arrays - 如何在数组中添加一个值？

Question

是否可以在数组前添加一个值？我知道如何连接两个数组，但如果我有一个数组和一个值(与数组的类型相同)，我可以将这个元素添加到数组之前吗？

Answer 1

在稳定的 Rust 中无法做到这一点；数组不能在运行时添加或删除值；它们的长度在编译时是固定的。

您更有可能需要 Vec 和 Vec::insert .

另见:

• How to set a Rust array length dynamically?
• What is the difference between a slice and an array?
• What is the difference between storing a Vec vs a Slice?
• Efficiently insert or replace multiple elements in the middle or at the beginning of a Vec?

---

在 nightly Rust 中，您可以使用不稳定的特性来构建一个稍微大一点的全新数组并将所有值移动过来:

// 1.52.0-nightly (2021-03-07 234781afe33d3f339b00)
#![allow(incomplete_features)]
#![feature(const_generics, const_evaluatable_checked)]

use std::{
    array::IntoIter,
    mem::{self, MaybeUninit},
    ptr,
};

fn prepend<T, const N: usize>(a: [T; N], v: T) -> [T; N + 1] {
    // # SAFETY
    //
    // Converting an uninitialized array to an array of
    // uninitialized values is always safe.
    // https://github.com/rust-lang/rust/issues/80908
    let mut xs: [MaybeUninit<T>; N + 1] = unsafe { MaybeUninit::uninit().assume_init() };

    let (head, tail) = xs.split_first_mut().unwrap();
    *head = MaybeUninit::new(v);
    for (x, v) in tail.iter_mut().zip(IntoIter::new(a)) {
        *x = MaybeUninit::new(v)
    }

    // # SAFETY
    //
    // We are effectively transmuting from an array of filled `MaybeUninit<T>` to
    // the array of `T`, but cannot actually use `transmute`:
    // https://github.com/rust-lang/rust/issues/61956
    unsafe {
        let tmp_xs = &mut xs as *mut [MaybeUninit<T>; N + 1] as *mut [T; N + 1];
        mem::forget(xs);
        ptr::read(tmp_xs)
    }
}

fn main() {
    let v = prepend([1, 2, 3], 4);
    assert_eq!([4, 1, 2, 3], v);
}

另见:

• Is it possible to control the size of an array using the type parameter of a generic?
• How do I move values out of an array one at a time?
• How do I collect into an array?