src/hello/print/fmt.md

# Formatting

We've seen that formatting is specified via a *format string*:

* `format!("{}", foo)` -> `"3735928559"`
* `format!("0x{:X}", foo)` -> [`"0xDEADBEEF"`][deadbeef]
* `format!("0o{:o}", foo)` -> `"0o33653337357"`

The same variable (`foo`) can be formatted differently depending on which
*argument type* is used: `X` vs `o` vs *unspecified*.

This formatting functionality is implemented via traits, and there is one trait
for each argument type. The most common formatting trait is `Display`, which
handles cases where the argument type is left unspecified: `{}` for instance.

```rust,editable
use std::fmt::{self, Formatter, Display};

struct City {
    name: &'static str,
    // Latitude
    lat: f32,
    // Longitude
    lon: f32,
}

impl Display for City {
    // `f` is a buffer, and this method must write the formatted string into it.
    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
        let lat_c = if self.lat >= 0.0 { 'N' } else { 'S' };
        let lon_c = if self.lon >= 0.0 { 'E' } else { 'W' };

        // `write!` is like `format!`, but it will write the formatted string
        // into a buffer (the first argument).
        write!(f, "{}: {:.3}°{} {:.3}°{}",
               self.name, self.lat.abs(), lat_c, self.lon.abs(), lon_c)
    }
}

#[derive(Debug)]
struct Color {
    red: u8,
    green: u8,
    blue: u8,
}

fn main() {
    for city in [
        City { name: "Dublin", lat: 53.347778, lon: -6.259722 },
        City { name: "Oslo", lat: 59.95, lon: 10.75 },
        City { name: "Vancouver", lat: 49.25, lon: -123.1 },
    ] {
        println!("{}", city);
    }
    for color in [
        Color { red: 128, green: 255, blue: 90 },
        Color { red: 0, green: 3, blue: 254 },
        Color { red: 0, green: 0, blue: 0 },
    ] {
        // Switch this to use {} once you've added an implementation
        // for fmt::Display.
        println!("{:?}", color);
    }
}
```

You can view a [full list of formatting traits][fmt_traits] and their argument
types in the [`std::fmt`][fmt] documentation.

### Activity

Add an implementation of the `fmt::Display` trait for the `Color` struct above
so that the output displays as:

```text
RGB (128, 255, 90) 0x80FF5A
RGB (0, 3, 254) 0x0003FE
RGB (0, 0, 0) 0x000000
```

Two hints if you get stuck:

* You [may need to list each color more than once][named_parameters].
* You can [pad with zeros to a width of 2][fmt_width] with `:0>2`.
For hexadecimals, you can use `:02X`.

Bonus:

* If you would like to experiment with [type casting][type_casting] in advance,
the formula for [calculating a color in the RGB color space][rgb_color] is
`RGB = (R * 65_536) + (G * 256) + B`, where `R is RED, G is GREEN, and B is BLUE`.
An unsigned 8-bit integer (`u8`) can only hold numbers up to 255. To cast `u8` to `u32`, you can write `variable_name as u32`.

### See also:

[`std::fmt`][fmt]

[rgb_color]: https://www.rapidtables.com/web/color/RGB_Color.html#rgb-format
[named_parameters]: https://doc.rust-lang.org/std/fmt/#named-parameters
[deadbeef]: https://en.wikipedia.org/wiki/Deadbeef#Magic_debug_values
[fmt]: https://doc.rust-lang.org/std/fmt/
[fmt_traits]: https://doc.rust-lang.org/std/fmt/#formatting-traits
[fmt_width]: https://doc.rust-lang.org/std/fmt/#width
[type_casting]: ../../types/cast.md