diff --git a/second-edition/nostarch/chapter03.md b/second-edition/nostarch/chapter03.md index b100a1385..06b832bfb 100644 --- a/second-edition/nostarch/chapter03.md +++ b/second-edition/nostarch/chapter03.md @@ -6,7 +6,7 @@ This chapter covers concepts that appear in almost every programming language and how they work in Rust. Many programming languages have much in common at their core. None of the concepts presented in this chapter are unique to Rust, -but we’ll discuss them in the context of Rust and explain their conventions. +but we’ll discuss them in the context of Rust and explain its conventions. Specifically, you’ll learn about variables, basic types, functions, comments, and control flow. These foundations will be in every Rust program, and learning @@ -14,8 +14,8 @@ them early will give you a strong core to start from. > ### Keywords > -> The Rust language has a set of *keywords* that have been reserved for use by -> the language only, much like other languages do. Keep in mind that you cannot +> The Rust language has a set of *keywords* that are reserved for use by +> the language only, much as in other languages. Keep in mind that you cannot > use these words as names of variables or functions. Most of the keywords have > special meanings, and you’ll be using them to do various tasks in your Rust > programs; a few have no current functionality associated with them but have @@ -24,18 +24,19 @@ them early will give you a strong core to start from. ## Variables and Mutability -As mentioned in Chapter 2, by default variables are *immutable*. This is one of -many nudges in Rust that encourages you to write your code in a way that takes -advantage of the safety and easy concurrency that Rust offers. However, you -still have the option to make your variables mutable. Let’s explore how and why -Rust encourages you to favor immutability, and why you might want to opt out. +As mentioned in Chapter 2, by default variables are immutable. This is one of +many nudges Rust gives you to write your code in a way that takes advantage of +the safety and easy concurrency that Rust offers. However, you still have the +option to make your variables mutable. Let’s explore how and why Rust +encourages you to favor immutability and why sometimes you might want to opt +out. -When a variable is immutable, that means once a value is bound to a name, you -can’t change that value. To illustrate, let’s generate a new project called -*variables* in your *projects* directory by using `cargo new --bin variables`. +When a variable is immutable, once a value is bound to a name, you can’t change +that value. To illustrate this, let’s generate a new project called *variables* +in your *projects* directory by using `cargo new --bin variables`. Then, in your new *variables* directory, open *src/main.rs* and replace its -code with the following: +code with the following code that won’t compile just yet: Filename: src/main.rs @@ -67,8 +68,8 @@ Even though compiler errors can be frustrating, they only mean your program isn’t safely doing what you want it to do yet; they do *not* mean that you’re not a good programmer! Experienced Rustaceans still get compiler errors. -The error indicates that the cause of the error is that we `cannot assign twice -to immutable variable x`, because we tried to assign a second value to the +The error indicates that the cause of the error is that you `cannot assign twice +to immutable variable x`, because you tried to assign a second value to the immutable `x` variable. It’s important that we get compile-time errors when we attempt to change a @@ -76,21 +77,21 @@ value that we previously designated as immutable because this very situation can lead to bugs. If one part of our code operates on the assumption that a value will never change and another part of our code changes that value, it’s possible that the first part of the code won’t do what it was designed to do. -This cause of bugs can be difficult to track down after the fact, especially -when the second piece of code changes the value only *sometimes*. +The cause of this kind of bug can be difficult to track down after the fact, +especially when the second piece of code changes the value only *sometimes*. -In Rust the compiler guarantees that when we state that a value won’t change, +In Rust, the compiler guarantees that when you state that a value won’t change, it really won’t change. That means that when you’re reading and writing code, -you don’t have to keep track of how and where a value might change, which can -make code easier to reason about. +you don’t have to keep track of how and where a value might change. Your code +is thus easier to reason through. -But mutability can be very useful. Variables are immutable only by default; we -can make them mutable by adding `mut` in front of the variable name. In -addition to allowing this value to change, it conveys intent to future readers -of the code by indicating that other parts of the code will be changing this -variable value. +But mutability can be very useful. Variables are immutable only by default; as +you did in Chapter 2, you can make them mutable by adding `mut` in front of the +variable name. In addition to allowing this value to change, `mut` conveys +intent to future readers of the code by indicating that other parts of the code +will be changing this variable value. -For example, change *src/main.rs* to the following: +For example, let’s change *src/main.rs* to the following: Filename: src/main.rs @@ -103,7 +104,7 @@ fn main() { } ``` -When we run this program, we get the following: +When we run the program now, we get this: ``` $ cargo run @@ -114,44 +115,43 @@ The value of x is: 5 The value of x is: 6 ``` -Using `mut`, we’re allowed to change the value that `x` binds to from `5` to -`6`. In some cases, you’ll want to make a variable mutable because it makes the -code more convenient to write than an implementation that only uses immutable -variables. +We’re allowed to change the value that `x` binds to from `5` to `6` when `mut` +is used. In some cases, you’ll want to make a variable mutable because it makes +the code more convenient to write than if it had only immutable variables. -There are multiple trade-offs to consider, in addition to the prevention of +There are multiple trade-offs to consider in addition to the prevention of bugs. For example, in cases where you’re using large data structures, mutating an instance in place may be faster than copying and returning newly allocated instances. With smaller data structures, creating new instances and writing in -a more functional programming style may be easier to reason about, so the lower +a more functional programming style may be easier to think through, so lower performance might be a worthwhile penalty for gaining that clarity. ### Differences Between Variables and Constants Being unable to change the value of a variable might have reminded you of another programming concept that most other languages have: *constants*. Like -immutable variables, constants are also values that are bound to a name and -are not allowed to change, but there are a few differences between constants -and variables. +immutable variables, constants are values that are bound to a name and are not +allowed to change, but there are a few differences between constants and +variables. -First, we aren’t allowed to use `mut` with constants: constants aren’t only -immutable by default, they’re always immutable. +First, you aren’t allowed to use `mut` with constants. Constants aren’t just +immutable by default—they’re always immutable. -We declare constants using the `const` keyword instead of the `let` keyword, +You declare constants using the `const` keyword instead of the `let` keyword, and the type of the value *must* be annotated. We’re about to cover types and type annotations in the next section, “Data Types,” so don’t worry about the -details right now, just know that we must always annotate the type. +details right now. Just know that you must always annotate the type. Constants can be declared in any scope, including the global scope, which makes them useful for values that many parts of code need to know about. -The last difference is that constants may only be set to a constant expression, +The last difference is that constants may be set only to a constant expression, not the result of a function call or any other value that could only be computed at runtime. Here’s an example of a constant declaration where the constant’s name is -`MAX_POINTS` and its value is set to 100,000. (Rust constant naming convention -is to use all upper case with underscores between words): +`MAX_POINTS` and its value is set to 100,000. (Rust’s constant naming +convention is to use all uppercase with underscores between words): ``` const MAX_POINTS: u32 = 100_000; @@ -170,12 +170,13 @@ hardcoded value needed to be updated in the future. ### Shadowing -As we saw in the guessing game tutorial in Chapter 2, we can declare a new -variable with the same name as a previous variable, and the new variable -*shadows* the previous variable. Rustaceans say that the first variable is -*shadowed* by the second, which means that the second variable’s value is what -we’ll see when we use the variable. We can shadow a variable by using the same -variable’s name and repeating the use of the `let` keyword as follows: +As you saw in the “Comparing the Guess to the Secret Number” section in Chapter +2, you can declare a new variable with the same name as a previous variable, +and the new variable shadows the previous variable. Rustaceans say that the +first variable is *shadowed* by the second, which means that the second +variable’s value is what appears when the variable is used. We can shadow a +variable by using the same variable’s name and repeating the use of the `let` +keyword as follows: Filename: src/main.rs @@ -193,9 +194,9 @@ fn main() { This program first binds `x` to a value of `5`. Then it shadows `x` by repeating `let x =`, taking the original value and adding `1` so the value of -`x` is then `6`. The third `let` statement also shadows `x`, taking the -previous value and multiplying it by `2` to give `x` a final value of `12`. -When you run this program, it will output the following: +`x` is then `6`. The third `let` statement also shadows `x`, multiplying the +previous value by `2` to give `x` a final value of `12`. When we run this +program, it will output the following: ``` $ cargo run @@ -205,14 +206,15 @@ $ cargo run The value of x is: 12 ``` -This is different than marking a variable as `mut`, because unless we use the -`let` keyword again, we’ll get a compile-time error if we accidentally try to -reassign to this variable. We can perform a few transformations on a value but -have the variable be immutable after those transformations have been completed. +Shadowing is different than marking a variable as `mut`, because we’ll get a +compile-time error if we accidentally try to reassign to this variable without +using the `let` keyword. By using `let`, we can perform a few transformations +on a value but have the variable be immutable after those transformations have +been completed. The other difference between `mut` and shadowing is that because we’re effectively creating a new variable when we use the `let` keyword again, we can -change the type of the value, but reuse the same name. For example, say our +change the type of the value but reuse the same name. For example, say our program asks a user to show how many spaces they want between some text by inputting space characters, but we really want to store that input as a number: @@ -221,10 +223,10 @@ let spaces = " "; let spaces = spaces.len(); ``` -This construct is allowed because the first `spaces` variable is a string type, +This construct is allowed because the first `spaces` variable is a string type and the second `spaces` variable, which is a brand-new variable that happens to have the same name as the first one, is a number type. Shadowing thus spares us -from having to come up with different names, like `spaces_str` and +from having to come up with different names, such as `spaces_str` and `spaces_num`; instead, we can reuse the simpler `spaces` name. However, if we try to use `mut` for this, as shown here, we’ll get a compile-time error: @@ -251,17 +253,16 @@ can have. ## Data Types -Every value in Rust is of a certain *type*, which tells Rust what kind of data -is being specified so it knows how to work with that data. In this section, -we’ll look at a number of types that are built into the language. We split the -types into two subsets: scalar and compound. +Every value in Rust is of a certain *data type*, which tells Rust what kind of +data is being specified so it knows how to work with that data. We’ll look at +two data type subsets: scalar and compound. -Throughout this section, keep in mind that Rust is a *statically typed* -language, which means that it must know the types of all variables at compile -time. The compiler can usually infer what type we want to use based on the -value and how we use it. In cases when many types are possible, such as when we -converted a `String` to a numeric type using `parse` in Chapter 2, we must add -a type annotation, like this: +Keep in mind that Rust is a *statically typed* language, which means that it +must know the types of all variables at compile time. The compiler can usually +infer what type we want to use based on the value and how we use it. In cases +when many types are possible, such as when we converted a `String` to a numeric +type using `parse` in the “Comparing the Guess to the Secret Number” section in +Chapter 2, we must add a type annotation, like this: ``` let guess: u32 = "42".parse().expect("Not a number!"); @@ -269,7 +270,7 @@ let guess: u32 = "42".parse().expect("Not a number!"); If we don’t add the type annotation here, Rust will display the following error, which means the compiler needs more information from us to know which -possible type we want to use: +type we want to use: ``` error[E0282]: type annotations needed @@ -282,38 +283,37 @@ error[E0282]: type annotations needed | consider giving `guess` a type ``` -You’ll see different type annotations as we discuss the various data types. +You’ll see different type annotations for other data types. ### Scalar Types A *scalar* type represents a single value. Rust has four primary scalar types: -integers, floating-point numbers, Booleans, and characters. You’ll likely -recognize these from other programming languages, but let’s jump into how they -work in Rust. +integers, floating-point numbers, Booleans, and characters. You may recognize +these from other programming languages. Let’s jump into how they work in Rust. #### Integer Types An *integer* is a number without a fractional component. We used one integer -type earlier in this chapter, the `u32` type. This type declaration indicates -that the value it’s associated with should be an unsigned integer (signed -integer types start with `i` instead of `u`) that takes up 32 bits of space. -Table 3-1 shows the built-in integer types in Rust. Each variant in the Signed -and Unsigned columns (for example, *i16*) can be used to declare the type of an -integer value. +type in Chapter 2, the `u32` type. This type declaration indicates that the +value it’s associated with should be an unsigned integer (signed integer types +start with `i`, instead of `u`) that takes up 32 bits of space. Table 3-1 shows +the built-in integer types in Rust. Each variant in the Signed and Unsigned +columns (for example, `i16`) can be used to declare the type of an integer +value. Table 3-1: Integer Types in Rust -| Length | Signed | Unsigned | -|--------|--------|----------| -| 8-bit | i8 | u8 | -| 16-bit | i16 | u16 | -| 32-bit | i32 | u32 | -| 64-bit | i64 | u64 | -| arch | isize | usize | +| Length | Signed | Unsigned | +|--------|---------|----------| +| 8-bit | `i8` | `u8` | +| 16-bit | `i16` | `u16` | +| 32-bit | `i32` | `u32` | +| 64-bit | `i64` | `u64` | +| arch | `isize` | `usize` | Each variant can be either signed or unsigned and has an explicit size. -Signed and unsigned refers to whether it’s possible for the number to be -negative or positive; in other words, whether the number needs to have a sign +*Signed* and *unsigned* refer to whether it’s possible for the number to be +negative or positive—in other words, whether the number needs to have a sign with it (signed) or whether it will only ever be positive and can therefore be represented without a sign (unsigned). It’s like writing numbers on paper: when the sign matters, a number is shown with a plus sign or a minus sign; however, @@ -323,13 +323,13 @@ unsure what this is, you can search for it online; an explanation is outside the scope of this book). Each signed variant can store numbers from -(2n - 1) to 2n - -1 - 1 inclusive, where `n` is the number of bits that variant uses. So an +1 - 1 inclusive, where *n* is the number of bits that variant uses. So an `i8` can store numbers from -(27) to 27 - 1, which equals -128 to 127. Unsigned variants can store numbers from 0 to 2n - 1, so a `u8` can store numbers from 0 to 28 - 1, which equals 0 to 255. Additionally, the `isize` and `usize` types depend on the kind of computer your -program is running on: 64-bits if you’re on a 64-bit architecture and 32-bits +program is running on: 64 bits if you’re on a 64-bit architecture and 32 bits if you’re on a 32-bit architecture. You can write integer literals in any of the forms shown in Table 3-2. Note @@ -347,9 +347,9 @@ Table 3-2: Integer Literals in Rust | Byte (`u8` only) | `b'A'` | So how do you know which type of integer to use? If you’re unsure, Rust’s -defaults are generally good choices, and integer types default to `i32`: it’s -generally the fastest, even on 64-bit systems. The primary situation in which -you’d use `isize` or `usize` is when indexing some sort of collection. +defaults are generally good choices, and integer types default to `i32`: this +type is generally the fastest, even on 64-bit systems. The primary situation in +which you’d use `isize` or `usize` is when indexing some sort of collection. #### Floating-Point Types @@ -376,7 +376,7 @@ Floating-point numbers are represented according to the IEEE-754 standard. The #### Numeric Operations -Rust supports the usual basic mathematical operations you’d expect for all of the +Rust supports the basic mathematical operations you’d expect for all of the number types: addition, subtraction, multiplication, division, and remainder. The following code shows how you’d use each one in a `let` statement: @@ -427,10 +427,10 @@ section. #### The Character Type -So far we’ve only worked with numbers, but Rust supports letters too. Rust’s +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 the `char` type is specified with -single quotes, as opposed to strings that use double quotes: +code shows one way to use it. (Note that the `char` type is specified with +single quotes, as opposed to strings, which use double quotes.) Filename: src/main.rs @@ -443,20 +443,19 @@ fn main() { ``` Rust’s `char` type represents a Unicode Scalar Value, which means it can -represent a lot more than just ASCII. Accented letters, Chinese/Japanese/Korean -ideographs, emoji, and zero width spaces are all valid `char` types in Rust. -Unicode Scalar Values range from `U+0000` to `U+D7FF` and `U+E000` to +represent a lot more than just ASCII. Accented letters; Chinese, Japanese, and +Korean ideographs; emoji; and zero-width spaces are all valid `char` types 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 the “Strings” section -in Chapter 8. +`char` is in Rust. We’ll discuss this topic in detail in “Strings” in Chapter 8. ### Compound Types -*Compound types* can group multiple values of other types into one type. Rust -has two primitive compound types: tuples and arrays. +*Compound types* can group multiple values into one type. Rust has two +primitive compound types: tuples and arrays. -#### Grouping Values into Tuples +#### The Tuple Type A tuple is a general way of grouping together some number of other values with a variety of types into one compound type. @@ -474,7 +473,7 @@ fn main() { } ``` -The variable `tup` binds to the entire tuple, since a tuple is considered a +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: @@ -496,9 +495,9 @@ 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 also access a -tuple element directly by using a period (`.`) followed by the index of the -value we want to access. For example: +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 @@ -518,11 +517,11 @@ This program creates a tuple, `x`, and then makes new variables for each element by using their index. As with most programming languages, the first index in a tuple is 0. -#### Arrays +#### 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 than arrays in some other languages because arrays in Rust have a +different from arrays in some other languages because arrays in Rust have a fixed length: once declared, they cannot grow or shrink in size. In Rust, the values going into an array are written as a comma-separated list @@ -538,11 +537,11 @@ fn main() { 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. They aren’t as -flexible as the vector type, though. The vector type 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. +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 @@ -556,7 +555,7 @@ let months = ["January", "February", "March", "April", "May", "June", "July", ##### Accessing Array Elements -An array is a single chunk of memory allocated on the stack. We can access +An array is a single chunk of memory allocated on the stack. You can access elements of an array using indexing, like this: Filename: src/main.rs @@ -605,7 +604,7 @@ thread '
' panicked at 'index out of bounds: the len is 5 but the index is note: Run with `RUST_BACKTRACE=1` for a backtrace. ``` -The compilation didn’t produce any errors, but the program results in a +The compilation didn’t produce any errors, but the program resulted in a *runtime* error and didn’t exit successfully. 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 the length, Rust will @@ -617,7 +616,7 @@ 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. -## How Functions Work +## Functions Functions are pervasive in Rust code. You’ve already seen one of the most important functions in the language: the `main` function, which is the entry @@ -673,12 +672,12 @@ 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, we 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. +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: @@ -732,13 +731,13 @@ fn another_function(x: i32, y: i32) { ``` This example creates a function with two parameters, both of which are `i32` -types. The function then prints out the values in both of its parameters. Note -that function parameters don’t all need to be the same type, they just happen -to be in this example. +types. The function then prints the values in both of its parameters. Note that +function parameters don’t all need to be the same type, they just happen to be +in this example. 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`: +project’s *src/main.rs* file with the preceding example and run it using `cargo +run`: ``` $ cargo run @@ -756,7 +755,7 @@ as the value for `y`, the two strings are printed with these values. 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 we have seen expressions as parts of statements. Because Rust is an +but you have seen an expression as part of statements. 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 @@ -779,7 +778,7 @@ fn main() { } ``` -Listing 3-1: A `main` function declaration containing one statement. +Listing 3-1: A `main` function declaration containing one statement Function definitions are also statements; the entire preceding example is a statement in itself. @@ -810,15 +809,15 @@ error: expected expression, found statement (`let`) ``` The `let y = 6` statement does not return a value, so there isn’t anything for -`x` to bind to. This is different than 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. +`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 something and make up most of the rest of the code that you’ll write in Rust. Consider a simple 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 that had the statement `let y = 6;`, `6` is an +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: @@ -849,10 +848,10 @@ This expression: 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, 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. +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 @@ -903,8 +902,9 @@ 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: +because it’s an expression whose value we want to return. + +Let’s look at another example: Filename: src/main.rs @@ -920,9 +920,9 @@ fn plus_one(x: i32) -> i32 { } ``` -Running this code will print `The value of x is: 6`. What happens if we place a +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: +expression to a statement, we’ll get an error. Filename: src/main.rs @@ -1013,15 +1013,15 @@ discuss in Chapter 14. ## Control Flow -Deciding whether or not to run some code depending on if a condition is true or -deciding to run some code repeatedly while a condition is true are basic +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 us to branch our code depending on conditions. We +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.” @@ -1051,7 +1051,9 @@ 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 +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 @@ -1085,8 +1087,7 @@ 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: +the condition isn’t a `bool`, we’ll get an error. For example: Filename: src/main.rs @@ -1114,9 +1115,9 @@ error[E0308]: mismatched types found type `{integer}` ``` -The error indicates that Rust expected a `bool` but got an integer. Rust will -not automatically try to convert non-Boolean types to a Boolean, unlike -languages such as Ruby and JavaScript. You must be explicit and always provide +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: @@ -1135,9 +1136,9 @@ fn main() { Running this code will print `number was something other than zero`. -#### Multiple Conditions with `else if` +#### Handling Multiple Conditions with `else if` -We can have multiple conditions by combining `if` and `else` in an `else if` +You can have multiple conditions by combining `if` and `else` in an `else if` expression. For example: Filename: src/main.rs @@ -1172,18 +1173,18 @@ 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. The -reason is that Rust will only execute the block for the first true condition, -and once it finds one, it won’t even check the rest. +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 +#### Using `if` in a `let` Statement Because `if` is an expression, we can use it on the right side of a `let` -statement, for instance in Listing 3-2: +statement, as in Listing 3-2: Filename: src/main.rs @@ -1200,8 +1201,7 @@ fn main() { } ``` -Listing 3-2: Assigning the result of an `if` expression -to a variable +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: @@ -1252,7 +1252,7 @@ error[E0308]: if and else have incompatible types 6 | | } else { 7 | | "six" 8 | | }; - | |_____^ expected integral variable, found reference + | |_____^ expected integral variable, found &str | = note: expected type `{integer}` found type `&str` @@ -1324,15 +1324,15 @@ program when the user won the game by guessing the correct number. #### 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, you -call `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. +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, and it’s called a `while` loop. The following example uses `while`: the -program loops three times, counting down each time. Then, after the loop, it -prints another message and exits: +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 @@ -1350,6 +1350,8 @@ fn main() { } ``` +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. @@ -1357,7 +1359,7 @@ 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 3-3: +such as an array. For example, let’s look at Listing 3-4: Filename: src/main.rs @@ -1374,13 +1376,12 @@ fn main() { } ``` -Listing 3-3: Looping through each element of a collection -using a `while` loop +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 out every -element in the array: +when `index < 5` is no longer true). Running this code will print every element +in the array: ``` $ cargo run @@ -1404,7 +1405,7 @@ code to perform the conditional check on every element 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 this code in Listing 3-4: +for each item in a collection. A `for` loop looks like this code in Listing 3-5: Filename: src/main.rs @@ -1418,17 +1419,16 @@ fn main() { } ``` -Listing 3-4: Looping through each element of a collection -using a `for` loop +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 3-3. More +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 3-3, if you removed an item from the `a` +For example, in the code in Listing 3-4, if you removed an item from the `a` array but forgot to update the condition to `while index < 4`, the code would -panic. Using the `for` loop, you don’t need to remember to change any other +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 @@ -1458,8 +1458,9 @@ This code is a bit nicer, isn’t it? ## Summary You made it! That was a sizable chapter: you learned about variables, scalar -and `if` expressions, and loops! If you want to practice with the concepts -discussed in this chapter, try building programs to do the following: +and compound data types, functions, comments, `if` expressions, and loops! If +you want to practice with the concepts discussed in this chapter, try building +programs to do the following: * Convert temperatures between Fahrenheit and Celsius. * Generate the nth Fibonacci number.