如何在元素上添加两个Rust数组？ [英] How do I add two Rust arrays element-wise?

问题描述

这是一个绝对的初学者问题，但是搜索半小时后，我找不到任何有用的东西。我有Rust 1.7.0和以下代码：

This is an absolute beginner question, but I can't find anything useful after searching for half an hour. I have Rust 1.7.0 and this code:

type coord = [i64; 3];

// add two coordinates ("vectors") pointwise, that is
// if z = add(a, b) then z[i] = a[i] + b[i] for i=0..2
fn add(a: coord, b: coord) -> coord {
    //???
}

我首先尝试的显而易见的方法是 a.zip （b）.map（|（u，v）| u + v），但这不起作用（无法压缩数组）， a.iter（ ）.zip（b.iter（））。map（|（u，v）| u + v），因为它无法将迭代器转换回数组。我可以看到为什么这通常无法正常工作，但在这种情况下，我们知道两者长度相同。

The obvious thing I tried first is a.zip(b).map(|(u,v)| u+v) but this doesn't work (can't zip arrays), nor does a.iter().zip(b.iter()).map(|(u,v)| u+v) because it can't convert the iterator back to an array. I can see why this doesn't work in general but in this case we know both things are the same length.

现在我正在做

fn add(a: coord, b: coord) -> coord {
    let mut z: coord = [0, 0, 0];
    for i in 0..2 {
        z[i] = a[i] + b[i];
    }
    z
}

但相比之下，它看起来很难看。我缺少什么？

but it looks ugly by comparison. What am I missing?

推荐答案

一种简单的方法是使用枚举迭代器方法，并通过分配获得的索引来填充 z 显而易见的方式：

One simple approach is to generate indices using the enumerate iterator method and fill z the "obvious" way, by assigning into the obtained indices:

type Coord = [i64; 3];

fn add(a: Coord, b: Coord) -> Coord {
    let mut z: Coord = [0, 0, 0];
    for (i, (aval, bval)) in a.iter().zip(&b).enumerate() {
        z[i] = aval + bval;
    }
    z
}

fn main() {
    let x: Coord = [1, 2, 3];
    let y: Coord = [1, 1, 1];
    assert!(add(x, y) == [2, 3, 4]);
}

在Rust中，通过注意到 iter（）在数组中生成引用，并且 iter_mut（）可用于生成可变引用。这样产生的代码与您尝试编写的代码非常相似：

In Rust, we can do better than that by noticing that iter() produces references into the array, and iter_mut() is available to produce mutable references. This results in code very similar to what you attempted to write:

fn add(a: Coord, b: Coord) -> Coord {
    let mut z: Coord = [0, 0, 0];
    for ((zref, aval), bval) in z.iter_mut().zip(&a).zip(&b) {
        *zval = aval + bval;
    }
    z
}

如果这种写入方式 z 重复执行不同的操作，您可以抽象创建新的 Coord 并将其填充到通用函数中的数据：

If this pattern of writing into z recurs with different operations, you can abstract the creation of new Coord and filling it with data into a generic function:

fn new_coord_from<F: Iterator<Item=i64>>(src: F) -> Coord {
    let mut result = [0; 3];
    for (rref, val) in result.iter_mut().zip(src) {
        *rref = val;
    }
    result
}

添加，然后看起来就像我们想要的那样：

add then looks just like we'd like it to:

fn add(a: Coord, b: Coord) -> Coord {
    new_coord_from(a.iter().zip(&b).map(|(a, b)| a + b))
}

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