src/primitives/tuples.md

# Tuples

A tuple is a collection of values of different types. Tuples are constructed
using parentheses `()`, and each tuple itself is a value with type signature
`(T1, T2, ...)`, where `T1`, `T2` are the types of its members. Functions can
use tuples to return multiple values, as tuples can hold any number of values.

```rust,editable
// Tuples can be used as function arguments and as return values.
fn reverse(pair: (i32, bool)) -> (bool, i32) {
    // `let` can be used to bind the members of a tuple to variables.
    let (int_param, bool_param) = pair;

    (bool_param, int_param)
}

// The following struct is for the activity.
#[derive(Debug)]
struct Matrix(f32, f32, f32, f32);

fn main() {
    // A tuple with a bunch of different types.
    let long_tuple = (1u8, 2u16, 3u32, 4u64,
                      -1i8, -2i16, -3i32, -4i64,
                      0.1f32, 0.2f64,
                      'a', true);

    // Values can be extracted from the tuple using tuple indexing.
    println!("Long tuple first value: {}", long_tuple.0);
    println!("Long tuple second value: {}", long_tuple.1);

    // Tuples can be tuple members.
    let tuple_of_tuples = ((1u8, 2u16, 2u32), (4u64, -1i8), -2i16);

    // Tuples are printable.
    println!("tuple of tuples: {:?}", tuple_of_tuples);

    // But long Tuples (more than 12 elements) cannot be printed.
    //let too_long_tuple = (1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13);
    //println!("Too long tuple: {:?}", too_long_tuple);
    // TODO ^ Uncomment the above 2 lines to see the compiler error

    let pair = (1, true);
    println!("Pair is {:?}", pair);

    println!("The reversed pair is {:?}", reverse(pair));

    // To create one element tuples, the comma is required to tell them apart
    // from a literal surrounded by parentheses.
    println!("One element tuple: {:?}", (5u32,));
    println!("Just an integer: {:?}", (5u32));

    // Tuples can be destructured to create bindings.
    let tuple = (1, "hello", 4.5, true);

    let (a, b, c, d) = tuple;
    println!("{:?}, {:?}, {:?}, {:?}", a, b, c, d);

    let matrix = Matrix(1.1, 1.2, 2.1, 2.2);
    println!("{:?}", matrix);
}
```

### Activity

1. *Recap*: Add the `fmt::Display` trait to the `Matrix` struct in the above
   example, so that if you switch from printing the debug format `{:?}` to the
   display format `{}`, you see the following output:

   ```text
   ( 1.1 1.2 )
   ( 2.1 2.2 )
   ```

   You may want to refer back to the example for [print display][print_display].
2. Add a `transpose` function using the `reverse` function as a template, which
   accepts a matrix as an argument, and returns a matrix in which two elements
   have been swapped. For example:

   ```rust,ignore
   println!("Matrix:\n{}", matrix);
   println!("Transpose:\n{}", transpose(matrix));
   ```

   Results in the output:

   ```text
   Matrix:
   ( 1.1 1.2 )
   ( 2.1 2.2 )
   Transpose:
   ( 1.1 2.1 )
   ( 1.2 2.2 )
   ```

[print_display]: ../hello/print/print_display.md