Enums

An enum is short for enumerations. They look very similar to a struct, but are different. Here is the difference:

  • Use a struct when you want one thing AND another thing.
  • Use an enum when you want one thing OR another thing.

So structs are for many things together, while enums are for many choices together.

To declare an enum, write enum and use a code block with the options, separated by commas. Just like a struct, the last part can have a comma or not. We will create an enum called ThingsInTheSky:

enum ThingsInTheSky {
    Sun,
    Stars,
}

fn main() {}

This is an enum because you can either see the sun, or the stars: you have to choose one. These are called variants.

// create the enum with two choices
enum ThingsInTheSky {
    Sun,
    Stars,
}

// With this function we can use an i32 to create ThingsInTheSky.
fn create_skystate(time: i32) -> ThingsInTheSky {
    match time {
        6..=18 => ThingsInTheSky::Sun, // Between 6 and 18 hours we can see the sun
        _ => ThingsInTheSky::Stars, // Otherwise, we can see stars
    }
}

// With this function we can match against the two choices in ThingsInTheSky.
fn check_skystate(state: &ThingsInTheSky) {
    match state {
        ThingsInTheSky::Sun => println!("I can see the sun!"),
        ThingsInTheSky::Stars => println!("I can see the stars!")
    }
}

fn main() {
    let time = 8; // it's 8 o'clock
    let skystate = create_skystate(time); // create_skystate returns a ThingsInTheSky
    check_skystate(&skystate); // Give it a reference so it can read the variable skystate
}

This prints I can see the sun!.

You can add data to an enum too.

enum ThingsInTheSky {
    Sun(String), // Now each variant has a string
    Stars(String),
}

fn create_skystate(time: i32) -> ThingsInTheSky {
    match time {
        6..=18 => ThingsInTheSky::Sun(String::from("I can see the sun!")), // Write the strings here
        _ => ThingsInTheSky::Stars(String::from("I can see the stars!")),
    }
}

fn check_skystate(state: &ThingsInTheSky) {
    match state {
        ThingsInTheSky::Sun(description) => println!("{}", description), // Give the string the name description so we can use it
        ThingsInTheSky::Stars(n) => println!("{}", n), // Or you can name it n. Or anything else - it doesn't matter
    }
}

fn main() {
    let time = 8; // it's 8 o'clock
    let skystate = create_skystate(time); // create_skystate returns a ThingsInTheSky
    check_skystate(&skystate); // Give it a reference so it can read the variable skystate
}

This prints the same thing: I can see the sun!

You can also "import" an enum so you don't have to type so much. Here's an example where we have to type Mood:: every time we match on our mood:

enum Mood {
    Happy,
    Sleepy,
    NotBad,
    Angry,
}

fn match_mood(mood: &Mood) -> i32 {
    let happiness_level = match mood {
        Mood::Happy => 10, // Here we type Mood:: every time
        Mood::Sleepy => 6,
        Mood::NotBad => 7,
        Mood::Angry => 2,
    };
    happiness_level
}

fn main() {
    let my_mood = Mood::NotBad;
    let happiness_level = match_mood(&my_mood);
    println!("Out of 1 to 10, my happiness is {}", happiness_level);
}

It prints Out of 1 to 10, my happiness is 7. Let's import so we can type less. To import everything, write *. Note: it's the same key as * for dereferencing but is completely different.

enum Mood {
    Happy,
    Sleepy,
    NotBad,
    Angry,
}

fn match_mood(mood: &Mood) -> i32 {
    use Mood::*; // We imported everything in Mood. Now we can just write Happy, Sleepy, etc.
    let happiness_level = match mood {
        Happy => 10, // We don't have to write Mood:: anymore
        Sleepy => 6,
        NotBad => 7,
        Angry => 2,
    };
    happiness_level
}

fn main() {
    let my_mood = Mood::Happy;
    let happiness_level = match_mood(&my_mood);
    println!("Out of 1 to 10, my happiness is {}", happiness_level);
}

Parts of an enum can also be turned into an integer. That's because Rust gives each arm of an enum a number that starts with 0 for its own use. You can do things with it if your enum doesn't have any other data in it.

enum Season {
    Spring, // If this was Spring(String) or something it wouldn't work
    Summer,
    Autumn,
    Winter,
}

fn main() {
    use Season::*;
    let four_seasons = vec![Spring, Summer, Autumn, Winter];
    for season in four_seasons {
        println!("{}", season as u32);
    }
}

This prints:

0
1
2
3

You can give it a different number though if you want - Rust doesn't care and can use it in the same way. Just add an = and your number to the variant that you want to have a number. You don't have to give all of them a number. But if you don't, Rust will just add 1 from the arm before to give it a number.

enum Star {
    BrownDwarf = 10,
    RedDwarf = 50,
    YellowStar = 100,
    RedGiant = 1000,
    DeadStar, // Think about this one. What number will it have?
}

fn main() {
    use Star::*;
    let starvec = vec![BrownDwarf, RedDwarf, YellowStar, RedGiant];
    for star in starvec {
        match star as u32 {
            size if size <= 80 => println!("Not the biggest star."), // Remember: size doesn't mean anything. It's just a name we chose so we can print it
            size if size >= 80 => println!("This is a good-sized star."),
            _ => println!("That star is pretty big!"),
        }
    }
    println!("What about DeadStar? It's the number {}.", DeadStar as u32);
}

This prints:

Not the biggest star.
Not the biggest star.
This is a good-sized star.
This is a good-sized star.
What about DeadStar? It's the number 1001.

DeadStar would have been number 4, but now it's 1001.

Enums to use multiple types

You know that items in a Vec, array, etc. all need the same type (only tuples are different). But you can actually use an enum to put different types in. Imagine we want to have a Vec with u32s or i32s. Of course, you can make a Vec<(u32, i32)> (a vec with (u32, i32) tuples) but we only want one each time. So here you can use an enum. Here is a simple example:

enum Number {
    U32(u32),
    I32(i32),
}

fn main() {}

So there are two variants: the U32 variant with a u32 inside, and the I32 variant with i32 inside. U32 and I32 are just names we made. They could have been UThirtyTwo or IThirtyTwo or anything else.

Now, if we put them into a Vec we just have a Vec<Number>, and the compiler is happy because it's all the same type. The compiler doesn't care that we have either u32 or i32 because they are all inside a single type called Number. And because it's an enum, you have to pick one, which is what we want. We will use the .is_positive() method to pick. If it's true then we will choose U32, and if it's false then we will choose I32.

Now the code looks like this:

enum Number {
    U32(u32),
    I32(i32),
}

fn get_number(input: i32) -> Number {
    let number = match input.is_positive() {
        true => Number::U32(input as u32), // change it to u32 if it's positive
        false => Number::I32(input), // otherwise just give the number because it's already i32
    };
    number
}


fn main() {
    let my_vec = vec![get_number(-800), get_number(8)];

    for item in my_vec {
        match item {
            Number::U32(number) => println!("It's a u32 with the value {}", number),
            Number::I32(number) => println!("It's an i32 with the value {}", number),
        }
    }
}

This prints what we wanted to see:

It's an i32 with the value -800
It's a u32 with the value 8