How do I compare a vector against a reversed version of itself?

Question

Why won\'t this compile?

fn isPalindrome(v: Vec) -> bool {
  return v.reverse() == v;
}

I get

Answer 1

Read up on the documentation for the function you are using:

Reverse the order of elements in a slice, in place.

Or check the function signature:

fn reverse(&mut self)

The return value of the method is the unit type, an empty tuple (). You can't compare that against a vector.

---

Stylistically, Rust uses 4 space indents, snake_case identifiers for functions and variables, and has an implicit return at the end of blocks. You should adjust to these conventions in a new language.

Additionally, you should take a &[T] instead of a Vec if you are not adding items to the vector.

To solve your problem, we will use iterators to compare the slice. You can get forward and backward iterators of a slice, which requires a very small amount of space compared to reversing the entire array. Iterator::eq allows you to do the comparison succinctly.

You also need to state that the T is comparable against itself, which requires Eq or PartialEq.

fn is_palindrome(v: &[T]) -> bool
where
    T: Eq,
{
    v.iter().eq(v.iter().rev())
}

fn main() {
    println!("{}", is_palindrome(&[1, 2, 3]));
    println!("{}", is_palindrome(&[1, 2, 1]));
}

If you wanted to do the less-space efficient version, you have to allocate a new vector yourself:

fn is_palindrome(v: &[T]) -> bool
where
    T: Eq + Clone,
{
    let mut reverse = v.to_vec();
    reverse.reverse();
    reverse == v
}

fn main() {
    println!("{}", is_palindrome(&[1, 2, 3]));
    println!("{}", is_palindrome(&[1, 2, 1]));
}

Note that we are now also required to Clone the items in the vector, so we add that trait bound to the method.