# rust abc(6): 字符串的简单使用

1. 目的

学习 Rust 语言中的字符串， 包括数据类型， 常用 API。

2. 数据类型

Rust 语言提供了两种字符串：

1. str 类型. 通常用它的 borrow 类型 &str.

2. 标准库的 String 类型.

2.1 str 类型

也叫做 string slice. 通常是使用它的 borrow 类型，也就是 &str. ¹

字符串字面量 (string literals) 的类型， 也是 str 类型（也就是 &'static str）.

字符串字面量 &str 是在编译时确定其值的字符串类型。

fn main() {
    let a:&str = "hello";
    println!("a {}", a);

    let b = "world";
    println!("b {}", b);
}

编译运行

zz@Legion-R7000P% ./u1
a hello
b world

2.2 标准库 String 类型

Rust 语言的 String 位于标准库²:

String 是一个 struct³:

pub struct String {
    vec: Vec<u8>,
}

其中 Vec 指的是 UTF-8 字符作为元素的容器。这里暂时不了解容器，简单理解为：和 C++ 的 vector 类似的东西。

尝试使用 String 创建字符串变量：

fn main() {
    let s1 = String::new(); // 创建空字符串
    println!("s1: |{}|", s1);

    let s2 = String::from("hello world"); // 从字符串字面量创建 String
    println!("s2: {}", s2);
}

3. 常用 API

3.1 len() 方法

无论是 &str 还是 String 类型的对象， 都支持 len() 方法：

u3.rs:

fn main() {
    let s1 = String::new();
    println!("s1: |{}|, s1.len(): {}", s1, s1.len());

    let s2 = String::from("hello world");
    println!("s2: {}, s2.len(): {}", s2, s2.len());

    let t1 = "";
    println!("t1: |{}|, t1.len(): {}", t1, t1.len());

    let t2 = "hello world";
    println!("t2: {}, t2.len(): {}", t2, t2.len());
}

编译运行结果：

zz@Legion-R7000P% rustc u3.rs
zz@Legion-R7000P% ./u3
s1: ||, s1.len(): 0
s2: hello world, s2.len(): 11
t1: ||, t1.len(): 0
t2: hello world, t2.len(): 11

3.2 is_empty() 方法

无论是 &str 还是 String 类型的对象， 都支持 is_empty() 方法：

u4.rs:

fn main() {
    let s1 = String::new();
    println!("s1: |{}|, s1.is_empty(): {}", s1, s1.is_empty());

    let s2 = String::from("hello world");
    println!("s2: {}, s2.is_empty(): {}", s2, s2.is_empty());

    let t1 = "";
    println!("t1: |{}|, t1.is_empty(): {}", t1, t1.is_empty());

    let t2 = "hello world";
    println!("t2: {}, t2.is_empty: {}", t2, t2.is_empty());
}

编译运行结果：

zz@Legion-R7000P% rustc u4.rs
zz@Legion-R7000P% ./u4
s1: ||, s1.is_empty(): true
s2: hello world, s2.is_empty(): false
t1: ||, t1.is_empty(): true
t2: hello world, t2.is_empty: false

3.3 starts_with() 方法

无论是 &str 还是 String 类型的对象， 都支持 starts_with() 方法， 也都支持 ends_with() 方法。这里以及 starts_with() 方法为例：

u5.rs:

fn main() {
    let s1 = String::from("Hello World");
    println!("s1: {}, s1.starts_with(\"Hello\"): {}", s1, s1.starts_with("Hello"));
    println!("s1: {}, s1.starts_with(\"hello\"): {}", s1, s1.starts_with("hello"));

    let t1 = "Hello World";
    println!("t1: {}, t1.starts_with(\"Hello\"): {}", t1, t1.starts_with("Hello"));
    println!("t1: {}, t1.starts_with(\"hello\"): {}", t1, t1.starts_with("hello"));
}

编译运行结果：

zz@Legion-R7000P% rustc u5.rs
zz@Legion-R7000P% ./u5   
s1: Hello World, s1.starts_with("Hello"): true
s1: Hello World, s1.starts_with("hello"): false
t1: Hello World, t1.starts_with("Hello"): true
t1: Hello World, t1.starts_with("hello"): false

3.4 find() 方法

无论是 &str 还是 String 类型的对象， 都支持 find() 方法, 返回的是匹配到给定模式时当前 &str/string 的第一个字符的索引. 举例如下：

这里本来想写一个简单的例子， 不过编译报错了， 看起来 rust 的 println! 宏罢工了， 不能直接处理字符串的 find() 方法返回的结果：

对应的代码是 u6.rs(这是一个错误的示范， 了解的大佬欢迎在评论区赐教)

fn main() {
    let s1 = String::from("Hello World");
    let idx1 = s1.find("ello");
    let idx2 = s1.find("world");
    println!("s1: {}, s1.find(\"ello\"): {}", s1, idx1);
    println!("s1: {}, s1.find(\"world\"): {}", s1, idx2);

    let t1 = "Hello World";
    let idx1 = t1.find("ello");
    let idx2 = t1.find("world");
    println!("t1: {}, t1.find(\"ello\"): {}", t1, idx1);
    println!("s1: {}, s1.find(\"world\"): {}", s1, idx2);
}

zz@Legion-R7000P% rustc u6.rs
error[E0277]: `Option<usize>` doesn't implement `std::fmt::Display`
 --> u6.rs:5:51
  |
5 |     println!("s1: {}, s1.find(\"ello\"): {}", s1, idx1);
  |                                                   ^^^^ `Option` cannot be formatted with the default formatter
  |
  = help: the trait `std::fmt::Display` is not implemented for `Option`
  = note: in format strings you may be able to use `{:?}` (or {:#?} for pretty-print) instead
  = note: this error originates in the macro `$crate::format_args_nl` which comes from the expansion of the macro `println` (in Nightly builds, run with -Z macro-backtrace for more info)

error[E0277]: `Option` doesn't implement `std::fmt::Display`
 --> u6.rs:6:52
  |
6 |     println!("s1: {}, s1.find(\"world\"): {}", s1, idx2);
  |                                                    ^^^^ `Option<usize>` cannot be formatted with the default formatter
  |
  = help: the trait `std::fmt::Display` is not implemented for `Option<usize>`
  = note: in format strings you may be able to use `{:?}` (or {:#?} for pretty-print) instead
  = note: this error originates in the macro `$crate::format_args_nl` which comes from the expansion of the macro `println` (in Nightly builds, run with -Z macro-backtrace for more info)

error[E0277]: `Option<usize>` doesn't implement `std::fmt::Display`
  --> u6.rs:11:51
   |
11 |     println!("t1: {}, t1.find(\"ello\"): {}", t1, idx1);
   |                                                   ^^^^ `Option` cannot be formatted with the default formatter
   |
   = help: the trait `std::fmt::Display` is not implemented for `Option`
   = note: in format strings you may be able to use `{:?}` (or {:#?} for pretty-print) instead
   = note: this error originates in the macro `$crate::format_args_nl` which comes from the expansion of the macro `println` (in Nightly builds, run with -Z macro-backtrace for more info)

error[E0277]: `Option` doesn't implement `std::fmt::Display`
  --> u6.rs:12:52
   |
12 |     println!("s1: {}, s1.find(\"world\"): {}", s1, idx2);
   |                                                    ^^^^ `Option<usize>` cannot be formatted with the default formatter
   |
   = help: the trait `std::fmt::Display` is not implemented for `Option<usize>`
   = note: in format strings you may be able to use `{:?}` (or {:#?} for pretty-print) instead
   = note: this error originates in the macro `$crate::format_args_nl` which comes from the expansion of the macro `println` (in Nightly builds, run with -Z macro-backtrace for more info)

error: aborting due to 4 previous errors

For more information about this error, try `rustc --explain E0277`.

4. References

---

1. https://doc.rust-lang.org/std/primitive.str.html ↩︎

2. https://doc.rust-lang.org/std/string/struct.String.html# ↩︎

3. https://doc.rust-lang.org/src/alloc/string.rs.html#365 ↩︎