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, 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 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
Three hints if you get stuck:
- The formula for calculating a color in the RGB color space is:
RGB = (R*65536)+(G*256)+B , (when R is RED, G is GREEN and B is BLUE)
. For more see RGB color format & calculation. - You may need to list each color more than once.
- You can pad with zeros to a width of 2 with
:0>2
.