Rust-Lang Formatting Syntax - Everything Technology

There has been a buzz about Rust lang for a while so today I decided to embark on the journey to learn Rust lang with a plan to spend one hour every day on the official website https://doc.rust-lang.org/rust-by-example/.

Most of the explanations and activities were fairly straightforward requiring you to refer to this documentation https://doc.rust-lang.org/std/fmt/.

The only question which I felt left me a bit lost was the last one where you had to display a number in Hex and with padding. After going through the docs and realising that the following message actually meant that the format syntax was wrong:

   Compiling playground v0.0.1 (/playground)
error: invalid format string: expected `'}'`, found `'0'`
  --> src/main.rs:33:57
   |
33 |         write!(f, "RGB ({red}, {green}, {blue}) 0x{red:X02}{green:02X}{blue:02X}",
   |                                                   -     ^ expected `}` in format string
   |                                                   |
   |                                                   because of this opening brace
   |
   = note: if you intended to print `{`, you can escape it using `{{`

error: aborting due to previous error

error: could not compile `playground`

To learn more, run the command again with --verbose.

I went ahead to read the syntax part of the documentation at https://doc.rust-lang.org/std/fmt/#syntax

In there I found out that the order in which the format specifications is given is important. For example the above error happens because hex format spec should be after the padding i.e {red:X02} -> {red:02X}.

If you are curious the correct final code looks as follows:

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,
}

impl Display for Color {
    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
        write!(f, "RGB ({red}, {green}, {blue}) 0x{red:02X}{green:02X}{blue:02X}",
               red=self.red, green=self.green, blue=self.blue)
    }
}

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);
    }
}

Hope this helped you if you were stuck in Rust-Lang formatting question. I am also streaming learning Rust live at https://www.twitch.tv/lougarou_ everyday at 8pm GMT+4 if you are interested to learn together with me.

By Lougarou