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.

XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

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 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