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

Question

Why won't this compile?

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

I get

error[E0308]: mismatched types
 --> src/main.rs:2:25
  |
2 |   return v.reverse() == v;
  |                         ^ expected (), found struct `std::vec::Vec`
  |
  = note: expected type `()`
             found type `std::vec::Vec<T>`

Answer 1

Since you only need to look at the front half and back half, you can use the DoubleEndedIterator trait (methods .next() and .next_back()) to look at pairs of front and back elements this way:

/// Determine if an iterable equals itself reversed
fn is_palindrome<I>(iterable: I) -> bool
where
    I: IntoIterator,
    I::Item: PartialEq,
    I::IntoIter: DoubleEndedIterator,
{
    let mut iter = iterable.into_iter();
    while let (Some(front), Some(back)) = (iter.next(), iter.next_back()) {
        if front != back {
            return false;
        }
    }

    true
}

(run in playground)

This version is a bit more general, since it supports any iterable that is double ended, for example slice and chars iterators.

It only examines each element once, and it automatically skips the remaining middle element if the iterator was of odd length.