# Formatting

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

- `format!("{}", foo)` -> `"3735928559"`
- `format!("0x{:X}", foo)` -> `"0xDEADBEEF"`
- `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.

    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, 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 },
        ].iter() {
            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 },
        ].iter() {
            // Switch this to use {} once you've added an implementation
            // for fmt::Display
            println!("{:?}", *color);
        }
    }

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

### Activity

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

    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,
- You can pad with zeros to a width of 2 with `:02`.

### See also

`std::fmt`