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< h1 id = "数组" > < a class = "header" href = "#数组" > 数组< / a > < / h1 >
< p > 在日常开发中，使用最广的数据结构之一就是数组，在 Rust 中，最常用的数组有两种，第一种是速度很快但是长度固定的 < code > array< / code > ，第二种是可动态增长的但是有性能损耗的 < code > Vector< / code > ，在本书中，我们称 < code > array< / code > 为数组，< code > Vector< / code > 为动态数组。< / p >
< p > 不知道你们发现没，这两个数组的关系跟 < code > & str< / code > 与 < code > String< / code > 的关系很像，前者是长度固定的字符串切片，后者是可动态增长的字符串。其实，在 Rust 中无论是 < code > String< / code > 还是 < code > Vector< / code > ，它们都是 Rust 的高级类型：集合类型，在后面章节会有详细介绍。< / p >
< p > 对于本章节，我们的重点还是放在数组 < code > array< / code > 上。数组的具体定义很简单：将多个类型相同的元素依次组合在一起，就是一个数组。结合上面的内容，可以得出数组的三要素：< / p >
< ul >
< li > 长度固定< / li >
< li > 元素必须有相同的类型< / li >
< li > 依次线性排列< / li >
< / ul >
< p > 这里再啰嗦一句，< strong > 我们这里说的数组是 Rust 的基本类型，是固定长度的，这点与其他编程语言不同，其它编程语言的数组往往是可变长度的，与 Rust 中的动态数组 < code > Vector< / code > 类似< / strong > ，希望读者大大牢记此点。< / p >
< h3 id = "创建数组" > < a class = "header" href = "#创建数组" > 创建数组< / a > < / h3 >
< p > 在 Rust 中，数组是这样定义的：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > fn main() {
let a = [1, 2, 3, 4, 5];
}< / code > < / pre >
< p > 数组语法跟 JavaScript 很像，也跟大多数编程语言很像。由于它的元素类型大小固定，且长度也是固定，因此< strong > 数组 < code > array< / code > 是存储在栈上< / strong > ，性能也会非常优秀。与此对应，< strong > 动态数组 < code > Vector< / code > 是存储在堆上< / strong > ，因此长度可以动态改变。当你不确定是使用数组还是动态数组时，那就应该使用后者，具体见< a href = "https://course.rs/basic/collections/vector.html" > 动态数组 Vector< / a > 。< / p >
< p > 举个例子，在需要知道一年中各个月份名称的程序中，你很可能希望使用的是数组而不是动态数组。因为月份是固定的，它总是只包含 12 个元素：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > let months = ["January", "February", "March", "April", "May", "June", "July",
"August", "September", "October", "November", "December"];
< span class = "boring" > }< / span > < / code > < / pre >
< p > 在一些时候，还需要为< strong > 数组声明类型< / strong > ，如下所示：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > let a: [i32; 5] = [1, 2, 3, 4, 5];
< span class = "boring" > }< / span > < / code > < / pre >
< p > 这里，数组类型是通过方括号语法声明，< code > i32< / code > 是元素类型，分号后面的数字 < code > 5< / code > 是数组长度，数组类型也从侧面说明了< strong > 数组的元素类型要统一，长度要固定< / strong > 。< / p >
< p > 还可以使用下面的语法初始化一个< strong > 某个值重复出现 N 次的数组< / strong > ： < / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > let a = [3; 5];
< span class = "boring" > }< / span > < / code > < / pre >
< p > < code > a< / code > 数组包含 < code > 5< / code > 个元素，这些元素的初始化值为 < code > 3< / code > ，聪明的读者已经发现，这种语法跟数组类型的声明语法其实是保持一致的：< code > [3; 5]< / code > 和 < code > [类型; 长度]< / code > 。< / p >
< p > 在元素重复的场景，这种写法要简单的多，否则你就得疯狂敲击键盘：< code > let a = [3, 3, 3, 3, 3];< / code > ，不过老板可能很喜欢你的这种疯狂编程的状态。< / p >
< h3 id = "访问数组元素" > < a class = "header" href = "#访问数组元素" > 访问数组元素< / a > < / h3 >
< p > 因为数组是连续存放元素的，因此可以通过索引的方式来访问存放其中的元素：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > fn main() {
let a = [9, 8, 7, 6, 5];
let first = a[0]; // 获取a数组第一个元素
let second = a[1]; // 获取第二个元素
}< / code > < / pre >
< p > 与许多语言类似，数组的索引下标是从 0 开始的。此处，< code > first< / code > 获取到的值是 < code > 9< / code > ， < code > second< / code > 是 < code > 8< / code > 。< / p >
< h4 id = "越界访问" > < a class = "header" href = "#越界访问" > 越界访问< / a > < / h4 >
< p > 如果使用超出数组范围的索引访问数组元素，会怎么样？下面是一个接收用户的控制台输入，然后将其作为索引访问数组元素的例子：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > use std::io;
fn main() {
let a = [1, 2, 3, 4, 5];
println!("Please enter an array index.");
let mut index = String::new();
// 读取控制台的输出
io::stdin()
.read_line(& mut index)
.expect("Failed to read line");
let index: usize = index
.trim()
.parse()
.expect("Index entered was not a number");
let element = a[index];
println!(
"The value of the element at index {} is: {}",
index, element
);
}< / code > < / pre >
< p > 使用 < code > cargo run< / code > 来运行代码，因为数组只有 5 个元素，如果我们试图输入 < code > 5< / code > 去访问第 6 个元素，则会访问到不存在的数组元素，最终程序会崩溃退出：< / p >
< pre > < code class = "language-console" > Please enter an array index.
5
thread 'main' panicked at 'index out of bounds: the len is 5 but the index is 5', src/main.rs:19:19
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
< / code > < / pre >
< p > 这就是数组访问越界，访问了数组中不存在的元素，导致 Rust 运行时错误。程序因此退出并显示错误消息，未执行最后的 < code > println!< / code > 语句。< / p >
< p > 当你尝试使用索引访问元素时， ， < strong > < em > panic< / em > < / strong > 。这种检查只能在运行时进行，比如在上面这种情况下，编译器无法在编译期知道用户运行代码时将输入什么值。< / p >
< p > 这种就是 Rust 的安全特性之一。在很多系统编程语言中，并不会检查数组越界问题，你会访问到无效的内存地址获取到一个风马牛不相及的值，最终导致在程序逻辑上出现大问题，而且这种问题会非常难以检查。< / p >
< h4 id = "数组元素为非基础类型" > < a class = "header" href = "#数组元素为非基础类型" > 数组元素为非基础类型< / a > < / h4 >
< p > 学习了上面的知识，很多朋友肯定觉得已经学会了 Rust 的数组类型，但现实会给我们一记重锤，实际开发中还会碰到一种情况，就是< strong > 数组元素是非基本类型< / strong > 的，这时候大家一定会这样写。< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > let array = [String::from("rust is good!"); 8];
println!("{:#?}", array);
< span class = "boring" > }< / span > < / code > < / pre >
< p > 然后你会惊喜的得到编译错误。< / p >
< pre > < code class = "language-console" > error[E0277]: the trait bound `String: std::marker::Copy` is not satisfied
--> src/main.rs:7:18
|
7 | let array = [String::from("rust is good!"); 8];
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `std::marker::Copy` is not implemented for `String`
|
= note: the `Copy` trait is required because this value will be copied for each element of the array
< / code > < / pre >
< p > 有些还没有看过特征的小伙伴，有可能不太明白这个报错，不过这个目前可以不提，我们就拿之前所学的< a href = "https://course.rs/basic/ownership/ownership.html" > 所有权< / a > 知识，就可以思考明白，前面几个例子都是 Rust 的基本类型，而< strong > 基本类型在 Rust 中赋值是以 Copy 的形式< / strong > ，这时候你就懂了吧，< code > let array=[3;5]< / code > 底层就是不断的Copy出来的， ， < / p >
< p > 接着就有小伙伴会这样写。< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > let array = [String::from("rust is good!"),String::from("rust is good!"),String::from("rust is good!")];
println!("{:#?}", array);
< span class = "boring" > }< / span > < / code > < / pre >
< p > 作为一个追求极致完美的Rust开发者， ！ < / p >
< p > < strong > 正确的写法< / strong > ，应该调用< code > std::array::from_fn< / code > < / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > let array: [String; 8] = std::array::from_fn(|_i| String::from("rust is good!"));
println!("{:#?}", array);
< span class = "boring" > }< / span > < / code > < / pre >
< h2 id = "数组切片" > < a class = "header" href = "#数组切片" > 数组切片< / a > < / h2 >
< p > 在之前的< a href = "https://course.rs/basic/compound-type/string-slice.html#切片slice" > 章节< / a > ，我们有讲到 < code > 切片< / code > 这个概念，它允许你引用集合中的部分连续片段，而不是整个集合，对于数组也是，数组切片允许我们引用数组的一部分：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > let a: [i32; 5] = [1, 2, 3, 4, 5];
let slice: & [i32] = & a[1..3];
assert_eq!(slice, & [2, 3]);
< span class = "boring" > }< / span > < / code > < / pre >
< p > 上面的数组切片 < code > slice< / code > 的类型是< code > & [i32]< / code > ，与之对比，数组的类型是< code > [i32;5]< / code > ，简单总结下切片的特点：< / p >
< ul >
< li > 切片的长度可以与数组不同，并不是固定的，而是取决于你使用时指定的起始和结束位置< / li >
< li > 创建切片的代价非常小，因为切片只是针对底层数组的一个引用< / li >
< li > 切片类型 [T] 拥有不固定的大小，而切片引用类型 & [T] 则具有固定的大小，因为 Rust 很多时候都需要固定大小数据类型，因此 & [T] 更有用，< code > & str< / code > 字符串切片也同理< / li >
< / ul >
< h2 id = "总结" > < a class = "header" href = "#总结" > 总结< / a > < / h2 >
< p > 最后，让我们以一个综合性使用数组的例子，来结束本章节的学习：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > fn main() {
// 编译器自动推导出one的类型
let one = [1, 2, 3];
// 显式类型标注
let two: [u8; 3] = [1, 2, 3];
let blank1 = [0; 3];
let blank2: [u8; 3] = [0; 3];
// arrays是一个二维数组， ，
let arrays: [[u8; 3]; 4] = [one, two, blank1, blank2];
// 借用arrays的元素用作循环中
for a in & arrays {
print!("{:?}: ", a);
// 将a变成一个迭代器，
// 你也可以直接用for n in a {}来进行循环
for n in a.iter() {
print!("\t{} + 10 = {}", n, n+10);
}
let mut sum = 0;
// 0..a.len,是一个 Rust 的语法糖， ，
for i in 0..a.len() {
sum += a[i];
}
println!("\t({:?} = {})", a, sum);
}
}< / code > < / pre >
< p > 做个总结，数组虽然很简单，但是其实还是存在几个要注意的点：< / p >
< ul >
< li > < strong > 数组类型容易跟数组切片混淆< / strong > ， < / li >
< li > < code > [u8; 3]< / code > 和< code > [u8; 4]< / code > 是不同的类型，数组的长度也是类型的一部分< / li >
< li > < strong > 在实际开发中，使用最多的是数组切片[T]< / strong > ，我们往往通过引用的方式去使用< code > & [T]< / code > ，因为后者有固定的类型大小< / li >
< / ul >
< p > 至此，关于数据类型部分，我们已经全部学完了，对于 Rust 学习而言，我们也迈出了坚定的第一步，后面将开始更高级特性的学习。未来如果大家有疑惑需要检索知识，一样可以继续回顾过往的章节，因为本书不仅仅是一门 Rust 的教程，还是一本厚重的 Rust 工具书。< / p >
< h2 id = "课后练习" > < a class = "header" href = "#课后练习" > 课后练习< / a > < / h2 >
< blockquote >
< p > < a href = "https://practice-zh.course.rs/compound-types/array.html" > Rust By Practice< / a > ，支持代码在线编辑和运行，并提供详细的< a href = "https://github.com/sunface/rust-by-practice/blob/master/solutions/compound-types/array.md" > 习题解答< / a > 。< / p >
< / blockquote >
< / main >
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