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[TOC]
Common Programming Concepts
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.
Specifically, you’ll learn about variables, basic types, functions, comments, and control flow. These foundations will be in every Rust program, and learning them early will give you a strong core to start from.
PROD: START BOX
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 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 been reserved for functionality that might be added to Rust in the future. You can find a list of the keywords in Appendix A.
PROD: END BOX
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.
When a variable is immutable, that means mean 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.
Then, in your new variables directory, open src/main.rs and replace its code with the following:
Filename: src/main.rs
fn main() {
let x = 5;
println!("The value of x is: {}", x);
x = 6;
println!("The value of x is: {}", x);
}
Save and run the program using cargo run. You should receive an error
message, as shown in this output:
$ cargo run
Compiling variables v0.0.1 (file:///projects/variables)
error[E0384]: re-assignment of immutable variable `x`
--> src/main.rs:4:5
|
2 | let x = 5;
| - first assignment to `x`
3 | println!("The value of x is: {}", x);
4 | x = 6;
| ^^^^^ re-assignment of immutable variable
This example shows how the compiler helps you find errors in your programs.
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 re-assignment of immutable variable, because we 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 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.
In Rust the compiler guarantees that when we 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.
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.
For example, change src/main.rs to the following:
Filename: src/main.rs
fn main() {
let mut x = 5;
println!("The value of x is: {}", x);
x = 6;
println!("The value of x is: {}", x);
}
When we run this program, we get the following:
$ cargo run
Compiling variables v0.1.0 (file:///projects/variables)
Running `target/debug/variables`
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.
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, always creating new instances and writing in a more functional programming style may be easier to reason about, so the lower performance penalty might be worth it to gain that clarity.
Differences Between Variables and Constants
Not being able to change the value of a variable might have reminded you of
another programming concept that most languages have: constants. Constants
are also values bound to a name that are not allowed to change, but there are a
few differences between constants and variables. First, using mut with
constants is not allowed: constants aren't only immutable by default, they're
always immutable. Constants are declared 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. Constants can be declared in any scope,
including the global scope, which makes them useful for a value that many parts
of your code need to know about. The last difference is that constants may only
be set to a constant expression, not the result of a function call or any other
value that could only be used 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:
const MAX_POINTS: u32 = 100_000;
Constants are valid for the entire lifetime of a program, within the scope they were declared in. That makes constants useful for values in your application domain that multiple part of the program might need to know about, such as the maximum number of points any player of a game is allowed to earn or the number of seconds in a year.
Documenting hardcoded values used throughout your program by naming them as constants is useful to convey the meaning of that value to future maintainers of the code. It also helps to have only one place in your code that you would need to change if the hardcoded value needed to be updated in the future.
Shadowing
As we saw in the guessing game tutorial in Chapter 2, we can declare new
variables with the same name as a previous variables, 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:
Filename: src/main.rs
fn main() {
let x = 5;
let x = x + 1;
let x = x * 2;
println!("The value of x is: {}", x);
}
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:
$ cargo run
Compiling variables v0.1.0 (file:///projects/variables)
Running `target/debug/variables`
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.
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
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:
let spaces = " ";
let spaces = spaces.len();
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
spaces_num; instead, we can reuse the simpler spaces name. However, if we
try to use mut for this, as shown here:
let mut spaces = " ";
spaces = spaces.len();
we’ll get a compile-time error because we’re not allowed to mutate a variable’s type:
error[E0308]: mismatched types
--> src/main.rs:3:14
|
3 | spaces = spaces.len();
| ^^^^^^^^^^^^ expected &str, found usize
|
= note: expected type `&str`
= note: found type `usize`
Now that we’ve explored how variables work, let’s look at more data types they 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.
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:
let guess: u32 = "42".parse().unwrap();
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:
error[E0282]: unable to infer enough type information about `_`
--> src/main.rs:2:5
|
2 | let guess = "42".parse().unwrap();
| ^^^^^ cannot infer type for `_`
|
= note: type annotations or generic parameter binding required
You’ll see different type annotations as we discuss the various 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.
Integer Types
An integer is a number without a fractional component. We used one integer
type earlier in this chapter, the i32 type. This type declaration indicates
that the value it’s associated with should be a signed integer (hence the i,
as opposed to a u for unsigned) for a 32-bit system. Table 3-1 shows the
built-in integer types in Rust. Each variant in the Signed and Unsigned columns
(for example, i32) can be used to declare the type of an integer value.
| 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 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, when it’s safe to assume the number is positive, it’s shown with no sign. Signed numbers are stored using two’s complement representation (if you’re 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
i8 can store numbers from -(27) to 27, 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
if you’re on a 32-bit architecture.
You can write integer literals in any of the forms shown in Table 3-2. Note
that all number literals except the byte literal allow a type suffix, such as
57u8, and _ as a visual separator, such as 1_000.
| Number literals | Example |
|---|---|
| Decimal | 98_222 |
| Hex | 0xff |
| Octal | 0o77 |
| Binary | 0b1111_0000 |
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.
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 it’s roughly the same speed as f32 but is capable of more precision.
It’s possible to use an f64 type on 32-bit systems, but it will be slower
than using an f32 type on those systems. Most of the time, trading potential
worse performance for better precision is a reasonable initial choice, and you
should benchmark your code if you suspect floating-point size is a problem in
your situation.
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 usual basic mathematic 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:
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;
// 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. 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 consume boolean values is through conditionals, such as an if
statement. We’ll cover how if statements work in Rust in the “Control Flow”
section.
The Character Type
So far we’ve only worked 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:
Filename: src/main.rs
fn main() {
let c = 'z';
let z = 'ℤ';
let heart_eyed_cat = '😻';
}
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
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.
Compound Types
Compound types can group multiple values of other types into one type. Rust has two primitive compound types: tuples and arrays.
Grouping Values into Tuples
A tuple is a general way of grouping together some number of other values with a variety of types into one compound type.
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, since 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 also 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 index. As with most programming languages, the first
index in a tuple is 0.
Arrays
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 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 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. 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.
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 items:
let months = ["January", "February", "March", "April", "May", "June", "July",
"August", "September", "October", "November", "December"];
Accessing Array Elements
An array is a single chunk of memory allocated on the stack. We 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 we try to access an element of an array that is past the end of the array? Say we change the example to the following:
Filename: src/main.rs
fn main() {
let a = [1, 2, 3, 4, 5];
let element = a[10];
println!("The value of element is: {}", element);
}
Running this code using cargo run produces the following result:
$ cargo run
Compiling arrays v0.1.0 (file:///projects/arrays)
Running `target/debug/arrays`
thread '<main>' panicked at 'index out of bounds: the len is 5 but the index is
10', src/main.rs:4
note: Run with `RUST_BACKTRACE=1` for a backtrace.
error: Process didn't exit successfully: `target/debug/arrays` (exit code: 101)
The compilation didn’t produce any errors, but the program results 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 panic, which is the term Rust uses when a program exits with an error.
This is the first example of Rust’s 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.
How Functions Work
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
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 braces 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)
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 Arguments
Functions can also take arguments. The following rewritten version of
another_function shows what arguments 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)
Running `target/debug/functions`
The value of x is: 5
The declaration of another_function has one argument 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 braces were in the format
string.
In function signatures, you must declare the type. 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 you mean.
When you want a function to have multiple arguments, separate them inside the function signature with commas, like this:
Filename: src/main.rs
fn main() {
another_function(5, 6);
}
fn another_function(x: i32, y: i32) {
println!("The value of x is: {}", x);
println!("The value of y is: {}", y);
}
This example creates a function with two arguments, both of which are i32
types. If your function has multiple arguments, the arguments don’t need to be
the same type, but they just happen to be in this example. The function then
prints out the values of both of its arguments.
Let’s try running this code. Replace the program currently in your function
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)
Running `target/debug/functions`
The value of x is: 5
The value of y is: 6
Because 5 is passed as the x argument and 6 is passed as the y
argument, the two strings are printed with these values.
Function Bodies
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 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.
Statements and Expressions
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 3-3, let y = 6; is a statement:
Filename: src/main.rs
fn main() {
let y = 6;
}
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:
Filename: src/main.rs
fn main() {
let x = (let y = 6);
}
When you run this program, you’ll get an error like this:
$ cargo run
Compiling functions v0.1.0 (file:///projects/functions)
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
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.
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 3-3 that had the statement let y = 6;, 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 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.
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. 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)
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 requires no arguments 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. What happens if we place a
semicolon at the end of the line containing x + 1, changing it from an
expression to a statement?
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 produces an error, as follows:
error[E0269]: not all control paths return a value
--> src/main.rs:7:1
|
7 | fn plus_one(x: i32) -> i32 {
| ^
|
help: consider removing this semicolon:
--> src/main.rs:8:10
|
8 | x + 1;
| ^
The main error message, “not all control paths return a value,” 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. 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, comments must start with two slashes and continue 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;
}
That’s all there is to comments. They’re not particularly complicated.
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
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
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
braces. 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)
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)
Running `target/debug/branches`
condition was false
It’s also worth noting that the condition in this code must be a bool. To
see what happens if the condition isn’t a bool, 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:
Compiling branches v0.1.0 (file:///projects/branches)
error[E0308]: mismatched types
--> src/main.rs:4:8
|
4 | if number {
| ^^^^^^ expected bool, found integral variable
|
= note: expected type `bool`
= note: found type `{integer}`
error: aborting due to previous error
Could not compile `branches`.
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
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.
Multiple Conditions with else if
We 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)
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. 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.
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, for instance in Listing 3-4:
Filename: src/main.rs
fn main() {
let condition = true;
let number = if condition {
5
} else {
6
};
println!("The value of number is: {}", number);
}
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)
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 3-4, the results of both the if arm and the else
arm were i32 integers. But what happens if the types are mismatched, as in
the following example?
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 run 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:
Compiling branches v0.1.0 (file:///projects/branches)
error[E0308]: if and else have incompatible types
--> src/main.rs:4:18
|
4 | let number = if condition {
| ^ expected integral variable, found reference
|
= note: expected type `{integer}`
= note: found type `&’static str`
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,
ctrl-C, to halt a program that is stuck in a continual loop. Give it a try:
$ cargo run
Compiling loops v0.1.0 (file:///projects/loops)
Running `target/debug/loops`
again!
again!
again!
again!
^Cagain!
The symbol ^C represents where you pressed ctrl-C. 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 halt signal.
Fortunately, Rust provides another, more reliable way to break out of a loop.
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.
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.
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:
Filename: src/main.rs
fn main() {
let mut number = 3;
while number != 0 {
println!("{}!", number);
number = number - 1;
}
println!("LIFTOFF!!!");
}
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:
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 = index + 1;
}
}
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:
$ cargo run
Compiling loops v0.1.0 (file:///projects/loops)
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 6 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 length is incorrect. It’s also slow, because the compiler needs to perform the conditional check on every element on every iteration through the loop.
As a more efficient alternative, you can use a for loop and execute some code
for each item in a collection. A for loop looks like this:
Filename: src/main.rs
fn main() {
let a = [10, 20, 30, 40, 50];
for element in a.iter() {
println!("the value is: {}", element);
}
}
When we run this code, we’ll see the same output as in Listing 3-5. 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 3-5, 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
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 3-5, 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?
Summary
You made it! That was a sizable chapter: you learned about variables, scalar
andif 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.
- Print the lyrics to the Christmas carol “The Twelve Days of Christmas,” taking advantage of the repetition in the song.
When you’re ready to move on, we’ll talk about a concept in Rust that doesn’t commonly exist in other programming languages: ownership.