The Rust language has a set of keywords that are reserved for use by the language only
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.
When a variable is immutable, once a value is bound to a name, you can’t change that value.
Like 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.
⇒ declare constants using the const keyword
mut with constants. Constants aren’t just immutable by default—they’re always immutable.const THREE_HOURS_IN_SECONDS: u32 = 60 * 60 * 3;
<aside> 💡 The compiler is able to evaluate a limited set of operations at compile time, which lets us choose to write out this value in a way that’s easier to understand and verify, rather than setting this constant to the value 10,800. See the Rust Reference’s section on constant evaluation for more information on what operations can be used when declaring constants.
</aside>
you can declare a new variable with the same name as a previous variable. Rustaceans say that the first variable is shadowed by the second, which means that the second variable is what the compiler will see when you use the name of the variable.
Shadowing is different from 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 program asks a user to show how many spaces they want between some text by inputting space characters, and then we want to store that input as a number:
let spaces = " ";
let spaces = spaces.len();
Compound types can group multiple values into one type. Rust has two primitive compound types: tuples and arrays.
A tuple is a general way of grouping several values with a variety of types into one compound type.
Tuples have a fixed length: once declared, they cannot grow or shrink in size.
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.
We can also access a tuple element directly by using a period (.) followed by the index of the value
<aside>
💡 The tuple without any values has a special name, a unit. This value and its corresponding type are both written () and represent an empty value or an empty return type. Expressions implicitly return the unit value if they don’t return any other value.
</aside>
every element of an array must have the same type. Unlike arrays in some other languages, arrays in Rust have a fixed length.
We write the values in an array as a comma-separated list inside square brackets:
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.
Another way of initialization of array,
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.
<aside> 💡 Invalid Array Element Access
Then 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.
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Every value in Rust is of a certain data type.
two data type subsets: scalar and compouned.
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, we need to mention the type. for example :
let guess: u32 = "42".parse().expect("Not a number!");
A scalar type represents a single value. Rust has four primary scalar types: integers, floating-point numbers, Booleans, and characters.
| Length | Signed | Unsigned |
|---|---|---|
| 8-bit | i8 |
u8 |
| 16-bit | i16 |
u16 |
| 32-bit | i32 |
u32 |
| 64-bit | i64 |
u64 |
| 128-bit | i128 |
u128 |
| arch | isize |
usize |
Signed and unsigned refer to whether the number can be negative—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).
Note that number literals that can be multiple numeric types allow a type suffix, such as 57u8, to designate the type. Number literals can also use _ as a visual separator to make the number easier to read, such as 1_000, which will have the same value as if you had specified 1000.
Table 3-2: Integer Literals in Rust
| --- | --- |
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 that range, such as 256, integer overflow will occur, which can result in one of two behaviors. 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 provided by the standard library for primitive numeric types:
wrapping_* methods, such as wrapping_add.Returning values can also transfer ownership. Listing 4-4 shows an example of a function that returns some value, with similar annotations as those in Listing 4-3.
Filename: src/main.rs
fn main() {
let s1 = gives_ownership(); // gives_ownership moves its return
// value into s1
let s2 = String::from("hello"); // s2 comes into scope
let s3 = takes_and_gives_back(s2); // s2 is moved into
// takes_and_gives_back, which also
// moves its return value into s3
} // Here, s3 goes out of scope and is dropped. s2 was moved, so nothing
// happens. s1 goes out of scope and is dropped.
fn gives_ownership() -> String { // gives_ownership will move its
// return value into the function
// that calls it
let some_string = String::from("yours"); // some_string comes into scope
some_string // some_string is returned and
// moves out to the calling
// function
}
// This function takes a String and returns one
fn takes_and_gives_back(a_string: String) -> String { // a_string comes into
// scope
a_string // a_string is returned and moves out to the calling function
}
Listing 4-4: Transferring ownership of return values
The ownership of a variable follows the same pattern every time: assigning a value to another variable moves it. When a variable that includes data on the heap goes out of scope, the value will be cleaned up by drop unless ownership of the data has been moved to another variable.
While this works, taking ownership and then returning ownership with every function is a bit tedious. What if we want to let a function use a value but not take ownership? It’s quite annoying that anything we pass in also needs to be passed back if we want to use it again, in addition to any data resulting from the body of the function that we might want to return as well.
Rust does let us return multiple values using a tuple, as shown in Listing 4-5.
Filename: src/main.rs
fn main() {
let s1 = String::from("hello");
let (s2, len) = calculate_length(s1);
println!("The length of '{}' is {}.", s2, len);
}
fn calculate_length(s: String) -> (String, usize) {
let length = s.len(); // len() returns the length of a String
(s, length)
}
Listing 4-5: Returning ownership of parameters