如何检查 Vec 中的值是否为无？

Question

I would like to use the fact that Vec.last() returns a None if there is nothing at that index but that index still exists. The problem is that I can't find a way to compare the result to anything inside of a while or if . I can however compare it if I use match , although that seems less efficient than using a while loop. Is there a way to compare it in an if or a while loop?

This is what I'm trying to do, but the self.particles.last() == None is invalid since you can't compare things to None except None as far as I can tell.

while vec.last() == None {
    num += 1;
}

Answer 1

I would like to use the fact that Vec.last() returns a None if there is nothing at that index but that index still exists.

You seem to be very confused about the behaviour of Vec::last . It's absolutely not going to return None if "that index still exists" (whatever that means). It's going to return None if the vec is empty, that's it.

So your loop is either never going to execute, or will loop forever.

This is what I'm trying to do, but the self.particles.last() == None is invalid since you can't compare things to None except None as far as I can tell.

 while vec.last() == None { num += 1; }

No? The issue would probably be that Option<T> is PartialEq iff T is PartialEq : https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=00f4ea97a0fad1cd74ef3b6ac6b74dc8

#[derive(PartialEq, Eq)]
struct Equatable;
struct NonEquatable;
fn main() {
    // compiles fine
    println!("{}", None == Some(Equatable));
    // can't compile
    // println!("{}", None == Some(NonEquatable));
}

Though it is obviously not the normal way to do it, you'd usually use Option::is_none , possibly matches! in some rare cases (but that's more for enums which don't have predicates).

Answer 2

You can use pattern matching syntax. What is that? Well, normally you'd write something like this to loop as long as you get Some values and simultaneously bind the items to a variable:

while let Some(elem) = vec.last() {
    ...
}

Some(elem) is a pattern. It turns out you can replace it with a pattern that has no variable: None .

while let None = vec.last() {
    ...
}

I admit let without a variable looks odd, and it looks like it's backwards having the None on the left, but it works.

(Also, I should point out this will be an infinite loop if you don't modify vec , yes?)