iterator - 如何在不收集临时向量的情况下对结果的迭代器执行迭代器计算？

Question

在此示例中，我正在寻找一种消除临时向量分配的方法:

fn doit<T: Iterator<Item = Result<i32, &'static str>>>(name: &str, iter: T) {
    println!(
        "{}: {:?}",
        name,
        iter.collect::<Result<Vec<_>, _>>()
            .map(|v| v.into_iter().min())
    );
}

fn main() {
    let without_errors = vec![Ok(1), Ok(2), Ok(3)];
    let with_errors = vec![Ok(1), Err("error"), Ok(2)];

    doit("without errors", without_errors.into_iter());
    doit("with errors", with_errors.into_iter());
}

这是具有错误处理主题的迭代器的一种变体，只是我不想创建一个集合(因此collect()不能完全完成工作)，但是我想对要迭代的元素执行进一步的操作。

请注意，这会产生错误的结果，因为Ok小于Err:

fn doit<T: Iterator<Item = Result<i32, &'static str>>>(name: &str, iter: T) {
    println!("{}: {:?}", name, iter.min());
}

偶然会为max()提供正确的结果，但不会停止对第一个错误的迭代。

Answer 1

Iterator::try_fold 提供了您所需的框架，并且从Rust 1.27(Playground)开始就可以使用:

fn fold_ok<I, T, E, F>(mut iter: I, f: F) -> Result<Option<T>, E>
where
    I: Iterator<Item = Result<T, E>>,
    T: Ord,
    F: Fn(T, T) -> T,
{
    iter.try_fold(None, |r, i| {
        let i = i?;
        Ok(Some(if let Some(r) = r { f(r, i) } else { i }))
    })
}

fn main() {
    let without_errors = vec![Ok(1), Ok(2), Ok(3)];
    let with_errors = vec![Ok(1), Err("error"), Ok(2)];

    fn doit<'r, T>(name: &str, iter: T)
    where
        T: Iterator<Item = &'r Result<i32, &'static str>> + Clone,
    {
        println!("{}: {:?}", name, fold_ok(iter.cloned(), ::std::cmp::min));
    }

    doit("without errors", without_errors.iter());
    doit("with errors", with_errors.iter());
}

在此之前，我认为您唯一的选择是手动迭代(Playground)

fn fold_ok<I, T, E, F>(mut iter: I, f: F) -> Result<Option<T>, E>
where
    I: Iterator<Item = Result<T, E>>,
    T: Ord,
    F: Fn(T, T) -> T,
{
    let mut result = match iter.next() {
        None => return Ok(None),
        Some(r) => r?,
    };

    for item in iter {
        result = f(result, item?);
    }

    Ok(Some(result))
}

fn main() {
    let without_errors = vec![Ok(1), Ok(2), Ok(3)];
    let with_errors = vec![Ok(1), Err("error"), Ok(2)];

    fn doit<'r, T>(name: &str, iter: T)
    where
        T: Iterator<Item = &'r Result<i32, &'static str>> + Clone,
    {
        println!(
            "{}: {:?}",
            name,
            fold_ok(iter.clone().cloned(), ::std::cmp::min)
        );
    }

    doit("without errors", without_errors.iter());
    doit("with errors", with_errors.iter());
}