一 創(chuàng)建項(xiàng)目

cargo new variables

二 變量可變性的實(shí)例錯(cuò)誤代碼

vim ./src/main.rs

fn main() {
    let x = 5;
    println!("The value of x is: {}", x);
    x = 6;
    println!("The value of x is: {}", x);
    
}

三 編譯查看錯(cuò)誤提示

cargo run

cargo run 
   Compiling variables v0.1.0 (/home/li/projects/variables)
error[E0384]: cannot assign twice to immutable variable `x`
 --> src/main.rs:4:5
  |
2 |     let x = 5;
  |         -
  |         |
  |         first assignment to `x`
  |         help: make this binding mutable: `mut x`
3 |     println!("The value of x is: {}", x);
4 |     x = 6;
  |     ^^^^^ cannot assign twice to immutable variable

error: aborting due to previous error

小結(jié)：默認(rèn)情況，let定義的變量是不可變的，如果可變需加關(guān)鍵字mut

四 增加mut關(guān)鍵字，讓變量可變

vim ./src/main.rs

fn main() {
    let mut x = 5;
    println!("The value of x is: {}", x);
    x = 6;
    println!("The value of x is: {}", x);
    
}

輸出

cargo run 
   Compiling variables v0.1.0 (/home/li/projects/variables)
    Finished dev [unoptimized + debuginfo] target(s) in 4.98s
     Running `target/debug/variables`
The value of x is: 5
The value of x is: 6

五 常量

使用const關(guān)鍵字定義常量
const MAX_POINTS: u32 = 100_000; //在數(shù)字字面值下面插入下劃線來提高數(shù)字可讀性，這里等價(jià)于 100000

六 隱藏（shadowing）變量

重復(fù)使用let關(guān)鍵字，定義同名變量來隱藏之前的變量。
實(shí)際上是創(chuàng)建一個(gè)新的變量。
可改變變量的類型和值

fn main() {
    let x = 5;
    println!("The value of x is: {}", x);
    let x = "6+1";
    println!("The value of x is: {}", x);
}

如果使用mut，則會(huì)報(bào)編譯錯(cuò)誤
因?yàn)閙ut不允許改變變量類型

fn main() {
    let mut x = 5;
    println!("The value of x is: {}", x);
    x = "6+1";
    println!("The value of x is: {}", x);
    
}

cargo run
   Compiling variables v0.1.0 (/home/li/projects/variables)
error[E0308]: mismatched types
 --> src/main.rs:4:9
  |
4 |     x = "6+1";
  |         ^^^^^ expected integer, found reference
  |
  = note: expected type `{integer}`
             found type `&'static str`

七總結(jié)

1 使用大型數(shù)據(jù)結(jié)構(gòu)時(shí)，適當(dāng)?shù)厥褂每勺冏兞?br> 2 對(duì)于較小的數(shù)據(jù)結(jié)構(gòu)，總是創(chuàng)建新實(shí)例
3 了解let，mut，const關(guān)鍵字用法