Jeromy's Notes

Basic Concepts

8 min read

Data Types

Scalar

Integer

Table 3-1: Integer Types in Rust

LengthSignedUnsigned
8-biti8u8
16-biti16u16
32-biti32u32
64-biti64u64
128-biti128u128
archisizeusize

Floating Point

Rust’s floating-point types are f32 and f64, which are 32 bits and 64 bits in size, respectively. The default type is f64 because on modern CPUs, it’s roughly the same speed as f32 but is capable of more precision. All floating-point types are signed.

Character Type

Rust’s char type is the language’s most primitive alphabetic type. Note that we specify char literals with single quotes, as opposed to string literals, which use double quotes. Rust’s char type is four bytes in size and represents a Unicode Scalar Value, which means it can represent a lot more than just ASCII

Compound

Tuple

A tuple is a general way of grouping together a number of values with a variety of types into one compound type. Tuples have a fixed length: once declared, they cannot grow or shrink in size.

We create a tuple by writing a comma-separated list of values inside parentheses. Each position in the tuple has a type, and the types of the different values in the tuple don’t have to be the same. We’ve added optional type annotations in this example:

Filename: src/main.rs

fn main() { let tup: (i32, f64, u8) = (500, 6.4, 1); }

The variable tup binds to the entire tuple because a tuple is considered a single compound element. To get the individual values out of a tuple, we can use pattern matching to destructure a tuple value, like this:

Filename: src/main.rs

fn main() { let tup = (500, 6.4, 1); let (x, y, z) = tup; println!("The value of y is: {y}"); }

This program first creates a tuple and binds it to the variable tup. It then uses a pattern with let to take tup and turn it into three separate variables, x, y, and z. This is called destructuring because it breaks the single tuple into three parts. Finally, the program prints the value of y, which is 6.4.

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 tup = (500, 6.4, 1);
 
    let (x, y, z) = tup;
 
    println!("The value of y is: {y}");
}

This program creates the tuple x and then accesses each element of the tuple using their respective indices. As with most programming languages, the first index in a tuple is 0.

The tuple without any values has a special name, 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.

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. 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:

Filename: src/main.rs

fn main() { let a = [1, 2, 3, 4, 5]; }

Arrays are useful when you want your data allocated on the stack rather than the heap (we will discuss the stack and the heap more in Chapter 4) or when you want to ensure you always have a fixed number of elements. An array isn’t as flexible as the vector type, though. A vector is a similar collection type provided by the standard library that is allowed to grow or shrink in size. If you’re unsure whether to use an array or a vector, chances are you should use a vector. Chapter 8 discusses vectors in more detail.

However, arrays are more useful when you know the number of elements will not need to change. For example, if you were using the names of the month in a program, you would probably use an array rather than a vector because you know it will always contain 12 elements:

let months = ["January", "February", "March", "April", "May", "June", "July", "August", "September", "October", "November", "December"];

You write an array’s type using square brackets with the type of each element, a semicolon, and then the number of elements in the array, like so:

let a: [i32; 5] = [1, 2, 3, 4, 5];

Here, i32 is the type of each element. After the semicolon, the number 5 indicates the array contains five elements.

You can also initialize an array to contain the same value for each element by specifying the initial value, followed by a semicolon, and then the length of the array in square brackets, as shown here:

let a = [3; 5];

The array named a will contain 5 elements that will all be set to the value 3 initially. This is the same as writing let a = [3, 3, 3, 3, 3]; but in a more concise way.

Accessing Array Elements

An array is a single chunk of memory of a known, fixed size that can be allocated on the stack. You can access elements of an array using indexing, like this:

Filename: src/main.rs

fn main() {
    let a = [1, 2, 3, 4, 5];
 
    let first = a[0];
    let second = a[1];
}

In this example, the variable named first will get the value 1 because that is the value at index [0] in the array. The variable named second will get the value 2 from index [1] in the array.

Invalid Array Element Access

Let’s see what happens if you try to access an element of an array that is past the end of the array. Say you run this code, similar to the guessing game in Chapter 2, to get an array index from the user:

Filename: src/main.rs

use std::io;
 
fn main() {
    let a = [1, 2, 3, 4, 5];
 
    println!("Please enter an array index.");
 
    let mut index = String::new();
 
    io::stdin()
        .read_line(&mut index)
        .expect("Failed to read line");
 
    let index: usize = index
        .trim()
        .parse()
        .expect("Index entered was not a number");
 
    let element = a[index];
 
    println!("The value of the element at index {index} is: {element}");
}

This code compiles successfully. If you run this code using cargo run and enter 0, 1, 2, 3, or 4, the program will print out the corresponding value at that index in the array. If you instead enter a number past the end of the array, such as 10, you’ll see output like this:

thread 'main' panicked at 'index out of bounds: the len is 5 but the index is 10', src/main.rs:19:19 note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

The program resulted in a runtime error at the point of using an invalid value in the indexing operation. The program exited with an error message and didn’t execute the final println! statement. When you attempt to access an element using indexing, Rust will check that the index you’ve specified is less than the array length. If the index is greater than or equal to the length, Rust will panic. This check has to happen at runtime, especially in this case, because the compiler can’t possibly know what value a user will enter when they run the code later.

This is an example of Rust’s memory safety principles in action. In many low-level languages, this kind of check is not done, and when you provide an incorrect index, invalid memory can be accessed. Rust protects you against this kind of error by immediately exiting instead of allowing the memory access and continuing. Chapter 9 discusses more of Rust’s error handling and how you can write readable, safe code that neither panics nor allows invalid memory access.

Variables and Mutability

  1. let: define an immutable variable
  2. let mut: define a mutable variable
  3. const: define a constant.

Functions

Rust code uses snake case as the conventional style for function and variable names, in which all letters are lowercase and underscores separate words.

fn func_name(p1:p1_type, p2:p2_type) -> return_type {
	do sth;
	return implicitly or obviously
}

Above is an example of function in Rust, return value can be returned as a single expression or as a return statement.

Comments

Literally same as C/C++, use separated // instead of /**/. While Rust has another kind of comments, which will be covered in Crates.

Control Flows

  • if
    1. if must have a boolean as condition. if (a) in c++ should be if a != 0 in Rust.
    2. if can be used in let statement: let var = if condition {} else {};.
  • loop
    • 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. break and continue are as the same as anywhere else. break can return expected value to the result of loop, e.g.: let result = loop { break 5;} loop can be labeled like: 'label_name: loop{}, and continue or break can exactly works on that label like break 'label_name.
    • while and for are the same as other languages.
    • collections should iterated by for, it can also be iterated in reverse by calling .rev() function.

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