# Compound Types

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

Array assignment and access:

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

Tuple assignment and access:

```rust,editable
fn main() {
    let t: (i8, bool) = (7, true);
    println!("1st index: {}", t.0);
    println!("2nd index: {}", t.1);
}
```

<details>

Key points:

Arrays:

* Arrays have elements of the same type, `T`, and length, `N`, which is a compile-time constant.
  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.

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>