comprehensive-rust/src/tuples-and-arrays/tuples-and-arrays.md

---
minutes: 10
---

# 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.

|        | Types                         | Literals                          |
| ------ | ----------------------------- | --------------------------------- |
| Arrays | `[T; N]`                      | `[20, 30, 40]`, `[0; 3]`          |
| Tuples | `()`, `(T,)`, `(T1, T2)`, ... | `()`, `('x',)`, `('x', 1.2)`, ... |

Array assignment and access:

<!-- mdbook-xgettext: skip -->

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

Tuple assignment and access:

<!-- mdbook-xgettext: skip -->

```rust,editable
fn main() {
    let t: (i8, bool) = (7, true);
    println!("t.0: {}", t.0);
    println!("t.1: {}", t.1);
}
```

<details>

Key points:

Arrays:

- 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.

Tuples:

- 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.

</details>