9.1 Print macros

The print macros are used to print on console, either on standard output (a.k.a.: stdout) or standard error (a.k.a.: stderr). The ones terminating with the suffix ln add a line feed character at the end of the printed string. See Table 1 for a sum-up of those macros.

All macros take a literal string as first argument, then zero or more values that are used to replace format string markers {}.

9.1.1Printing of simple objects¶

Printing a simple string:

println!("Hello, world!");

Hello, world!

Using unnamed markers:

println!("my values: {} {}", 12.34, 789);

my values: 12.34 789

Using named markers:

println!("my_values: {a} - {b}", b=12.34, a=789);

my_values: 789 - 12.34

Printing a variable:

let my_var = 123;
println!("my_var: {my_var}");

my_var: 123

Insert leading characters:

let my_var = 123;
println!("my_var: {my_var:05}");

my_var: 00123

Setting precision on a float:

println!("x: {:.5}", 0.123456789);

x: 0.12346

Using a variable for precision:

let prec = 3;
println!("x: {:.*}", prec, 0.123456789);

mismatched types

9.1.2Printing of complex objects¶

Printing a complex object requires a special formatter. The easiest way is to use the Debug formatter that is included in core objects and objects from the standard library. For custom types, the Debug trait can easily be implemented using the derive() attribute as we will see later.

Let us define a tuple object with three values:

:clear

let t = (1.5, "abc", true);

The Debug formatter being implemented for tuple objects, we can use the special debug formatter operator ? to print our tuple:

println!("{t:?}");

Using the # option, we can also enable the pretty-print output:

println!("{t:#?}");