Compound Types

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);

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.

  • We can use literals to assign values to arrays.

  • In the main function, the print statement asks for the debug implementation with the ? format parameter: {} gives the default output, {:?} gives the debug output. We could also have used {a} and {a:?} without specifying the value after the format string.

  • 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.