Compound types: tuples and arrays

Why compound types?

Often one value is not enough. A train has a number and a name; a week has seven temperatures. A compound type groups several values into one. Rust has two built-in ones: the tuple and the array. Later you will meet structs (named groups) and Vec (a list that can grow).

Tuples

A tuple is a fixed-size group of values that can have different types. You write it with round brackets. Its type lists each part's type.

fn main() {
    let train: (u32, &str, f32) = (12951, "Mumbai Rajdhani", 15.5);
    println!("{} {}", train.0, train.1);   // by position, starting at 0

    let (number, name, hours) = train;     // destructuring
    println!("{name} ({number}) takes {hours} hours");
}

Destructuring means unpacking the parts of a value into separate variables in one line. You can ignore a part with an underscore: let (number, _, _) = train;.

The unit type ()

A tuple with no values, written (), is called the unit type. It means "nothing useful". A function that returns nothing actually returns (). You will see it in error messages like "expected i32, found ()".

Arrays

An array holds a fixed number of values that all have the same type. Its type is written [type; length]. The length is part of the type, so [i32; 3] and [i32; 4] are different types. Arrays live on the stack, which makes them fast (module 3 explains the stack).

fn main() {
    let temps: [i32; 7] = [31, 33, 30, 29, 34, 35, 32];
    let zeros = [0; 5];                 // [0, 0, 0, 0, 0]
    println!("Monday: {}", temps[0]);
    println!("Days: {}", temps.len());
    println!("{:?}", zeros);
    for t in temps {
        print!("{t} ");
    }
    println!();
}

Out of bounds

let marks = [78, 85, 91];
let i = 5;
println!("{}", marks[i]);
// thread 'main' panicked: index out of bounds: the len is 3 but the index is 5

If the index is a literal number the compiler can see, it may even refuse to compile. For indexes that come from users, use marks.get(i), which returns Some(value) or None instead of crashing (see module 5).

Arrays or Vec?

Common mistakes

• Writing temps[7] for the 7th item. Indexes start at 0, so the last of 7 items is temps[6].
• Expecting an array to grow with push. Arrays are fixed; use a Vec.
• Using {} to print an array or tuple. They have no Display format; use {:?}.
• Mixing types in an array, like [1, "two"]. All items must have the same type; use a tuple or a struct instead.