13.2 while/let pattern

The while/let pattern allows us to stay in a loop, as long as some matching is true.

Let us define a tagged union to register measurement values and the absence values:

:clear

enum Measure {
  Value(f64),
  Nil,
}

We define then a function that generates some Measure values depending on an index:

fn get_measure(i: usize) -> Measure {
  if i < 10 {
    Measure::Value(1.12 * i as f64)
  }
  else {
    Measure::Nil
  }
}

Let us initialize a global index:

let mut i: usize = 0;

And use it to iterate over the Measure values returned by the get_measure() function. We use the while/let construct to extract the float values returned by the function. As soon as a Nil is returned, we will quit the loop:

while let Measure::Value(x) = get_measure(i) {
  print!("{x} ");
  i += 1;
}
println!();

0 1.12 2.24 3.3600000000000003 4.48 5.6000000000000005 6.720000000000001 7.840000000000001 8.96 10.080000000000002