diff options
Diffstat (limited to '')
| -rw-r--r-- | src/doc/rust-by-example/src/hello/print.md | 66 |
1 files changed, 35 insertions, 31 deletions
diff --git a/src/doc/rust-by-example/src/hello/print.md b/src/doc/rust-by-example/src/hello/print.md index cce838fc2..55f6ed520 100644 --- a/src/doc/rust-by-example/src/hello/print.md +++ b/src/doc/rust-by-example/src/hello/print.md @@ -1,12 +1,14 @@ # Formatted print -Printing is handled by a series of [`macros`][macros] defined in [`std::fmt`][fmt] -some of which include: +Printing is handled by a series of [`macros`][macros] defined in +[`std::fmt`][fmt] some of which include: * `format!`: write formatted text to [`String`][string] -* `print!`: same as `format!` but the text is printed to the console (io::stdout). +* `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 `print!` but the text is printed to the standard error (io::stderr). +* `eprint!`: same as `print!` 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 @@ -20,7 +22,7 @@ fn main() { // Positional arguments can be used. Specifying an integer inside `{}` // determines which additional argument will be replaced. Arguments start - // at 0 immediately after the format string + // at 0 immediately after the format string. println!("{0}, this is {1}. {1}, this is {0}", "Alice", "Bob"); // As can named arguments. @@ -29,13 +31,13 @@ fn main() { subject="the quick brown fox", verb="jumps over"); - // Different formatting can be invoked by specifying the format character after a - // `:`. - println!("Base 10: {}", 69420); //69420 - println!("Base 2 (binary): {:b}", 69420); //10000111100101100 - println!("Base 8 (octal): {:o}", 69420); //207454 - println!("Base 16 (hexadecimal): {:x}", 69420); //10f2c - println!("Base 16 (hexadecimal): {:X}", 69420); //10F2C + // Different formatting can be invoked by specifying the format character + // after a `:`. + println!("Base 10: {}", 69420); // 69420 + println!("Base 2 (binary): {:b}", 69420); // 10000111100101100 + println!("Base 8 (octal): {:o}", 69420); // 207454 + println!("Base 16 (hexadecimal): {:x}", 69420); // 10f2c + println!("Base 16 (hexadecimal): {:X}", 69420); // 10F2C // You can right-justify text with a specified width. This will @@ -43,32 +45,31 @@ fn main() { println!("{number:>5}", number=1); // You can pad numbers with extra zeroes, - //and left-adjust by flipping the sign. This will output "10000". + // and left-adjust by flipping the sign. This will output "10000". println!("{number:0<5}", number=1); - // You can use named arguments in the format specifier by appending a `$` + // You can use named arguments in the format specifier by appending a `$`. println!("{number:0>width$}", number=1, width=5); - // Rust even checks to make sure the correct number of arguments are - // used. + // Rust even checks to make sure the correct number of arguments are used. println!("My name is {0}, {1} {0}", "Bond"); // FIXME ^ Add the missing argument: "James" // Only types that implement fmt::Display can be formatted with `{}`. User- - // defined types do not implement fmt::Display by default + // defined types do not implement fmt::Display by default. - #[allow(dead_code)] + #[allow(dead_code)] // disable `dead_code` which warn against unused module struct Structure(i32); // This will not compile because `Structure` does not implement - // fmt::Display + // fmt::Display. //println!("This struct `{}` won't print...", Structure(3)); // TODO ^ Try uncommenting this line // For Rust 1.58 and above, you can directly capture the argument from a // surrounding variable. Just like the above, this will output - // " 1". 4 white spaces and a "1". + // " 1", 4 white spaces and a "1". let number: f64 = 1.0; let width: usize = 5; println!("{number:>width$}"); @@ -80,7 +81,7 @@ 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. + 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. @@ -88,21 +89,22 @@ 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]. +In *line 46*, `#[allow(dead_code)]` is an [attribute] which only apply to the module after it. + ### Activities - * Fix the issue in the above code (see FIXME) so that it runs without - error. - * Try uncommenting the line that attempts to format the `Structure` struct (see TODO) - * Add a `println!` macro call 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`][fmt] documentation for setting the number of - decimals to display) +* Fix the issue in the above code (see FIXME) so that it runs without + error. +* Try uncommenting the line that attempts to format the `Structure` struct + (see TODO) +* Add a `println!` macro call 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`][fmt] documentation for setting the number of decimals to display) ### See also: -[`std::fmt`][fmt], [`macros`][macros], [`struct`][structs], -and [`traits`][traits] +[`std::fmt`][fmt], [`macros`][macros], [`struct`][structs], [`traits`][traits], and [`dead_code`][dead_code] [fmt]: https://doc.rust-lang.org/std/fmt/ [macros]: ../macros.md @@ -111,3 +113,5 @@ and [`traits`][traits] [traits]: https://doc.rust-lang.org/std/fmt/#formatting-traits [`ToString`]: https://doc.rust-lang.org/std/string/trait.ToString.html [convert]: ../conversion/string.md +[attribute]: ../attribute.md +[dead_code]: ../attribute/unused.md |