shows one way to use it. (Note that `char` literals are specified with
|
|||
|
single quotes, as opposed to string literals, which use double quotes.)
|
|||
|
|
|||
|
Filename: src/main.rs
|
|||
|
|
|||
|
```
|
|||
|
fn main() {
|
|||
|
let c = 'z';
|
|||
|
let z = 'ℤ';
|
|||
|
let heart_eyed_cat = '😻';
|
|||
|
}
|
|||
|
```
|
|||
|
|
|||
|
Rust’s `char` type is four bytes in size and represents a Unicode Scalar Value,
|
|||
|
which means it can represent a lot more than just ASCII. Accented letters;
|
|||
|
Chinese, Japanese, and Korean characters; emoji; and zero-width spaces are all
|
|||
|
valid `char` values in Rust. Unicode Scalar Values range from `U+0000` to
|
|||
|
`U+D7FF` and `U+E000` to `U+10FFFF` inclusive. However, a “character” isn’t
|
|||
|
really a concept in Unicode, so your human intuition for what a “character” is
|
|||
|
may not match up with what a `char` is in Rust. We’ll discuss this topic in
|
|||
|
detail in “Storing UTF-8 Encoded Text with Strings” in Chapter 8.
|
|||
|
|
|||
|
### Compound Types
|
|||
|
|
|||
|
*Compound types* can group multiple values into one type. Rust has two
|
|||
|
primitive compound types: tuples and arrays.
|
|||
|
|
|||
|
#### The Tuple Type
|
|||
|
|
|||
|
A tuple is a general way of grouping together a number of values with a variety
|
|||
|
of types into one compound type. Tuples have a fixed length: once declared,
|
|||
|
they cannot grow or shrink in size.
|
|||
|
|
|||
|
We create a tuple by writing a comma-separated list of values inside
|
|||
|
parentheses. Each position in the tuple has a type, and the types of the
|
|||
|
different values in the tuple don’t have to be the same. We’ve added optional
|
|||
|
type annotations in this example:
|
|||
|
|
|||
|
Filename: src/main.rs
|
|||
|
|
|||
|
```
|
|||
|
fn main() {
|
|||
|
let tup: (i32, f64, u8) = (500, 6.4, 1);
|
|||
|
}
|
|||
|
```
|
|||
|
|
|||
|
The variable `tup` binds to the entire tuple, because a tuple is considered a
|
|||
|
single compound element. To get the individual values out of a tuple, we can
|
|||
|
use pattern matching to destructure a tuple value, like this:
|
|||
|
|
|||
|
Filename: src/main.rs
|
|||
|
|
|||
|
```
|
|||
|
fn main() {
|
|||
|
let tup = (500, 6.4, 1);
|
|||
|
|
|||
|
let (x, y, z) = tup;
|
|||
|
|
|||
|
println!("The value of y is: {}", y);
|
|||
|
}
|
|||
|
```
|
|||
|
|
|||
|
This program first creates a tuple and binds it to the variable `tup`. It then
|
|||
|
uses a pattern with `let` to take `tup` and turn it into three separate
|
|||
|
variables, `x`, `y`, and `z`. This is called *destructuring*, because it breaks
|
|||
|
the single tuple into three parts. Finally, the program prints the value of
|
|||
|
`y`, which is `6.4`.
|
|||
|
|
|||
|
In addition to destructuring through pattern matching, we can access a tuple
|
|||
|
element directly by using a period (`.`) followed by the index of the value we
|
|||
|
want to access. For example:
|
|||
|
|
|||
|
Filename: src/main.rs
|
|||
|
|
|||
|
```
|
|||
|
fn main() {
|
|||
|
let x: (i32, f64, u8) = (500, 6.4, 1);
|
|||
|
|
|||
|
let five_hundred = x.0;
|
|||
|
|
|||
|
let six_point_four = x.1;
|
|||
|
|
|||
|
let one = x.2;
|
|||
|
}
|
|||
|
```
|
|||
|
|
|||
|
This program creates a tuple, `x`, and then makes new variables for each
|
|||
|
element by using their respective indices. As with most programming languages,
|
|||
|
the first index in a tuple is 0.
|
|||
|
|
|||
|
The tuple without any values, `()`, is a special type that has only one value,
|
|||
|
also written `()`. The type is called the *unit type* and the value is called
|
|||
|
the *unit value*. Expressions implicitly return the unit value if they don’t
|
|||
|
return any other value.
|
|||
|
|
|||
|
#### The Array Type
|
|||
|
|
|||
|
Another way to have a collection of multiple values is with an *array*. Unlike
|
|||
|
a tuple, every element of an array must have the same type. Arrays in Rust are
|
|||
|
different from arrays in some other languages because arrays in Rust have a
|
|||
|
fixed length, like tuples.
|
|||
|
|
|||
|
In Rust, the values going into an array are written as a comma-separated list
|
|||
|
inside square brackets:
|
|||
|
|
|||
|
Filename: src/main.rs
|
|||
|
|
|||
|
```
|
|||
|
fn main() {
|
|||
|
let a = [1, 2, 3, 4, 5];
|
|||
|
}
|
|||
|
```
|
|||
|
|
|||
|
Arrays are useful when you want your data allocated on the stack rather than
|
|||
|
the heap (we will discuss the stack and the heap more in Chapter 4) or when
|
|||
|
you want to ensure you always have a fixed number of elements. An array isn’t
|
|||
|
as flexible as the vector type, though. A vector is a similar collection type
|
|||
|
provided by the standard library that *is* allowed to grow or shrink in size.
|
|||
|
If you’re unsure whether to use an array or a vector, you should probably use a
|
|||
|
vector. Chapter 8 discusses vectors in more detail.
|
|||
|
|
|||
|
An example of when you might want to use an array rather than a vector is in a
|
|||
|
program that needs to know the names of the months of the year. It’s very
|
|||
|
unlikely that such a program will need to add or remove months, so you can use
|
|||
|
an array because you know it will always contain 12 elements:
|
|||
|
|
|||
|
```
|
|||
|
let months = ["January", "February", "March", "April", "May", "June", "July",
|
|||
|
"August", "September", "October", "November", "December"];
|
|||
|
```
|
|||
|
|
|||
|
You would write an array’s type by using square brackets, and within the
|
|||
|
brackets include the type of each element, a semicolon, and then the number of
|
|||
|
elements in the array, like so:
|
|||
|
|
|||
|
```
|
|||
|
let a: [i32; 5] = [1, 2, 3, 4, 5];
|
|||
|
```
|
|||
|
|
|||
|
Here, `i32` is the type of each element. After the semicolon, the number `5`
|
|||
|
indicates the array contains five elements.
|
|||
|
|
|||
|
Writing an array’s type this way looks similar to an alternative syntax for
|
|||
|
initializing an array: if you want to create an array that contains the same
|
|||
|
value for each element, you can specify the initial value, followed by a
|
|||
|
semicolon, and then the length of the array in square brackets, as shown here:
|
|||
|
|
|||
|
```
|
|||
|
let a = [3; 5];
|
|||
|
```
|
|||
|
|
|||
|
The array named `a` will contain `5` elements that will all be set to the value
|
|||
|
`3` initially. This is the same as writing `let a = [3, 3, 3, 3, 3];` but in a
|
|||
|
more concise way.
|
|||
|
|
|||
|
##### Accessing Array Elements
|
|||
|
|
|||
|
An array is a single chunk of memory of a known, fixed size that can be
|
|||
|
allocated on the stack. You can access elements of an array using indexing,
|
|||
|
like this:
|
|||
|
|
|||
|
Filename: src/main.rs
|
|||
|
|
|||
|
```
|
|||
|
fn main() {
|
|||
|
let a = [1, 2, 3, 4, 5];
|
|||
|
|
|||
|
let first = a[0];
|
|||
|
let second = a[1];
|
|||
|
}
|
|||
|
```
|
|||
|
|
|||
|
In this example, the variable named `first` will get the value `1`, because
|
|||
|
that is the value at index `[0]` in the array. The variable named `second` will
|
|||
|
get the value `2` from index `[1]` in the array.
|
|||
|
|
|||
|
##### Invalid Array Element Access
|
|||
|
|
|||
|
What happens if you try to access an element of an array that is past the end
|
|||
|
of the array? Say you change the example to the following, which uses code
|
|||
|
similar to the guessing game in Chapter 2 to get an array index from the user:
|
|||
|
|
|||
|
Filename: src/main.rs
|
|||
|
|
|||
|
```
|
|||
|
use std::io;
|
|||
|
|
|||
|
fn main() {
|
|||
|
let a = [1, 2, 3, 4, 5];
|
|||
|
|
|||
|
println!("Please enter an array index.");
|
|||
|
|
|||
|
let mut index = String::new();
|
|||
|
|
|||
|
io::stdin()
|
|||
|
.read_line(&mut index)
|
|||
|
.expect("Failed to read line");
|
|||
|
|
|||
|
let index: usize = index
|
|||
|
.trim()
|
|||
|
.parse()
|
|||
|
.expect("Index entered was not a number");
|
|||
|
|
|||
|
let element = a[index];
|
|||
|
|
|||
|
println!(
|
|||
|
"The value of the element at index {} is: {}",
|
|||
|
index, element
|
|||
|
);
|
|||
|
}
|
|||
|
```
|
|||
|
|
|||
|
This code compiles successfully. If you run this code using `cargo run` and
|
|||
|
enter 0, 1, 2, 3, or 4, the program will print out the corresponding value at
|
|||
|
that index in the array. If you instead enter a number past the end of the
|
|||
|
array, such as 10, you’ll see output like this:
|
|||
|
|
|||
|
```
|
|||
|
thread 'main' panicked at 'index out of bounds: the len is 5 but the index is 10', src/main.rs:19:19
|
|||
|
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
|
|||
|
```
|
|||
|
|
|||
|
The program resulted in a *runtime* error at the point of using an invalid
|
|||
|
value in the indexing operation. The program exited with an error message and
|
|||
|
didn’t execute the final `println!` statement. When you attempt to access an
|
|||
|
element using indexing, Rust will check that the index you’ve specified is less
|
|||
|
than the array length. If the index is greater than or equal to the length,
|
|||
|
Rust will panic. This check has to happen at runtime, especially in this case,
|
|||
|
because the compiler can’t possibly know what value a user will enter when they
|
|||
|
run the code later.
|
|||
|
|
|||
|
This is an example of Rust’s memory safety principles in action. In many
|
|||
|
low-level languages, this kind of check is not done, and when you provide an
|
|||
|
incorrect index, invalid memory can be accessed. Rust protects you against this
|
|||
|
kind of error by immediately exiting instead of allowing the memory access and
|
|||
|
continuing. Chapter 9 discusses more of Rust’s error handling.
|
|||
|
|
|||
|
## Functions
|
|||
|
|
|||
|
Functions are prevalent in Rust code. You’ve already seen one of the most
|
|||
|
important functions in the language: the `main` function, which is the entry
|
|||
|
point of many programs. You’ve also seen the `fn` keyword, which allows you to
|
|||
|
declare new functions.
|
|||
|
|
|||
|
Rust code uses *snake case* as the conventional style for function and variable
|
|||
|
names. In snake case, all letters are lowercase and underscores separate words.
|
|||
|
Here’s a program that contains an example function definition:
|
|||
|
|
|||
|
Filename: src/main.rs
|
|||
|
|
|||
|
```
|
|||
|
fn main() {
|
|||
|
println!("Hello, world!");
|
|||
|
|
|||
|
another_function();
|
|||
|
}
|
|||
|
|
|||
|
fn another_function() {
|
|||
|
println!("Another function.");
|
|||
|
}
|
|||
|
```
|
|||
|
|
|||
|
Function definitions in Rust start with `fn` and have a set of parentheses
|
|||
|
after the function name. The curly brackets tell the compiler where the
|
|||
|
function body begins and ends.
|
|||
|
|
|||
|
We can call any function we’ve defined by entering its name followed by a set
|
|||
|
of parentheses. Because `another_function` is defined in the program, it can be
|
|||
|
called from inside the `main` function. Note that we defined `another_function`
|
|||
|
*after* the `main` function in the source code; we could have defined it before
|
|||
|
as well. Rust doesn’t care where you define your functions, only that they’re
|
|||
|
defined somewhere.
|
|||
|
|
|||
|
Let’s start a new binary project named *functions* to explore functions
|
|||
|
further. Place the `another_function` example in *src/main.rs* and run it. You
|
|||
|
should see the following output:
|
|||
|
|
|||
|
```
|
|||
|
$ cargo run
|
|||
|
Compiling functions v0.1.0 (file:///projects/functions)
|
|||
|
Finished dev [unoptimized + debuginfo] target(s) in 0.28s
|
|||
|
Running `target/debug/functions`
|
|||
|
Hello, world!
|
|||
|
Another function.
|
|||
|
```
|
|||
|
|
|||
|
The lines execute in the order in which they appear in the `main` function.
|
|||
|
First, the “Hello, world!” message prints, and then `another_function` is
|
|||
|
called and its message is printed.
|
|||
|
|
|||
|
### Function Parameters
|
|||
|
|
|||
|
Functions can also be defined to have *parameters*, which are special variables
|
|||
|
that are part of a function’s signature. When a function has parameters, you
|
|||
|
can provide it with concrete values for those parameters. Technically, the
|
|||
|
concrete values are called *arguments*, but in casual conversation, people tend
|
|||
|
to use the words *parameter* and *argument* interchangeably for either the
|
|||
|
variables in a function’s definition or the concrete values passed in when you
|
|||
|
call a function.
|
|||
|
|
|||
|
The following rewritten version of `another_function` shows what parameters
|
|||
|
look like in Rust:
|
|||
|
|
|||
|
Filename: src/main.rs
|
|||
|
|
|||
|
```
|
|||
|
fn main() {
|
|||
|
another_function(5);
|
|||
|
}
|
|||
|
|
|||
|
fn another_function(x: i32) {
|
|||
|
println!("The value of x is: {}", x);
|
|||
|
}
|
|||
|
```
|
|||
|
|
|||
|
Try running this program; you should get the following output:
|
|||
|
|
|||
|
```
|
|||
|
$ cargo run
|
|||
|
Compiling functions v0.1.0 (file:///projects/functions)
|
|||
|
Finished dev [unoptimized + debuginfo] target(s) in 1.21s
|
|||
|
Running `target/debug/functions`
|
|||
|
The value of x is: 5
|
|||
|
```
|
|||
|
|
|||
|
The declaration of `another_function` has one parameter named `x`. The type of
|
|||
|
`x` is specified as `i32`. When `5` is passed to `another_function`, the
|
|||
|
`println!` macro puts `5` where the pair of curly brackets were in the format
|
|||
|
string.
|
|||
|
|
|||
|
In function signatures, you *must* declare the type of each parameter. This is
|
|||
|
a deliberate decision in Rust’s design: requiring type annotations in function
|
|||
|
definitions means the compiler almost never needs you to use them elsewhere in
|
|||
|
the code to figure out what type you mean.
|
|||
|
|
|||
|
When you want a function to have multiple parameters, separate the parameter
|
|||
|
declarations with commas, like this:
|
|||
|
|
|||
|
Filename: src/main.rs
|
|||
|
|
|||
|
```
|
|||
|
fn main() {
|
|||
|
print_labeled_measurement(5, 'h');
|
|||
|
}
|
|||
|
|
|||
|
fn print_labeled_measurement(value: i32, unit_label: char) {
|
|||
|
println!("The measurement is: {}{}", value, unit_label);
|
|||
|
}
|
|||
|
```
|
|||
|
|
|||
|
This example creates a function named `print_labeled_measurement` with two
|
|||
|
parameters. The first parameter is named `value` and is an `i32`. The second is
|
|||
|
named `unit_label` and is type `char`. The function then prints text containing
|
|||
|
both the `value` and the `unit_label`.
|
|||
|
|
|||
|
Let’s try running this code. Replace the program currently in your *functions*
|
|||
|
project’s *src/main.rs* file with the preceding example and run it using `cargo
|
|||
|
run`:
|
|||
|
|
|||
|
```
|
|||
|
$ cargo run
|
|||
|
Compiling functions v0.1.0 (file:///projects/functions)
|
|||
|
Finished dev [unoptimized + debuginfo] target(s) in 0.31s
|
|||
|
Running `target/debug/functions`
|
|||
|
The measurement is: 5h
|
|||
|
```
|
|||
|
|
|||
|
Because we called the function with `5` as the value for `value` and `'h'` as
|
|||
|
the value for `unit_label`, the program output contains those values.
|
|||
|
|
|||
|
### Function Bodies Contain Statements and Expressions
|
|||
|
|
|||
|
Function bodies are made up of a series of statements optionally ending in an
|
|||
|
expression. So far, we’ve only covered functions without an ending expression,
|
|||
|
but you have seen an expression as part of a statement. Because Rust is an
|
|||
|
expression-based language, this is an important distinction to understand.
|
|||
|
Other languages don’t have the same distinctions, so let’s look at what
|
|||
|
statements and expressions are and how their differences affect the bodies of
|
|||
|
functions.
|
|||
|
|
|||
|
We’ve actually already used statements and expressions. *Statements* are
|
|||
|
instructions that perform some action and do not return a value. *Expressions*
|
|||
|
evaluate to a resulting value. Let’s look at some examples.
|
|||
|
|
|||
|
Creating a variable and assigning a value to it with the `let` keyword is a
|
|||
|
statement. In Listing 3-1, `let y = 6;` is a statement.
|
|||
|
|
|||
|
Filename: src/main.rs
|
|||
|
|
|||
|
```
|
|||
|
fn main() {
|
|||
|
let y = 6;
|
|||
|
}
|
|||
|
```
|
|||
|
|
|||
|
Listing 3-1: A `main` function declaration containing one statement
|
|||
|
|
|||
|
Function definitions are also statements; the entire preceding example is a
|
|||
|
statement in itself.
|
|||
|
|
|||
|
Statements do not return values. Therefore, you can’t assign a `let` statement
|
|||
|
to another variable, as the following code tries to do; you’ll get an error:
|
|||
|
|
|||
|
Filename: src/main.rs
|
|||
|
|
|||
|
```
|
|||
|
fn main() {
|
|||
|
let x = (let y = 6);
|
|||
|
}
|
|||
|
```
|
|||
|
|
|||
|
When you run this program, the error you’ll get looks like this:
|
|||
|
|
|||
|
```
|
|||
|
$ cargo run
|
|||
|
Compiling functions v0.1.0 (file:///projects/functions)
|
|||
|
error[E0658]: `let` expressions in this position are experimental
|
|||
|
--> src/main.rs:2:14
|
|||
|
|
|
|||
|
2 | let x = (let y = 6);
|
|||
|
| ^^^^^^^^^
|
|||
|
|
|
|||
|
= note: see issue #53667 <https://github.com/rust-lang/rust/issues/53667> for more information
|
|||
|
= help: you can write `matches!(<expr>, <pattern>)` instead of `let <pattern> = <expr>`
|
|||
|
|
|||
|
error: expected expression, found statement (`let`)
|
|||
|
--> src/main.rs:2:14
|
|||
|
|
|
|||
|
2 | let x = (let y = 6);
|
|||
|
| ^^^^^^^^^
|
|||
|
|
|
|||
|
= note: variable declaration using `let` is a statement
|
|||
|
|
|||
|
warning: unnecessary parentheses around assigned value
|
|||
|
--> src/main.rs:2:13
|
|||
|
|
|
|||
|
2 | let x = (let y = 6);
|
|||
|
| ^^^^^^^^^^^ help: remove these parentheses
|
|||
|
|
|
|||
|
= note: `#[warn(unused_parens)]` on by default
|
|||
|
```
|
|||
|
|
|||
|
The `let y = 6` statement does not return a value, so there isn’t anything for
|
|||
|
`x` to bind to. This is different from what happens in other languages, such as
|
|||
|
C and Ruby, where the assignment returns the value of the assignment. In those
|
|||
|
languages, you can write `x = y = 6` and have both `x` and `y` have the value
|
|||
|
`6`; that is not the case in Rust.
|
|||
|
|
|||
|
Expressions evaluate to a value and make up most of the rest of the code that
|
|||
|
you’ll write in Rust. Consider a math operation, such as `5 + 6`, which is an
|
|||
|
expression that evaluates to the value `11`. Expressions can be part of
|
|||
|
statements: in Listing 3-1, the `6` in the statement `let y = 6;` is an
|
|||
|
expression that evaluates to the value `6`. Calling a function is an
|
|||
|
expression. Calling a macro is an expression. The block that we use to create
|
|||
|
new scopes, `{}`, is an expression, for example:
|
|||
|
|
|||
|
Filename: src/main.rs
|
|||
|
|
|||
|
```
|
|||
|
fn main() {
|
|||
|
let x = 5;
|
|||
|
|
|||
|
let y = {
|
|||
|
let x = 3;
|
|||
|
x + 1
|
|||
|
};
|
|||
|
|
|||
|
println!("The value of y is: {}", y);
|
|||
|
}
|
|||
|
```
|
|||
|
|
|||
|
This expression:
|
|||
|
|
|||
|
```
|
|||
|
{
|
|||
|
let x = 3;
|
|||
|
x + 1
|
|||
|
}
|
|||
|
```
|
|||
|
|
|||
|
is a block that, in this case, evaluates to `4`. That value gets bound to `y`
|
|||
|
as part of the `let` statement. Note the `x + 1` line without a semicolon at
|
|||
|
the end, which is unlike most of the lines you’ve seen so far. Expressions do
|
|||
|
not include ending semicolons. If you add a semicolon to the end of an
|
|||
|
expression, you turn it into a statement, which will then not return a value.
|
|||
|
Keep this in mind as you explore function return values and expressions next.
|
|||
|
|
|||
|
### Functions with Return Values
|
|||
|
|
|||
|
Functions can return values to the code that calls them. We don’t name return
|
|||
|
values, but we do declare their type after an arrow (`->`). In Rust, the return
|
|||
|
value of the function is synonymous with the value of the final expression in
|
|||
|
the block of the body of a function. You can return early from a function by
|
|||
|
using the `return` keyword and specifying a value, but most functions return
|
|||
|
the last expression implicitly. Here’s an example of a function that returns a
|
|||
|
value:
|
|||
|
|
|||
|
Filename: src/main.rs
|
|||
|
|
|||
|
```
|
|||
|
fn five() -> i32 {
|
|||
|
5
|
|||
|
}
|
|||
|
|
|||
|
fn main() {
|
|||
|
let x = five();
|
|||
|
|
|||
|
println!("The value of x is: {}", x);
|
|||
|
}
|
|||
|
```
|
|||
|
|
|||
|
There are no function calls, macros, or even `let` statements in the `five`
|
|||
|
function—just the number `5` by itself. That’s a perfectly valid function in
|
|||
|
Rust. Note that the function’s return type is specified too, as `-> i32`. Try
|
|||
|
running this code; the output should look like this:
|
|||
|
|
|||
|
```
|
|||
|
$ cargo run
|
|||
|
Compiling functions v0.1.0 (file:///projects/functions)
|
|||
|
Finished dev [unoptimized + debuginfo] target(s) in 0.30s
|
|||
|
Running `target/debug/functions`
|
|||
|
The value of x is: 5
|
|||
|
```
|
|||
|
|
|||
|
The `5` in `five` is the function’s return value, which is why the return type
|
|||
|
is `i32`. Let’s examine this in more detail. There are two important bits:
|
|||
|
first, the line `let x = five();` shows that we’re using the return value of a
|
|||
|
function to initialize a variable. Because the function `five` returns a `5`,
|
|||
|
that line is the same as the following:
|
|||
|
|
|||
|
```
|
|||
|
let x = 5;
|
|||
|
```
|
|||
|
|
|||
|
Second, the `five` function has no parameters and defines the type of the
|
|||
|
return value, but the body of the function is a lonely `5` with no semicolon
|
|||
|
because it’s an expression whose value we want to return.
|
|||
|
|
|||
|
Let’s look at another example:
|
|||
|
|
|||
|
Filename: src/main.rs
|
|||
|
|
|||
|
```
|
|||
|
fn main() {
|
|||
|
let x = plus_one(5);
|
|||
|
|
|||
|
println!("The value of x is: {}", x);
|
|||
|
}
|
|||
|
|
|||
|
fn plus_one(x: i32) -> i32 {
|
|||
|
x + 1
|
|||
|
}
|
|||
|
```
|
|||
|
|
|||
|
Running this code will print `The value of x is: 6`. But if we place a
|
|||
|
semicolon at the end of the line containing `x + 1`, changing it from an
|
|||
|
expression to a statement, we’ll get an error.
|
|||
|
|
|||
|
Filename: src/main.rs
|
|||
|
|
|||
|
```
|
|||
|
fn main() {
|
|||
|
let x = plus_one(5);
|
|||
|
|
|||
|
println!("The value of x is: {}", x);
|
|||
|
}
|
|||
|
|
|||
|
fn plus_one(x: i32) -> i32 {
|
|||
|
x + 1;
|
|||
|
}
|
|||
|
```
|
|||
|
|
|||
|
Compiling this code produces an error, as follows:
|
|||
|
|
|||
|
```
|
|||
|
$ cargo run
|
|||
|
Compiling functions v0.1.0 (file:///projects/functions)
|
|||
|
error[E0308]: mismatched types
|
|||
|
--> src/main.rs:7:24
|
|||
|
|
|
|||
|
7 | fn plus_one(x: i32) -> i32 {
|
|||
|
| -------- ^^^ expected `i32`, found `()`
|
|||
|
| |
|
|||
|
| implicitly returns `()` as its body has no tail or `return` expression
|
|||
|
8 | x + 1;
|
|||
|
| - help: consider removing this semicolon
|
|||
|
```
|
|||
|
|
|||
|
The main error message, “mismatched types,” reveals the core issue with this
|
|||
|
code. The definition of the function `plus_one` says that it will return an
|
|||
|
`i32`, but statements don’t evaluate to a value, which is expressed by `()`,
|
|||
|
the unit type. Therefore, nothing is returned, which contradicts the function
|
|||
|
definition and results in an error. In this output, Rust provides a message to
|
|||
|
possibly help rectify this issue: it suggests removing the semicolon, which
|
|||
|
would fix the error.
|
|||
|
|
|||
|
## Comments
|
|||
|
|
|||
|
All programmers strive to make their code easy to understand, but sometimes
|
|||
|
extra explanation is warranted. In these cases, programmers leave notes, or
|
|||
|
*comments*, in their source code that the compiler will ignore but people
|
|||
|
reading the source code may find useful.
|
|||
|
|
|||
|
Here’s a simple comment:
|
|||
|
|
|||
|
```
|
|||
|
// hello, world
|
|||
|
```
|
|||
|
|
|||
|
In Rust, the idiomatic comment style starts a comment with two slashes, and the
|
|||
|
comment continues until the end of the line. For comments that extend beyond a
|
|||
|
single line, you’ll need to include `//` on each line, like this:
|
|||
|
|
|||
|
```
|
|||
|
// So we’re doing something complicated here, long enough that we need
|
|||
|
// multiple lines of comments to do it! Whew! Hopefully, this comment will
|
|||
|
// explain what’s going on.
|
|||
|
```
|
|||
|
|
|||
|
Comments can also be placed at the end of lines containing code:
|
|||
|
|
|||
|
Filename: src/main.rs
|
|||
|
|
|||
|
```
|
|||
|
fn main() {
|
|||
|
let lucky_number = 7; // I’m feeling lucky today
|
|||
|
}
|
|||
|
```
|
|||
|
|
|||
|
But you’ll more often see them used in this format, with the comment on a
|
|||
|
separate line above the code it’s annotating:
|
|||
|
|
|||
|
Filename: src/main.rs
|
|||
|
|
|||
|
```
|
|||
|
fn main() {
|
|||
|
// I’m feeling lucky today
|
|||
|
let lucky_number = 7;
|
|||
|
}
|
|||
|
```
|
|||
|
|
|||
|
Rust also has another kind of comment, documentation comments, which we’ll
|
|||
|
discuss in the “Publishing a Crate to Crates.io” section of Chapter 14.
|
|||
|
|
|||
|
## Control Flow
|
|||
|
|
|||
|
Deciding whether or not to run some code depending on if a condition is true
|
|||
|
and deciding to run some code repeatedly while a condition is true are basic
|
|||
|
building blocks in most programming languages. The most common constructs that
|
|||
|
let you control the flow of execution of Rust code are `if` expressions and
|
|||
|
loops.
|
|||
|
|
|||
|
### `if` Expressions
|
|||
|
|
|||
|
An `if` expression allows you to branch your code depending on conditions. You
|
|||
|
provide a condition and then state, “If this condition is met, run this block
|
|||
|
of code. If the condition is not met, do not run this block of code.”
|
|||
|
|
|||
|
Create a new project called *branches* in your *projects* directory to explore
|
|||
|
the `if` expression. In the *src/main.rs* file, input the following:
|
|||
|
|
|||
|
Filename: src/main.rs
|
|||
|
|
|||
|
```
|
|||
|
fn main() {
|
|||
|
let number = 3;
|
|||
|
|
|||
|
if number < 5 {
|
|||
|
println!("condition was true");
|
|||
|
} else {
|
|||
|
println!("condition was false");
|
|||
|
}
|
|||
|
}
|
|||
|
```
|
|||
|
|
|||
|
All `if` expressions start with the keyword `if`, which is followed by a
|
|||
|
condition. In this case, the condition checks whether or not the variable
|
|||
|
`number` has a value less than 5. The block of code we want to execute if the
|
|||
|
condition is true is placed immediately after the condition inside curly
|
|||
|
brackets. Blocks of code associated with the conditions in `if` expressions are
|
|||
|
sometimes called *arms*, just like the arms in `match` expressions that we
|
|||
|
discussed in the “Comparing the Guess to the Secret Number” section of Chapter
|
|||
|
2.
|
|||
|
|
|||
|
Optionally, we can also include an `else` expression, which we chose
|
|||
|
to do here, to give the program an alternative block of code to execute should
|
|||
|
the condition evaluate to false. If you don’t provide an `else` expression and
|
|||
|
the condition is false, the program will just skip the `if` block and move on
|
|||
|
to the next bit of code.
|
|||
|
|
|||
|
Try running this code; you should see the following output:
|
|||
|
|
|||
|
```
|
|||
|
$ cargo run
|
|||
|
Compiling branches v0.1.0 (file:///projects/branches)
|
|||
|
Finished dev [unoptimized + debuginfo] target(s) in 0.31s
|
|||
|
Running `target/debug/branches`
|
|||
|
condition was true
|
|||
|
```
|
|||
|
|
|||
|
Let’s try changing the value of `number` to a value that makes the condition
|
|||
|
`false` to see what happens:
|
|||
|
|
|||
|
```
|
|||
|
let number = 7;
|
|||
|
```
|
|||
|
|
|||
|
Run the program again, and look at the output:
|
|||
|
|
|||
|
```
|
|||
|
$ cargo run
|
|||
|
Compiling branches v0.1.0 (file:///projects/branches)
|
|||
|
Finished dev [unoptimized + debuginfo] target(s) in 0.31s
|
|||
|
Running `target/debug/branches`
|
|||
|
condition was false
|
|||
|
```
|
|||
|
|
|||
|
It’s also worth noting that the condition in this code *must* be a `bool`. If
|
|||
|
the condition isn’t a `bool`, we’ll get an error. For example, try running the
|
|||
|
following code:
|
|||
|
|
|||
|
Filename: src/main.rs
|
|||
|
|
|||
|
```
|
|||
|
fn main() {
|
|||
|
let number = 3;
|
|||
|
|
|||
|
if number {
|
|||
|
println!("number was three");
|
|||
|
}
|
|||
|
}
|
|||
|
```
|
|||
|
|
|||
|
The `if` condition evaluates to a value of `3` this time, and Rust throws an
|
|||
|
error:
|
|||
|
|
|||
|
```
|
|||
|
$ cargo run
|
|||
|
Compiling branches v0.1.0 (file:///projects/branches)
|
|||
|
error[E0308]: mismatched types
|
|||
|
--> src/main.rs:4:8
|
|||
|
|
|
|||
|
4 | if number {
|
|||
|
| ^^^^^^ expected `bool`, found integer
|
|||
|
```
|
|||
|
|
|||
|
The error indicates that Rust expected a `bool` but got an integer. Unlike
|
|||
|
languages such as Ruby and JavaScript, Rust will not automatically try to
|
|||
|
convert non-Boolean types to a Boolean. You must be explicit and always provide
|
|||
|
`if` with a Boolean as its condition. If we want the `if` code block to run
|
|||
|
only when a number is not equal to `0`, for example, we can change the `if`
|
|||
|
expression to the following:
|
|||
|
|
|||
|
Filename: src/main.rs
|
|||
|
|
|||
|
```
|
|||
|
fn main() {
|
|||
|
let number = 3;
|
|||
|
|
|||
|
if number != 0 {
|
|||
|
println!("number was something other than zero");
|
|||
|
}
|
|||
|
}
|
|||
|
```
|
|||
|
|
|||
|
Running this code will print `number was something other than zero`.
|
|||
|
|
|||
|
#### Handling Multiple Conditions with `else if`
|
|||
|
|
|||
|
You can have multiple conditions by combining `if` and `else` in an `else if`
|
|||
|
expression. For example:
|
|||
|
|
|||
|
Filename: src/main.rs
|
|||
|
|
|||
|
```
|
|||
|
fn main() {
|
|||
|
let number = 6;
|
|||
|
|
|||
|
if number % 4 == 0 {
|
|||
|
println!("number is divisible by 4");
|
|||
|
} else if number % 3 == 0 {
|
|||
|
println!("number is divisible by 3");
|
|||
|
} else if number % 2 == 0 {
|
|||
|
println!("number is divisible by 2");
|
|||
|
} else {
|
|||
|
println!("number is not divisible by 4, 3, or 2");
|
|||
|
}
|
|||
|
}
|
|||
|
```
|
|||
|
|
|||
|
This program has four possible paths it can take. After running it, you should
|
|||
|
see the following output:
|
|||
|
|
|||
|
```
|
|||
|
$ cargo run
|
|||
|
Compiling branches v0.1.0 (file:///projects/branches)
|
|||
|
Finished dev [unoptimized + debuginfo] target(s) in 0.31s
|
|||
|
Running `target/debug/branches`
|
|||
|
number is divisible by 3
|
|||
|
```
|
|||
|
|
|||
|
When this program executes, it checks each `if` expression in turn and executes
|
|||
|
the first body for which the condition holds true. Note that even though 6 is
|
|||
|
divisible by 2, we don’t see the output `number is divisible by 2`, nor do we
|
|||
|
see the `number is not divisible by 4, 3, or 2` text from the `else` block.
|
|||
|
That’s because Rust only executes the block for the first true condition, and
|
|||
|
once it finds one, it doesn’t even check the rest.
|
|||
|
|
|||
|
Using too many `else if` expressions can clutter your code, so if you have more
|
|||
|
than one, you might want to refactor your code. Chapter 6 describes a powerful
|
|||
|
Rust branching construct called `match` for these cases.
|
|||
|
|
|||
|
#### Using `if` in a `let` Statement
|
|||
|
|
|||
|
Because `if` is an expression, we can use it on the right side of a `let`
|
|||
|
statement, as in Listing 3-2.
|
|||
|
|
|||
|
Filename: src/main.rs
|
|||
|
|
|||
|
```
|
|||
|
fn main() {
|
|||
|
let condition = true;
|
|||
|
let number = if condition { 5 } else { 6 };
|
|||
|
|
|||
|
println!("The value of number is: {}", number);
|
|||
|
}
|
|||
|
```
|
|||
|
|
|||
|
Listing 3-2: Assigning the result of an `if` expression
|
|||
|
to a variable
|
|||
|
|
|||
|
The `number` variable will be bound to a value based on the outcome of the `if`
|
|||
|
expression. Run this code to see what happens:
|
|||
|
|
|||
|
```
|
|||
|
$ cargo run
|
|||
|
Compiling branches v0.1.0 (file:///projects/branches)
|
|||
|
Finished dev [unoptimized + debuginfo] target(s) in 0.30s
|
|||
|
Running `target/debug/branches`
|
|||
|
The value of number is: 5
|
|||
|
```
|
|||
|
|
|||
|
Remember that blocks of code evaluate to the last expression in them, and
|
|||
|
numbers by themselves are also expressions. In this case, the value of the
|
|||
|
whole `if` expression depends on which block of code executes. This means the
|
|||
|
values that have the potential to be results from each arm of the `if` must be
|
|||
|
the same type; in Listing 3-2, the results of both the `if` arm and the `else`
|
|||
|
arm were `i32` integers. If the types are mismatched, as in the following
|
|||
|
example, we’ll get an error:
|
|||
|
|
|||
|
Filename: src/main.rs
|
|||
|
|
|||
|
```
|
|||
|
fn main() {
|
|||
|
let condition = true;
|
|||
|
|
|||
|
let number = if condition { 5 } else { "six" };
|
|||
|
|
|||
|
println!("The value of number is: {}", number);
|
|||
|
}
|
|||
|
```
|
|||
|
|
|||
|
When we try to compile this code, we’ll get an error. The `if` and `else` arms
|
|||
|
have value types that are incompatible, and Rust indicates exactly where to
|
|||
|
find the problem in the program:
|
|||
|
|
|||
|
```
|
|||
|
$ cargo run
|
|||
|
Compiling branches v0.1.0 (file:///projects/branches)
|
|||
|
error[E0308]: `if` and `else` have incompatible types
|
|||
|
--> src/main.rs:4:44
|
|||
|
|
|
|||
|
4 | let number = if condition { 5 } else { "six" };
|
|||
|
| - ^^^^^ expected integer, found `&str`
|
|||
|
| |
|
|||
|
| expected because of this
|
|||
|
```
|
|||
|
|
|||
|
The expression in the `if` block evaluates to an integer, and the expression in
|
|||
|
the `else` block evaluates to a string. This won’t work because variables must
|
|||
|
have a single type. Rust needs to know at compile time what type the `number`
|
|||
|
variable is, definitively, so it can verify at compile time that its type is
|
|||
|
valid everywhere we use `number`. Rust wouldn’t be able to do that if the type
|
|||
|
of `number` was only determined at runtime; the compiler would be more complex
|
|||
|
and would make fewer guarantees about the code if it had to keep track of
|
|||
|
multiple hypothetical types for any variable.
|
|||
|
|
|||
|
### Repetition with Loops
|
|||
|
|
|||
|
It’s often useful to execute a block of code more than once. For this task,
|
|||
|
Rust provides several *loops*. A loop runs through the code inside the loop
|
|||
|
body to the end and then starts immediately back at the beginning. To
|
|||
|
experiment with loops, let’s make a new project called *loops*.
|
|||
|
|
|||
|
Rust has three kinds of loops: `loop`, `while`, and `for`. Let’s try each one.
|
|||
|
|
|||
|
#### Repeating Code with `loop`
|
|||
|
|
|||
|
The `loop` keyword tells Rust to execute a block of code over and over again
|
|||
|
forever or until you explicitly tell it to stop.
|
|||
|
|
|||
|
As an example, change the *src/main.rs* file in your *loops* directory to look
|
|||
|
like this:
|
|||
|
|
|||
|
Filename: src/main.rs
|
|||
|
|
|||
|
```
|
|||
|
fn main() {
|
|||
|
loop {
|
|||
|
println!("again!");
|
|||
|
}
|
|||
|
}
|
|||
|
```
|
|||
|
|
|||
|
When we run this program, we’ll see `again!` printed over and over continuously
|
|||
|
until we stop the program manually. Most terminals support a keyboard shortcut,
|
|||
|
<span class="keystroke">ctrl-c</span>, to interrupt a program that is stuck in
|
|||
|
a continual loop. Give it a try:
|
|||
|
|
|||
|
```
|
|||
|
$ cargo run
|
|||
|
Compiling loops v0.1.0 (file:///projects/loops)
|
|||
|
Finished dev [unoptimized + debuginfo] target(s) in 0.29s
|
|||
|
Running `target/debug/loops`
|
|||
|
again!
|
|||
|
again!
|
|||
|
again!
|
|||
|
again!
|
|||
|
^Cagain!
|
|||
|
```
|
|||
|
|
|||
|
The symbol `^C` represents where you pressed <span class="keystroke">ctrl-c
|
|||
|
</span>. You may or may not see the word `again!` printed after the `^C`,
|
|||
|
depending on where the code was in the loop when it received the interrupt
|
|||
|
signal.
|
|||
|
|
|||
|
Fortunately, Rust provides a way to break out of a loop from code. You can
|
|||
|
place the `break` keyword within the loop to tell the program when to stop
|
|||
|
executing the loop. Recall that we did this in the guessing game in the
|
|||
|
“Quitting After a Correct Guess” section of Chapter 2 to exit the program when
|
|||
|
the user won the game by guessing the correct number.
|
|||
|
|
|||
|
We also used `continue` in the guessing game. The `continue` keyword within a
|
|||
|
loop tells the program to skip over any remaining code in this iteration of the
|
|||
|
loop and go to the next iteration.
|
|||
|
|
|||
|
If you have loops within loops, `break` and `continue` apply to the innermost
|
|||
|
loop at that point. You can optionally specify a *loop label* on a loop and
|
|||
|
then use the label with `break` or `continue` to have those keywords applied to
|
|||
|
the labeled loop instead of the innermost loop. Here’s an example with two
|
|||
|
nested loops:
|
|||
|
|
|||
|
```
|
|||
|
fn main() {
|
|||
|
let mut count = 0;
|
|||
|
'counting_up: loop {
|
|||
|
println!("count = {}", count);
|
|||
|
let mut remaining = 10;
|
|||
|
|
|||
|
loop {
|
|||
|
println!("remaining = {}", remaining);
|
|||
|
if remaining == 9 {
|
|||
|
break;
|
|||
|
}
|
|||
|
if count == 2 {
|
|||
|
break 'counting_up;
|
|||
|
}
|
|||
|
remaining -= 1;
|
|||
|
}
|
|||
|
|
|||
|
count += 1;
|
|||
|
}
|
|||
|
println!("End count = {}", count);
|
|||
|
}
|
|||
|
```
|
|||
|
|
|||
|
The outer loop has the label `'counting_up`, and it will count up from 0 to 2.
|
|||
|
The inner loop without a label counts down from 10 to 9. The first `break` that
|
|||
|
doesn’t specify a label will exit the inner loop only. The `break
|
|||
|
'counting_up;` statement will exit the outer loop. This code prints:
|
|||
|
|
|||
|
```
|
|||
|
Compiling loops v0.1.0 (file:///projects/loops)
|
|||
|
Finished dev [unoptimized + debuginfo] target(s) in 0.58s
|
|||
|
Running `target/debug/loops`
|
|||
|
count = 0
|
|||
|
remaining = 10
|
|||
|
remaining = 9
|
|||
|
count = 1
|
|||
|
remaining = 10
|
|||
|
remaining = 9
|
|||
|
count = 2
|
|||
|
remaining = 10
|
|||
|
End count = 2
|
|||
|
```
|
|||
|
|
|||
|
#### Returning Values from Loops
|
|||
|
|
|||
|
One of the uses of a `loop` is to retry an operation you know might fail, such
|
|||
|
as checking whether a thread has completed its job. However, you might need to
|
|||
|
pass the result of that operation to the rest of your code. To do this, you can
|
|||
|
add the value you want returned after the `break` expression you use to stop
|
|||
|
the loop; that value will be returned out of the loop so you can use it, as
|
|||
|
shown here:
|
|||
|
|
|||
|
```
|
|||
|
fn main() {
|
|||
|
let mut counter = 0;
|
|||
|
|
|||
|
let result = loop {
|
|||
|
counter += 1;
|
|||
|
|
|||
|
if counter == 10 {
|
|||
|
break counter * 2;
|
|||
|
}
|
|||
|
};
|
|||
|
|
|||
|
println!("The result is {}", result);
|
|||
|
}
|
|||
|
```
|
|||
|
|
|||
|
Before the loop, we declare a variable named `counter` and initialize it to
|
|||
|
`0`. Then we declare a variable named `result` to hold the value returned from
|
|||
|
the loop. On every iteration of the loop, we add `1` to the `counter` variable,
|
|||
|
and then check whether the counter is equal to `10`. When it is, we use the
|
|||
|
`break` keyword with the value `counter * 2`. After the loop, we use a
|
|||
|
semicolon to end the statement that assigns the value to `result`. Finally, we
|
|||
|
print the value in `result`, which in this case is 20.
|
|||
|
|
|||
|
#### Conditional Loops with `while`
|
|||
|
|
|||
|
It’s often useful for a program to evaluate a condition within a loop. While
|
|||
|
the condition is true, the loop runs. When the condition ceases to be true, the
|
|||
|
program calls `break`, stopping the loop. This loop type could be implemented
|
|||
|
using a combination of `loop`, `if`, `else`, and `break`; you could try that
|
|||
|
now in a program, if you’d like.
|
|||
|
|
|||
|
However, this pattern is so common that Rust has a built-in language construct
|
|||
|
for it, called a `while` loop. Listing 3-3 uses `while`: the program loops
|
|||
|
three times, counting down each time, and then, after the loop, it prints
|
|||
|
another message and exits.
|
|||
|
|
|||
|
Filename: src/main.rs
|
|||
|
|
|||
|
```
|
|||
|
fn main() {
|
|||
|
let mut number = 3;
|
|||
|
|
|||
|
while number != 0 {
|
|||
|
println!("{}!", number);
|
|||
|
|
|||
|
number -= 1;
|
|||
|
}
|
|||
|
|
|||
|
println!("LIFTOFF!!!");
|
|||
|
}
|
|||
|
```
|
|||
|
|
|||
|
Listing 3-3: Using a `while` loop to run code while a condition holds true
|
|||
|
|
|||
|
This construct eliminates a lot of nesting that would be necessary if you used
|
|||
|
`loop`, `if`, `else`, and `break`, and it’s clearer. While a condition holds
|
|||
|
true, the code runs; otherwise, it exits the loop.
|
|||
|
|
|||
|
#### Looping Through a Collection with `for`
|
|||
|
|
|||
|
You could use the `while` construct to loop over the elements of a collection,
|
|||
|
such as an array. For example, let’s look at Listing 3-4.
|
|||
|
|
|||
|
Filename: src/main.rs
|
|||
|
|
|||
|
```
|
|||
|
fn main() {
|
|||
|
let a = [10, 20, 30, 40, 50];
|
|||
|
let mut index = 0;
|
|||
|
|
|||
|
while index < 5 {
|
|||
|
println!("the value is: {}", a[index]);
|
|||
|
|
|||
|
index += 1;
|
|||
|
}
|
|||
|
}
|
|||
|
```
|
|||
|
|
|||
|
Listing 3-4: Looping through each element of a collection using a `while` loop
|
|||
|
|
|||
|
Here, the code counts up through the elements in the array. It starts at index
|
|||
|
`0`, and then loops until it reaches the final index in the array (that is,
|
|||
|
when `index < 5` is no longer true). Running this code will print every element
|
|||
|
in the array:
|
|||
|
|
|||
|
```
|
|||
|
$ cargo run
|
|||
|
Compiling loops v0.1.0 (file:///projects/loops)
|
|||
|
Finished dev [unoptimized + debuginfo] target(s) in 0.32s
|
|||
|
Running `target/debug/loops`
|
|||
|
the value is: 10
|
|||
|
the value is: 20
|
|||
|
the value is: 30
|
|||
|
the value is: 40
|
|||
|
the value is: 50
|
|||
|
```
|
|||
|
|
|||
|
All five array values appear in the terminal, as expected. Even though `index`
|
|||
|
will reach a value of `5` at some point, the loop stops executing before trying
|
|||
|
to fetch a sixth value from the array.
|
|||
|
|
|||
|
But this approach is error prone; we could cause the program to panic if the
|
|||
|
index value or test condition are incorrect. It’s also slow, because the
|
|||
|
compiler adds runtime code to perform the conditional check of whether the
|
|||
|
index is within the bounds of the array on every iteration through the loop.
|
|||
|
|
|||
|
As a more concise alternative, you can use a `for` loop and execute some code
|
|||
|
for each item in a collection. A `for` loop looks like the code in Listing 3-5.
|
|||
|
|
|||
|
Filename: src/main.rs
|
|||
|
|
|||
|
```
|
|||
|
fn main() {
|
|||
|
let a = [10, 20, 30, 40, 50];
|
|||
|
|
|||
|
for element in a {
|
|||
|
println!("the value is: {}", element);
|
|||
|
}
|
|||
|
}
|
|||
|
```
|
|||
|
|
|||
|
Listing 3-5: Looping through each element of a collection using a `for` loop
|
|||
|
|
|||
|
When we run this code, we’ll see the same output as in Listing 3-4. More
|
|||
|
importantly, we’ve now increased the safety of the code and eliminated the
|
|||
|
chance of bugs that might result from going beyond the end of the array or not
|
|||
|
going far enough and missing some items.
|
|||
|
|
|||
|
For example, in the code in Listing 3-4, if you changed the definition of the
|
|||
|
`a` array to have four elements but forgot to update the condition to `while
|
|||
|
index < 4`, the code would panic. Using the `for` loop, you wouldn’t need to
|
|||
|
remember to change any other code if you changed the number of values in the
|
|||
|
array.
|
|||
|
|
|||
|
The safety and conciseness of `for` loops make them the most commonly used loop
|
|||
|
construct in Rust. Even in situations in which you want to run some code a
|
|||
|
certain number of times, as in the countdown example that used a `while` loop
|
|||
|
in Listing 3-3, most Rustaceans would use a `for` loop. The way to do that
|
|||
|
would be to use a `Range`, which is a type provided by the standard library
|
|||
|
that generates all numbers in sequence starting from one number and ending
|
|||
|
before another number.
|
|||
|
|
|||
|
Here’s what the countdown would look like using a `for` loop and another method
|
|||
|
we’ve not yet talked about, `rev`, to reverse the range:
|
|||
|
|
|||
|
Filename: src/main.rs
|
|||
|
|
|||
|
```
|
|||
|
fn main() {
|
|||
|
for number in (1..4).rev() {
|
|||
|
println!("{}!", number);
|
|||
|
}
|
|||
|
println!("LIFTOFF!!!");
|
|||
|
}
|
|||
|
```
|
|||
|
|
|||
|
This code is a bit nicer, isn’t it?
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## Summary
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You made it! That was a sizable chapter: you learned about variables, scalar
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and compound data types, functions, comments, `if` expressions, and loops! If
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you want to practice with the concepts discussed in this chapter, try building
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programs to do the following:
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|
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* Convert temperatures between Fahrenheit and Celsius.
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* Generate the nth Fibonacci number.
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* Print the lyrics to the Christmas carol “The Twelve Days of Christmas,”
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taking advantage of the repetition in the song.
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|
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When you’re ready to move on, we’ll talk about a concept in Rust that *doesn’t*
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commonly exist in other programming languages: ownership.
|