Rust, a code snippet introduction

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A basic introduction in Rust, with code snippets taken from the The Rust Programming Language and Rust by Example books, to present in a 40 minutes slot.

You can execute most of the snippets directly on Rust Playground

Hello world!

// This is the main function
fn main() {
    // Statements here are executed when the compiled binary is called

    // Print text to the console
    println!("Hello World!");
}

Use the rust compiler rustc to compile the code

rustc hello.rs

rustc will produce a hello binary that can be executed.

./hello

Println macro

fn main() {
    // In general, the `{}` will be automatically replaced with any
    // arguments. These will be stringified.
    println!("{} days", 31);

    // Without a suffix, 31 becomes an i32. You can change what type 31 is
    // by providing a suffix. The number 31i64 for example has the type i64.

    // There are various optional patterns this works with. Positional
    // arguments can be used.
    println!("{0}, this is {1}. {1}, this is {0}", "Alice", "Bob");

    // As can named arguments.
    println!("{subject} {verb} {object}",
             object="the lazy dog",
             subject="the quick brown fox",
             verb="jumps over");

    // Special formatting can be specified after a `:`.
    println!("{} of {:b} people know binary, the other half doesn't", 1, 2);

    // You can right-align text with a specified width. This will output
    // "     1". 5 white spaces and a "1".
    println!("{number:>width$}", number=1, width=6);

    // You can pad numbers with extra zeroes. This will output "000001".
    println!("{number:0>width$}", number=1, width=6);

    // Rust even checks to make sure the correct number of arguments are
    // used.
    println!("My name is {0}, {1} {0}", "Bond");
    // FIXME ^ Add the missing argument: "James"
}

Fix the compilation error


Variables

const STARTING_MISSILES: i32 = 8;
const READY_AMOUNT: i32 = 2;

fn main() {
    let mut missiles: i32 = STARTING_MISSILES;
    let ready: i32 = READY_AMOUNT;
    println!("Firing {} of my {} missiles...", ready, missiles);
    missiles = missiles - ready;
    println!("{} missiles left", missiles);
}

Mutability

Variable bindings are immutable by default, but this can be overridden using the mut modifier.

fn main() {
    let _immutable_binding = 1;
    let mut mutable_binding = 1;

    println!("Before mutation: {}", mutable_binding);

    // Ok
    mutable_binding += 1;

    println!("After mutation: {}", mutable_binding);

    // Error!
    _immutable_binding += 1;
    // FIXME ^ Comment out this line
}

Variables Scope

fn main() {
    let s = "world";
    {
        let s = "hello";
        println!("{}", s);
    }
    println!("{}", s);
}

Try to guess the output


Primitives

Scalar Types

  • signed integers: i8, i16, i32, i64, i128 and isize (pointer size)
  • unsigned integers: u8, u16, u32, u64, u128 and usize (pointer size)
  • floating point: f32, f64
  • char Unicode scalar values like 'a', 'α' and '∞' (4 bytes each)
  • bool either true or false
  • and the unit type (), whose only possible value is an empty tuple: ()

Compound Types

  • arrays like [1, 2, 3]
  • tuples like (1, true)

Snippet example

fn main() {
    // Variables can be type annotated.
    let logical: bool = true;

    let a_float: f64 = 1.0;  // Regular annotation
    let an_integer   = 5i32; // Suffix annotation

    // Or a default will be used.
    let default_float   = 3.0; // `f64`
    let default_integer = 7;   // `i32`

    // A type can also be inferred from context
    let mut inferred_type = 12; // Type i64 is inferred from another line
    inferred_type = 4294967296i64;

    // A mutable variable's value can be changed.
    let mut mutable = 12; // Mutable `i32`
    mutable = 21;

    // Error! The type of a variable can't be changed.
    mutable = true;

    // Variables can be overwritten with shadowing.
    let mutable = true;
}

Tuples

A tuple is a collection of values of different types. Tuples are constructed using parentheses (), and each tuple itself is a value with type signature (T1, T2, ...), where T1, T2 are the types of its members.

Functions can use tuples to return multiple values, as tuples can hold any number of values.

fn main() {
    let user = ("Adrian", 38);
    println!("User {} is {} years old", user.0, user.1);

    // tuples within tuples
    let employee = (("Adrian", 38), "die Mobiliar");
    println!("User {} is {} years old and works for {}", employee.0.1, employee.0.1, employee.1);
}


Functions

fn main() {
    println!("Sum result {}", sum(5,8));
}

fn sum(x:i32, y:i32) -> i32 {
    let z = {
         x + y
    };
   z
}


Ownership

Stack & Heap

  • both parts of memory, but structured differently
  • Stack (organized & fast)
    • data is stored in order
    • all data stored on the stack must have a known, fixed size
    • LIFO (Last In First Out) - push data on the stack and pop data of the stack
  • Heap (less organized & slow)
    • stores data with an unkonwn/variable size at compile time or a size that might change
    • unordered storage
    • data is allocated and a pointer (known, fixed size stored on the stack) the location/space on the heap is returned

Ownership rules

Ownership is Rust’s most unique feature, and it enables Rust to make memory safety guarantees without needing a garbage collector.

  1. Each value in Rust has a variable that’s called its owner.
  2. Only one owner of a value
  3. When the owner goes out of scope, the value will be dropped.

Take ownership - snippet

#[allow(unused_variables)]
fn main() {
    let s1 = String::from("hello");
    let s2 = s1;

    println!("{}, world!", s1);
}

Fix snippet above

Another move situation.

fn main() {
    let s1 = String::from("hello");
    do_stuff_with_string(s1);

    println!("{}, world!", s1);
}

#[allow(unused_variables)]
fn do_stuff_with_string(s: String) {
    //do_stuff
}

Fix

fn main() {
    let s1 = String::from("hello");
    do_stuff_with_string(&s1);

    println!("{}, world!", s1);
}

#[allow(unused_variables)]
fn do_stuff_with_string(s: &String) {
    //do_stuff
}

reference pointer

Structs

A struct, or structure, is a custom data type that lets you name and package together multiple related values that make up a meaningful group. If you’re familiar with an object-oriented language, a struct is like an object’s data attributes.

#[derive(Debug)]
struct Rectangle {
    width: u32,
    height: u32,
}

impl Rectangle {
    fn area(&self) -> u32 {
        self.width * self.height
    }
}

fn main() {
    let rect1 = Rectangle {
        width: 30,
        height: 50,
    };

    println!(
        "The area of the rectangle is {} square pixels.",
        rect1.area()
    );
}

Note:

  • to define the function within the context of Rectangle, we start an impl (implementation) block.
  • we’ve chosen &self here for the same reason we used &Rectangle in the function version: we don’t want to take ownership, and we just want to read the data in the struct, not write to it.

Traits

A trait tells the Rust compiler about functionality a particular type has and can share with other types. We can use traits to define shared behavior in an abstract way. We can use trait bounds to specify that a generic can be any type that has certain behavior.

Note: Traits are similar to a feature often called interfaces in other languages, although with some differences.

pub trait Summary {
    fn summarize(&self) -> String;
}

pub struct NewsArticle {
    pub headline: String,
    pub location: String,
    pub author: String,
    pub content: String,
}

impl Summary for NewsArticle {
    fn summarize(&self) -> String {
        format!("{}, by {} ({})", self.headline, self.author, self.location)
    }
}

pub struct Tweet {
    pub username: String,
    pub content: String,
    pub reply: bool,
    pub retweet: bool,
}

impl Summary for Tweet {
    fn summarize(&self) -> String {
        format!("{}: {}", self.username, self.content)
    }
}


fn main() {
    let tweet = Tweet {
        username: String::from("horse_ebooks"),
        content: String::from(
            "of course, as you probably already know, people",
        ),
        reply: false,
        retweet: false,
    };

    println!("1 new tweet: {}", tweet.summarize());
}

Enums

#[derive(Debug)]
enum UsState {
    Alabama,
    Alaska,
    // --snip--
}

enum Coin {
    Penny,
    Nickel,
    Dime,
    Quarter(UsState),
}

fn value_in_cents(coin: Coin) -> u8 {
    match coin {
        Coin::Penny => 1,
        Coin::Nickel => 5,
        Coin::Dime => 10,
        Coin::Quarter(state) => {
            println!("State quarter from {:?}!", state);
            25
        }
    }
}

fn main() {
    let value = value_in_cents(Coin::Quarter(UsState::Alaska));
    println!("{}", value);
}

Control flow

  • no brackets in if conditions
if num == 5 {
    msg = "five";
} else if num == 4 {
    msg = "four";
} else {
    msg = "other";
}

can be rewritten to(if is an expression not a statement):

msg = if num == 5 {
    "five"
} else if num == 4 {
    "four"
} else {
    "other"
}; //Note the semicolon at the end
//all the branches must return the same type (rust is strongly typed)

Unconditional loop

loop {
    break;
}

'outer_loop: loop { //define loop label "tick outer_loop"
    loop {
        loop {
            break 'outer_loop;
        }
    }
}

while must_evaluate_to_bool_expresion() { //no type coercion in Rust
    // do while stuff
}

syntachtic sugar to

loop {
    if !must_evaluate_to_bool_expresion() { break }
    //do stuff
}

No do while, but

loop {
    //do stuff
    if !must_evaluate_to_bool_expresion() { break }
}

for iterates over iterable values

for num in [1, 2, 3].iter() {
    // do stuff with num
}

let array = [(1,2), (3,4)];
for (x, y) in array.iter() { //destructuring
    // do stuff with x and y // x, y are local
}

Ranges

for num in 0..50 { //start inclusive, end exclusive
    // do stuff with num
}

for num in 0..=50 { //both start and end inclusive
    // do stuff with num
}

Collections

Vectors

fn main() {

/*
    let mut v = Vec::new();

    v.push(1);
    v.push(2);
    v.push(3);
    v.push(4);
    v.push(5);
*/

    let v = vec![1, 2, 3, 4, 5];

    let third: &i32 = &v[2];
    println!("The third element is {}", third);

    match v.get(2) {
        Some(third) => println!("The third element is {}", third),
        None => println!("There is no third element."),
    }

    //uncomment to panic the compiler
    //let does_not_exist = &v[100];
    //let does_not_exist = v.get(100);
}

Hashmaps

fn main() {
    use std::collections::HashMap;

    let mut scores = HashMap::new();
    scores.insert(String::from("Blue"), 10);

    scores.entry(String::from("Yellow")).or_insert(50);
    scores.entry(String::from("Blue")).or_insert(50);

    println!("{:?}", scores);
}

Generics

struct Point<T> {
    x: T,
    y: T,
}

#[allow(unused_variables)]
fn main() {
    let integer = Point { x: 5, y: 10 };
    let float = Point { x: 1.0, y: 4.0 };

    let wont_work = Point { x: 5, y: 4.0 }; // Try fix me
}

Rust Module System

Rust has a number of features that allow you to manage your code’s organization, including which details are exposed, which details are private, and what names are in each scope in your programs. These features, sometimes collectively referred to as the module system, include:

  • Packages: A Cargo feature that lets you build, test, and share crates
  • Crates: A tree of modules that produces a library or executable
  • Modules and use: Let you control the organization, scope, and privacy of paths
  • Paths: A way of naming an item, such as a struct, function, or module
crate
 └── front_of_house
     ├── hosting
     │   ├── add_to_waitlist
     │   └── seat_at_table
     └── serving
         ├── take_order
         ├── serve_order
         └── take_payment
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About the Author

Adrian Matei (aka adixchen)

Adrian Matei (aka adixchen)
Life force expressing itself as a coding capable human being
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