Tuples and Arrays

Tuples and arrays are the first “compound” types we have seen. All elements of an array have the same type, while tuples can accommodate different types. Both types have a size fixed at compile time.

Arrays[T; N][20, 30, 40], [0; 3]
Tuples(), (T,), (T1, T2), …(), ('x',), ('x', 1.2), …

Array assignment and access:

fn main() {
    let mut a: [i8; 10] = [42; 10];
    a[5] = 0;
    println!("a: {a:?}");

Tuple assignment and access:

fn main() {
    let t: (i8, bool) = (7, true);
    println!("t.0: {}", t.0);
    println!("t.1: {}", t.1);
This slide should take about 10 minutes.

Key points:


  • A value of the array type [T; N] holds N (a compile-time constant) elements of the same type T. Note that the length of the array is part of its type, which means that [u8; 3] and [u8; 4] are considered two different types. Slices, which have a size determined at runtime, are covered later.

  • Try accessing an out-of-bounds array element. Array accesses are checked at runtime. Rust can usually optimize these checks away, and they can be avoided using unsafe Rust.

  • We can use literals to assign values to arrays.

  • The println! macro asks for the debug implementation with the ? format parameter: {} gives the default output, {:?} gives the debug output. Types such as integers and strings implement the default output, but arrays only implement the debug output. This means that we must use debug output here.

  • Adding #, eg {a:#?}, invokes a “pretty printing” format, which can be easier to read.


  • Like arrays, tuples have a fixed length.

  • Tuples group together values of different types into a compound type.

  • Fields of a tuple can be accessed by the period and the index of the value, e.g. t.0, t.1.

  • The empty tuple () is also known as the “unit type”. It is both a type, and the only valid value of that type — that is to say both the type and its value are expressed as (). It is used to indicate, for example, that a function or expression has no return value, as we’ll see in a future slide.

    • You can think of it as void that can be familiar to you from other programming languages.