Why does't Rust match an empty array?

Question

I'm trying to match on an array in Rust:

let s = "x,y,z";
let v: Vec<_> = s.split(',').collect();
let (x, y, z) = match &v[..] {
    [x, y, z] => (x,y,z),
    [] => return Err("empty"),
    _ => return Err("wrong length"),
};
Ok(v)

When passing "" as s, I would expect to receive an Err("empty"), but I receive Err("wrong length") instead. What am I doing wrong?

Next time you could try `dbg!(&v[..])` or use a [debugger](https://stackoverflow.com/questions/25385173/what-is-a-debugger-and-how-can-it-help-me-diagnose-problems) to see what value you're actually getting. — cafce25, Dec 09 '22 at 17:48

score 4 · Accepted Answer · answered Dec 09 '22 at 17:48

s.split(sep) will always return at least one element: if the separator sep doesn't appear in the input string, it will just yield a single element that is the entire string. More generally, if the separator sep appears N times in the string s, the iterator will always yield N+1 elements.

So when you call "".split(','), you will get an iterator of a single element "" because the separator appears 0 times.

Here are two different approaches you can use if you want to avoid this:

// filter any empty segments, for example:
//   "" => [] instead of [""]
//   "x,y" => ["x", "y"]
//   "x,,y" => ["x", "y"] instead of ["x", "", "y"]
let v: Vec<_> = s.split(',').filter(|item| !item.is_empty()).collect();

// return empty array if input is empty, otherwise return all segments (even if empty):
//   "" => [] instead of [""]
//   "x,y" => ["x", "y"]
//   "x,,y" => ["x", "", "y"]
let v: Vec<_> = if s.is_empty() {
    vec![]
} else {
    s.split(',').collect()
};

Why does't Rust match an empty array?

1 Answers1