如何避免在Iterator :: flat_map中进行分配? [英] How do I avoid allocations in Iterator::flat_map?

问题描述

我有一个整数的 Vec ，我想创建一个新的 Vec ，其中包含那些整数和这些整数的平方.我可以必须这样做:

I have a Vec of integers and I want to create a new Vec which contains those integers and squares of those integers. I could do this imperatively:

let v = vec![1, 2, 3];
let mut new_v = Vec::new(); // new instead of with_capacity for simplicity sake.
for &x in v.iter() {
    new_v.push(x);
    new_v.push(x * x);
}
println!("{:?}", new_v);

但是我想使用迭代器.我想出了这段代码:

but I want to use iterators. I came up with this code:

let v = vec![1, 2, 3];
let new_v: Vec<_> = v.iter()
    .flat_map(|&x| vec![x, x * x])
    .collect();
println!("{:?}", new_v);

，但它在 flat_map 函数中分配了一个中间 Vec .

but it allocates an intermediate Vec in the flat_map function.

如何在没有分配的情况下使用 flat_map ?

How to use flat_map without allocations?

推荐答案

您可以使用

You can use an ArrayVec for this.

let v = vec![1, 2, 3];
let new_v: Vec<_> = v.iter()
    .flat_map(|&x| ArrayVec::from([x, x * x]))
    .collect();

使数组成为按值迭代器，这样就无需讨论 ArrayVec ，请参见

Making arrays be by-value iterators, so that you wouldn't need ArrayVec has been discussed, see https://github.com/rust-lang/rust/issues/25725 and the linked PRs.

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