67 lines
2.5 KiB
Markdown
67 lines
2.5 KiB
Markdown
|
# Formatted output
|
||
|
|
|
||
|
|
```rust,editable,ignore,mdbook-runnable
|
||
|
|
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.
|
||
|
|
|
||
|
|
// 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`][fmt] contains many [`traits`][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`][string].
|