rust - 如何从嵌套迭代器中收集数据？

Question

我正在尝试从嵌套迭代器中进行收集，但出现了 FromIterator not implemented 错误。这是一个例子:

#[derive(PartialEq)]
enum DayStatus {
    Normal,
    Abnormal,
}

struct Week {
    days: Vec<Day>,
}

struct Day {
    status: DayStatus,
}

struct Month {
    weeks: Vec<Week>,
}

fn get_abnormal_days(month: Month) -> Vec<Day> {
    // assume we have a month: Month which is filled
    month
        .weeks
        .iter()
        .map(|w| w.days.iter().filter(|d| d.status == DayStatus::Abnormal))
        .collect()
}

fn main() {}

给我:

 error[E0277]: the trait bound `std::vec::Vec<Day>: std::iter::FromIterator<std::iter::Filter<std::slice::Iter<'_, Day>, [closure@src/main.rs:24:39: 24:74]>>` is not satisfied
  --> src/main.rs:25:10
   |
25 |         .collect()
   |          ^^^^^^^ a collection of type `std::vec::Vec<Day>` cannot be built from an iterator over elements of type `std::iter::Filter<std::slice::Iter<'_, Day>, [closure@src/main.rs:24:39: 24:74]>`
   |
   = help: the trait `std::iter::FromIterator<std::iter::Filter<std::slice::Iter<'_, Day>, [closure@src/main.rs:24:39: 24:74]>>` is not implemented for `std::vec::Vec<Day>`

我可以尝试impl FromIterator 但它必须来自的类型似乎太内在而无法处理。我想我没有调用正确的 collect 或 map 但我看不到我遗漏了什么

我第一次尝试返回 &[Day] 但也失败了。

Answer 1

要取消嵌套迭代器，请使用 flat_map 而不是 map。

此外，您需要使用 Copy 类型或使用 into_iter 来迭代拥有的值而不仅仅是引用。

工作示例:

#[derive(PartialEq)]
enum DayStatus {
    Normal,
    Abnormal,
}

struct Week {
    days: Vec<Day>,
}

struct Day {
    status: DayStatus,
}

struct Month {
    weeks: Vec<Week>,
}

fn get_abnormal_days(month: Month) -> Vec<Day> {
    // assume we have a month: Month which is filled
    month
        .weeks
        .into_iter()
        .flat_map(|w| {
            w.days
                .into_iter()
                .filter(|d| d.status == DayStatus::Abnormal)
        })
        .collect()
}

fn main() {}