Formatted print

Printing is handled by a series of macros defined in std::fmt some of which include:

  • format!: write formatted text to String
  • print!: same as format! but the text is printed to the console (io::stdout).
  • println!: same as print! but a newline is appended.
  • eprint!: same as format! but the text is printed to the standard error (io::stderr).
  • eprintln!: same as eprint!but a newline is appended.

All parse text in the same fashion. As a plus, Rust checks formatting correctness at compile time.

fn main() {
    // In general, the `{}` will be automatically replaced with any
    // arguments. These will be stringified.
    println!("{} days", 31);

    // Positional arguments can be used. Specifying an integer inside `{}`
    // determines which additional argument will be replaced. Arguments start
    // at 0 immediately after the format string
    println!("{0}, this is {1}. {1}, this is {0}", "Alice", "Bob");

    // As can named arguments.
    println!("{subject} {verb} {object}",
             object="the lazy dog",
             subject="the quick brown fox",
             verb="jumps over");

    // Different formatting can invoked by specified format character after a
    // `:`.
    println!("Base 10 repr:               {}",   69420);
    println!("Base 2 (binary) repr:       {:b}", 69420);
    println!("Base 8 (octal) repr:        {:o}", 69420);
    println!("Base 16 (hexadecimal) repr: {:x}", 69420);
    println!("Base 16 (hexadecimal) repr: {:X}", 69420);

    // You can right-align text with a specified width. This will output
    // "     1". 5 white spaces and a "1".
    println!("{number:>5}", number=1);

    // You can pad numbers with extra zeroes. This will output "000001".
    println!("{number:0>5}", number=1);

    // You can use named arguments in the format specifier by appending a `$`
    println!("{number:0>width$}", number=1, width=5);


    // Rust even checks to make sure the correct number of arguments are
    // used.
    println!("My name is {0}, {1} {0}", "Bond");
    // FIXME ^ Add the missing argument: "James"

    // Only types that implement fmt::Display can be formatted with `{}`. User-
    // defined types to not implement fmt::Display by default

    #[allow(dead_code)]
    struct Structure(i32);

    // This will not compile because `Structure` does not implement
    // fmt::Display
    println!("This struct `{}` won't print...", Structure(3));
    // FIXME ^ Comment out this line.

    // For Rust 1.58 and above, you can directly capture the argument from
    // surrounding variable. Just like the above, this will output
    // "     1". 5 white spaces and a "1".
    let number: f64 = 1.0;
    let width: usize = 6;
    println!("{number:>width$}");
}

std::fmt contains many traits which govern the display of text. The base form of two important ones are listed below:

  • fmt::Debug: Uses the {:?} marker. Format text for debugging purposes.
  • fmt::Display: Uses the {} marker. Format text in a more elegant, user friendly fashion.

Here, we used fmt::Display because the std library provides implementations for these types. To print text for custom types, more steps are required.

Implementing the fmt::Display trait automatically implements the ToString trait which allows us to convert the type to String.

Activities

  • Fix the two issues in the above code (see FIXME) so that it runs without error.
  • Add a println! macro call that prints: Pi is roughly 3.142 by controlling the number of decimal places shown. For the purposes of this exercise, use let pi = 3.141592 as an estimate for pi. (Hint: you may need to check the std::fmt documentation for setting the number of decimals to display)

See also:

std::fmt, macros, struct, and traits