rust - 如何在未知大小的 Vec> 上创建一个迭代器，它等效于迭代器在内部 Vec 上的笛卡尔积？

Question

我想设计一个类似于示例 here 的函数除了我的情况，iproduct 的参数数量在编译时是未知的。正如 here 所解释的那样，这在 python 中很容易完成。 .

我已经尝试使用 itertools crate，特别是 iproduct!、multi_cartesian_product 和 cartesian_product 但目前正在与类型系统，以及不确定它是否是正确的路径。

pub fn product_of_lists(lists: &Vec<Vec<u16>>) -> std::slice::Iter<'_, u16> {
    // generate cross products between lists
    assert!(lists.len() > 2);
    let mut product_iter = lists[0].iter();
    for (en, list) in lists.iter().enumerate() {
        if en > 0{
            product_iter = iproduct!(product_iter, list.iter());
        }
    }
    product_iter
}

error[E0308]: mismatched types
  --> src/testcode.rs:44:28
   |
10 |             product_iter = iproduct!(product_iter, list.iter());
   |                            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected struct `std::slice::Iter`, found struct `itertools::adaptors::Product`
   |
   = note: expected type `std::slice::Iter<'_, _>`
              found type `itertools::adaptors::Product<std::slice::Iter<'_, _>, std::slice::Iter<'_, u16>>`
   = note: this error originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)

Answer 1

doc已经回答你了:

use itertools::Itertools; // 0.8.0

pub fn product_of_lists(lists: &[Vec<u16>]) -> impl Iterator<Item = Vec<&u16>> {
    // generate cross products between lists
    assert!(lists.len() > 2);
    lists.iter().map(|x| x.iter()).multi_cartesian_product()
}

或

use itertools::Itertools; // 0.8.0

pub fn product_of_lists<'a, L, I>(lists: L) -> impl Iterator<Item = Vec<&'a u16>>
where
    L: IntoIterator<Item = I>,
    I: IntoIterator<Item = &'a u16>,
    <I as IntoIterator>::IntoIter: Clone,
{
    lists
        .into_iter()
        .map(IntoIterator::into_iter)
        .multi_cartesian_product()
}