问题

我正在编写埃拉托斯特尼筛法，为此你想要用奇数索引`true`和偶数索引`false`来开始一个布尔向量。目前我用以下代码来实现这个目标：

```rust
let mut is_prime: Vec<bool> = vec![true; capacity];
for i in (3..capacity).step_by(2) {
    is_prime[i] = false;
}

然而，这并不适用于创建交替的true和false序列。我该如何实现这个目标？

注意： 我知道在这种情况下这不会对性能产生巨大影响，但我觉得这是一个有趣的问题，而且我怀疑可能存在某些情况下这会产生影响。

<details>
<summary>英文:</summary>

I am writing a sieve of Eratosthenes, for this you want to start a vector of booleans with odd indices `true` and even indices `false`. Currently my code for getting that is:

``` rust
let mut is_prime: Vec&lt;bool&gt; = vec![true; capacity];
is_prime.iter_mut().step_by(2).for_each(|m| *m = false);

However that doesn't work to create an alternating true false sequence. How can I achieve this?

Note: I know this won't have a massive impact on performance in this case, I but I thought it was an interesting problem and suspect there may be cases where it would make a difference.

答案1

得分: 6

你可以结合一些迭代器工具来高效、方便地完成这个任务：

let mut is_prime: Vec<_> = std::iter::repeat([true, false])
    .flatten()
    .take(capacity)
    .collect();

注意迭代器确切知道它的长度，这使得 collect 能够提前分配足够的容量来构建一个向量。

let mut is_prime: Vec<_> = std::iter::repeat([true, false])
    .flatten()
    .take(capacity)
    .collect();

答案2

得分: 5

你可以使用 cycle、take 和 collect 来完成：

let mut sieve: Vec<_> = [false, true].into_iter().cycle().take(capacity).collect();

Playground

let mut sieve: Vec<_> = [ false, true ].into_iter().cycle().take (capacity).collect();