# 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. A plus is that the formatting correctness will be checked at compile time.

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

        // Without a suffix, 31 becomes an i32. You can change what type 31 is,
        // with a suffix.

        // There are various optional patterns this works with. Positional
        // arguments can be used.
        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");

        // Special formatting can be specified after a `:`.
        println!("{} of {:b} people know binary, the other half doesn't", 1, 2);

        // You can right-align text with a specified width. This will output
        // "     1". 5 white spaces and a "1".
        println!("{number:>width$}", number=1, width=6);

        // You can pad numbers with extra zeroes. This will output "000001".
        println!("{number:>0width$}", number=1, width=6);

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

        // Create a structure which contains an `i32`. Name it `Structure`.
        #[allow(dead_code)]
        struct Structure(i32);

        // However, custom types such as this structure require more complicated
        // handling. This will not work.
        println!("This struct `{}` won't print...", Structure(3));
        // FIXME ^ Comment out this line.
    }

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

### 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`