# Rust by Example

## 1.2 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.
• println!: same as print! 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.
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)