Out parameter. Often in more complex Rust programs we want to have out parameters in a function. With "out" parameters, values assigned are retrieved outside the function.
Mut references. In Rust we do not have an "out" keyword, but we can pass local variables (like usize) as a mutable reference. Then we dereference the arguments and modify the value.
Example. Here we have a str that contains an ampersand, and several period characters. We want to count these characters in a single function, and retrieve the counts.
Thus We implement test_characters. We use a for-loop over the string chars, and match each char.
Info We pass the ampersand and period local variables to test_characters. We then dereference them and increment them.
fn test_characters(value: &str, ampersand: &mut usize, period: &mut usize) {
// Use mutable references as out parameters.
for v in value.chars() {
match v {
'&' => *ampersand += 1,
'.' => *period += 1,
_ => (),
}
}
}
fn main() {
let data = "A cat & a dog...";
let mut ampersand = 0;
let mut period = 0;
test_characters(&data, &mut ampersand, &mut period);
// Print final counts.
println!("{ampersand} {period}");
}1 3
Multiple return values. Sometimes it is more elegant to return a tuple containing multiple values from a function. This can sometimes make programs more clear.
However Using "out" parameters (mut references) can be a better solution in other situations.
It is possible to implement "out" parameters in Rust programs with mut references. The syntax is confusing at first, but can be memorized with practice.
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