如何将值推送到 Rust 中的 2D Vec 中?

问题描述

这是对 2D Vec 的一个非常简单的尝试.我正在尝试向顶级 Vec 中的最后一个条目添加一个元素:

Here is a really simple attempt at a 2D Vec. I'm trying to add an element to the last entry in the top-level Vec:

fn main() {
    let mut vec_2d = vec![vec![]];
    if let Some(v) = vec_2d.last() {
        v.push(1);
    }
    println!("{:?}", vec_2d);
}

我收到此错误:

error[E0596]: cannot borrow `*v` as mutable, as it is behind a `&` reference
 --> src/main.rs:4:9
  |
3 |     if let Some(v) = vec_2d.last() {
  |                 - help: consider changing this to be a mutable reference: `&mut std::vec::Vec<i32>`
4 |         v.push(1);
  |         ^ `v` is a `&` reference, so the data it refers to cannot be borrowed as mutable

我也试过 Some(ref v) 和 Some(ref mut v) 得到相同的结果.我找不到任何专门描述此错误的文档.这里的正确方法是什么?

I've also tried Some(ref v) and Some(ref mut v) with the same results. I can't find any documentation that describes this error specifically. What is the right approach here?

对类似问题的回答推荐了更像 Some(&mut v).然后我收到这些错误:

An answer to a similar question recommends something more like Some(&mut v). Then I get these errors:

error[E0308]: mismatched types
 --> src/main.rs:3:17
  |
3 |     if let Some(&mut v) = vec_2d.last() {
  |                 ^^^^^^ types differ in mutability
  |
  = note: expected type `&std::vec::Vec<_>`
             found type `&mut _`
  = help: did you mean `mut v: &&std::vec::Vec<_>`?

如果我尝试 Some(&ref mut v) 我得到:

If I try Some(&ref mut v) I get:

error[E0596]: cannot borrow data in a `&` reference as mutable
 --> src/main.rs:3:18
  |
3 |     if let Some(&ref mut v) = vec_2d.last() {
  |                  ^^^^^^^^^ cannot borrow as mutable

推荐答案

使用 last_mut;无需更改模式.

Grab a mutable reference to the last element with last_mut; no need to change patterns.

fn main() {
    let mut vec_2d = vec![vec![]];
    if let Some(v) = vec_2d.last_mut() {
        v.push(1);
    }
    println!("{:?}", vec_2d);
}

这篇关于如何将值推送到 Rust 中的 2D Vec 中?的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持！