从枚举内的一个框中提取属性

huangapple
go评论95阅读模式
英文:

Extracting an attribute from a Box inside an enum

问题

  1. #[derive(Clone, Debug)]
  2. pub struct Node<T> {
  3.     value: T,
  4.     left: Addr<T>,
  5.     right: Addr<T>,
  6. }
  8. #[derive(Clone, Debug)]
  9. pub enum Addr<T> {
  10.     Addr(Box<Node<T>>),
  11.     None,
  12. }
  14. impl<T> Addr<T> {
  15.     pub fn is_none(&self) -> bool {
  16.         matches!(&*self, Addr::None)
  17.     }
  18. }
  20. impl<T> Node<T> {
  21.     fn node_insert(mut n: Addr<T>, val: T) {
  22.         let nd = Box::new(Node {
  23.             value: val,
  24.             left: Addr::None,
  25.             right: Addr::None,
  26.         });
  28.         if n.is_none() {
  29.             n = Addr::Addr(nd);
  30.         } else {
  31.             // You should access the left attribute like this:
  32.             if let Addr::Addr(node) = n {
  33.                 // Access the left attribute of the Node struct
  34.                 node.left = node_insert(node.left, val);
  35.             }
  36.         }
  37.     }
  38. }

在 node_insert 函数中，要访问 Node 结构的 left 属性，您应该像上面的示例代码中那样进行访问，使用 if let 来将 n 解构为 Addr::Addr(node)，然后您可以访问 node 的 left 属性并调用 node_insert 递归。

英文: 
  1. #[derive(Clone, Debug)]
  2. pub struct Node&lt;T&gt; {
  3.     value: T,
  4.     left: Addr&lt;T&gt;,
  5.     right: Addr&lt;T&gt;,
  6. }
  8. #[derive(Clone, Debug)]
  9. pub enum Addr&lt;T&gt; {
  10.     Addr(Box&lt;Node&lt;T&gt;&gt;),
  11.     None,
  12. }
  14. impl&lt;T&gt; Addr&lt;T&gt; {
  15.     pub fn is_none(&amp;self) -&gt; bool {
  16.         matches!(&amp;*self, Addr::None)
  17.     }
  18. }
  19. impl&lt;T&gt; Node&lt;T&gt; {
  20.     fn node_insert(mut n: Addr&lt;T&gt;, val: T) {
  21.         let nd = Box::new(Node {
  22.             value: val,
  23.             left: Addr::None,
  24.             right: Addr::None,
  25.         });
  27.         if n.is_none() {
  28.             n = Addr::Addr(nd);
  29.         } else {
  30.             node_insert(n.left, val);
  31.         }
  32.     }
  33. }

I'm trying to implement a simple binary tree, and in the node_insert function I can't figure out how to access the left attribute of the Node struct when calling node_insert recursively.

Nothing I've done worked and I couldn't find any answer to help with it, probably because its somehow specific.

答案1

得分: 0

使用模式匹配：

  1. fn node_insert(mut n: Addr<T>, val: T) {
  2.     match n {
  3.         Addr::None => {
  4.             n = Addr::Addr(Box::new(Node {
  5.                 value: val,
  6.                 left: Addr::None,
  7.                 right: Addr::None,
  8.             }))
  9.         }
  10.         Addr::Addr(n) => Self::node_insert(n.left, val),
  11.     }
  12. }

然而，编译器正确地发出警告：

  1. warning: value assigned to `n` is never read
  2.   --> src/lib.rs:23:13
  3.    |
  4. 23 |             n = Addr::Addr(Box::new(Node {
  5.    |             ^
  6.    |
  7.    = help: maybe it is overwritten before being read?
  8.    = note: `#[warn(unused_assignments)]` on by default

由于您拥有 n，您正在写入一个局部变量而不是树。相反，您应该获取 &mut Addr：

  1. fn node_insert(n: &mut Addr<T>, val: T) {
  2.     match n {
  3.         Addr::None => {
  4.             *n = Addr::Addr(Box::new(Node {
  5.                 value: val,
  6.                 left: Addr::None,
  7.                 right: Addr::None,
  8.             }))
  9.         }
  10.         Addr::Addr(n) => Self::node_insert(&mut n.left, val),
  11.     }
  12. }
英文:

Use pattern matching:

  1. fn node_insert(mut n: Addr&lt;T&gt;, val: T) {
  2.     match n {
  3.         Addr::None =&gt; {
  4.             n = Addr::Addr(Box::new(Node {
  5.                 value: val,
  6.                 left: Addr::None,
  7.                 right: Addr::None,
  8.             }))
  9.         }
  10.         Addr::Addr(n) =&gt; Self::node_insert(n.left, val),
  11.     }
  12. }

However, the compiler rightfully warns:

  1. warning: value assigned to `n` is never read
  2.   --&gt; src/lib.rs:23:13
  3.    |
  4. 23 |             n = Addr::Addr(Box::new(Node {
  5.    |             ^
  6.    |
  7.    = help: maybe it is overwritten before being read?
  8.    = note: `#[warn(unused_assignments)]` on by default

Since you took ownership of n, you're writing to a local variable and not to the tree. Instead, you should take &amp;mut Addr:

  1. fn node_insert(n: &amp;mut Addr&lt;T&gt;, val: T) {
  2.     match n {
  3.         Addr::None =&gt; {
  4.             *n = Addr::Addr(Box::new(Node {
  5.                 value: val,
  6.                 left: Addr::None,
  7.                 right: Addr::None,
  8.             }))
  9.         }
  10.         Addr::Addr(n) =&gt; Self::node_insert(&amp;mut n.left, val),
  11.     }
  12. }

huangapple
  • 本文由 发表于 2023年2月26日 20:43:25
  • 转载请务必保留本文链接：https://go.coder-hub.com/75572054.html
  • binary-tree
  • enums
  • rust
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