Question

我有一个向量a，我需要复制此向量的一部分，比如a[n..n+3]，k次。

例如：

a = vec![0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
n = 2
k = 3

然后我想生成：

b = vec![2, 3, 4, 2, 3, 4, 2, 3, 4]

在之前的帮助下，我最终得到了以下内容：

a[n..n+3].iter().cloned().cycle().take(3 * k).collect()

这会是Rust-idiomatic吗？有更优选的方法吗？

Answer 1

好的，所以看完Which is more idiomatic? Functional, imperative or a mix?之后并在下面运行基准


    #![feature(test)]
    extern crate test;

    use test::Bencher;

    #[bench]
    fn bench_target_func(b: &mut Bencher) {
        let a = vec![0, 1, 2, 3, 4, 5, 6, 7, 8, 9];
        let n = 2;
        let k = 3;
        b.iter( || {
            let b: Vec = a[n..n+3].iter().cloned().cycle().take(3 * k).collect();
        });
    }

    #[bench]
    fn bench_target_imper(b: &mut Bencher) {
        let a = vec![0, 1, 2, 3, 4, 5, 6, 7, 8, 9];
        let n = 2;
        let k = 3;
        b.iter( || {
            let mut b: Vec = Vec::with_capacity(k * 3);
            let mut it = a[n..n+3].iter().cloned().cycle();
            for _ in 0..k*3 {
                b.push(it.next().unwrap());
            }
        });
    }

    #[bench]
    fn bench_target_imper2(b: &mut Bencher) {
        let a = vec![0, 1, 2, 3, 4, 5, 6, 7, 8, 9];
        let n = 2;
        let k = 3;

        b.iter(|| {
            let mut b = Vec::with_capacity(3 * k);
            for _ in 0..k {
                b.extend_from_slice(&a[n..n + 3]);
            }
        });
    }

    #[bench]
    fn bench_target_func2(b: &mut Bencher) {
        let a = vec![0, 1, 2, 3, 4, 5, 6, 7, 8, 9];
        let n = 2;
        let k = 3;

        b.iter(|| {
            let b : Vec = (0..k).flat_map(|_| a[n..n+3].iter().cloned()).collect();
        });
    }

    fn main() {
        println!("Hello, world!");
    }

我得到了以下结果：


    test bench_target_func   ... bench:          31 ns/iter (+/- 0)
    test bench_target_func2  ... bench:          97 ns/iter (+/- 1)
    test bench_target_imper  ... bench:          37 ns/iter (+/- 0)
    test bench_target_imper2 ... bench:          29 ns/iter (+/- 0)

看来flat_map要慢得多。

Answer 2

对我来说，你的例子似乎是Rust-idiomatic，但我会这样做：

let a = vec![0, 1, 2, 3, 4, 5, 6, 7, 8, 9];
let n = 2;
let k = 3;

let b : Vec<_> = (0..k).flat_map(|_| a[n..n+3].iter().cloned()).collect();
println!("{:?}", b);

我还提出了以下详细的方法，在某些情况下可能会有用。

let a = vec![0, 1, 2, 3, 4, 5, 6, 7, 8, 9];
let n = 2;
let k = 3;

let mut b = Vec::with_capacity(3 * k);
for _ in 0..k {
    b.extend_from_slice(&a[n..n+3]);
}
println!("{:?}", b);

如何将一片矢量k复制到另一个矢量？

2 个答案: