Formatted print
Printing is handled by a series of macros
defined in std::fmt
some of which include:
format!
: write formatted text toString
print!
: same asformat!
but the text is printed to the console (io::stdout).println!
: same asprint!
but a newline is appended.eprint!
: same asformat!
but the text is printed to the standard error (io::stderr).eprintln!
: same aseprint!
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); // Without a suffix, 31 becomes an i32. You can change what type 31 is // by providing a suffix. The number 31i64 for example has the type i64. // 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$}", number=1, width=6); // You can pad numbers with extra zeroes. This will output "000001". println!("{number:0>width$}", number=1, width=6); // 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" // Create a structure named `Structure` which contains an `i32`. #[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, 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, uselet pi = 3.141592
as an estimate for pi. (Hint: you may need to check thestd::fmt
documentation for setting the number of decimals to display)