Pattern match - same elements array

1

For an array of fixed size, how do I use pattern matching to check if all elements have a specific value? For example, the below code does not work. Is there an alternative?

#![allow(unused)]
// You can extend a String with some chars:
fn main() {
    let arr: [u8; 5] = [0, 0, 0, 0, 0];
    match arr {
        [0; 5] => println!("All elements are zeroes"),
        [1; 5] => println!("All elements are ones"),
        _ => println!("Something else!")
    }
}
2

Just use ordinary equality: Playground.

Alternatively, write a helper function.

1 Like
3

Thanks. I am aware of these two options. What I am looking for is a flexible check where I can use match where I can check for equality of all the elements in the array as well as do pattern matching as well - in the same match/pattern (based on certain pattern(s) - for example the head or tail being some specific values).

4

There's no "all elements are the same" pattern. Both of the above options allow you to still use the original array and perform additional pattern matching on it.

5

For that you can use a match that checks specific patterns, and then uses variable name patterns with an if condition, like this.

6

You can also match against a named constant:

  const ONES: [i32; 5] = [1; 5];

  match [0; 5] {
    ONES => dbg!("ones"),
    _ => dbg!("something else")
  };

There are also inline const patterns but they’re unstable as of now.

9 Likes
7

Okay. Got it. Thanks.

As I understand there is no way to use [0; 5] without an if condition in match. It would be really nice to have such a syntax for array match pattern.

8

Just for completeness. This works as well.

#![allow(unused)]
// You can extend a String with some chars:
fn main() {
    let arr: [u8; 5] = [0, 0, 0, 0, 0];
    match arr {
        [0, 0, 0, 0, 0] => println!("All elements are zeroes"),
        [1, 1, 1, 1, 1] => println!("All elements are ones"),
        _ => println!("Something else!")
    }
}

Of course that's a bit unhandy if you have a larger number of elements than 5.

5 Likes
9

Yes, yes. This works perfectly, but you are right. If I have too many values in the array, it gets clumsy. That's why I felt, a syntax like

match arr {
    [0; 32] => println!("All elements are zero"),
    ... 
}

will be so nice. Thoughts @nikomatsakis ??

11

There is an unstable feature:

#![feature(inline_const_pat)]

fn main() {
    let arr: [u8; 5] = [0, 0, 0, 0, 0];
    match arr {
        const { [0; 5] } => println!("All elements are zeroes"),
        const { [1; 5] } => println!("All elements are ones"),
        _ => println!("Something else!")
    }
}
7 Likes
12

I don't think pinging Niko for something like this is going to be productive (or polite).

2 Likes
13

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