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用于Vec的DoubleEndedIterator

rust

Harry · 技术社区 · 4 月前

如何检查给定的列表/范围是否是另一个范围/列表的后缀。我试图使用 DoubleEndedIterator 提供向后迭代便利的特性。但是 Vec Rust标准库中的集合未实现 DoubleEndedIterator 特质。

use std::iter::Peekable;

pub fn ends_with<'a, 'b, FirstList, FirstItemType, SecondList, SecondItemType, BinaryPredicate>(
    first_list: FirstList,
    second_list: SecondList,
    binary_predicate: BinaryPredicate) -> bool
where
    FirstList: IntoIterator<Item = &'a FirstItemType>,
    FirstList: DoubleEndedIterator<Item = &'a FirstItemType>,
    FirstItemType: 'a,
    SecondList: IntoIterator<Item = &'b SecondItemType>,
    SecondList: DoubleEndedIterator<Item = &'b SecondItemType>,
    SecondItemType: 'b,
    BinaryPredicate: Fn(&FirstItemType, &SecondItemType) -> bool
{
    let mut f_itr = first_list.rev().into_iter().peekable();
    let mut s_itr = second_list.rev().into_iter().peekable();

    while let (Some(f), Some(s)) = (f_itr.peek(), s_itr.peek()) {
        if !binary_predicate(*f, *s) {
            return false;
        }
        let _ = f_itr.next();
        let _ = s_itr.next();
    }

    f_itr.peek().is_some() || s_itr.peek().is_none()
}

fn main() {
    let first_list = vec![1, 2, 3, 4, 5, 6];
    let second_list = vec![4, 5, 6];

    let res = ends_with(&first_list, &second_list, |first_item, second_item| {
        *first_item == *second_item
    });

    assert!(res);
}

错误

error[E0277]: the trait bound `&Vec<{integer}>: DoubleEndedIterator` is not satisfied
  --> src/main.rs:34:29
   |
34 |     let mut res = ends_with(&first_list, &second_list, |first_item, second_item| {
   |                   --------- ^^^^^^^^^^^ the trait `DoubleEndedIterator` is not implemented for `&Vec<{integer}>`
   |                   |
   |                   required by a bound introduced by this call
   |
note: required by a bound in `ends_with`
  --> src/main.rs:9:16
   |
3  | pub fn ends_with<'a, 'b, FirstList, FirstItemType, SecondList, SecondItemType, BinaryPredicate>(
   |        --------- required by a bound in this function
...
9  |     FirstList: DoubleEndedIterator<Item = &'a FirstItemType>,
   |                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `ends_with`

error[E0277]: the trait bound `&Vec<{integer}>: DoubleEndedIterator` is not satisfied
  --> src/main.rs:34:42
   |
34 |     let mut res = ends_with(&first_list, &second_list, |first_item, second_item| {
   |                   ---------              ^^^^^^^^^^^^ the trait `DoubleEndedIterator` is not implemented for `&Vec<{integer}>`
   |                   |
   |                   required by a bound introduced by this call
   |
note: required by a bound in `ends_with`
  --> src/main.rs:12:17
   |
3  | pub fn ends_with<'a, 'b, FirstList, FirstItemType, SecondList, SecondItemType, BinaryPredicate>(
   |        --------- required by a bound in this function
...
12 |     SecondList: DoubleEndedIterator<Item = &'b SecondItemType>,
   |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `ends_with`

For more information about this error, try `rustc --explain E0277`.

1 回复 | 直到 4 月前

Francis Gagné 4 月前

你想把 DoubleEndedIterator 绑定在迭代器类型上,而不是可迭代类型(即实现的类型 IntoIterator ). 例如,对于 FirstList ,您需要将绑定写为 FirstList::IntoIter: DoubleEndedIterator<Item = &'a FirstItemType>, . IntoIter 是在上定义的关联类型 IntoIterator 特质。

也, first_list.rev().into_iter().peekable() 需要更改为 first_list.into_iter().rev().peekable() 因为 rev() 未由定义 IntoIterator 特质。

pub fn ends_with<'a, 'b, FirstList, FirstItemType, SecondList, SecondItemType, BinaryPredicate>(
    first_list: FirstList,
    second_list: SecondList,
    binary_predicate: BinaryPredicate,
) -> bool
where
    FirstList: IntoIterator<Item = &'a FirstItemType>,
    FirstList::IntoIter: DoubleEndedIterator<Item = &'a FirstItemType>,
    FirstItemType: 'a,
    SecondList: IntoIterator<Item = &'b SecondItemType>,
    SecondList::IntoIter: DoubleEndedIterator<Item = &'b SecondItemType>,
    SecondItemType: 'b,
    BinaryPredicate: Fn(&FirstItemType, &SecondItemType) -> bool,
{
    let mut f_itr = first_list.into_iter().rev().peekable();
    let mut s_itr = second_list.into_iter().rev().peekable();

    while let (Some(f), Some(s)) = (f_itr.peek(), s_itr.peek()) {
        if !binary_predicate(*f, *s) {
            return false;
        }
        let _ = f_itr.next();
        let _ = s_itr.next();
    }

    f_itr.peek().is_some() || s_itr.peek().is_none()
}

fn main() {
    let first_list = vec![1, 2, 3, 4, 5, 6];
    let second_list = vec![4, 5, 6];

    let res = ends_with(&first_list, &second_list, |first_item, second_item| {
        *first_item == *second_item
    });

    assert!(res);
}