变量4：exercises/variables/variables4.rs

题目

rust

// variables4.rs
//
// Execute `rustlings hint variables4` or use the `hint` watch subcommand for a
// hint.

// I AM NOT DONE

fn main() {
    let x = 3;
    println!("Number {}", x);
    x = 5; // don't change this line
    println!("Number {}", x);
}

题目解析

编译器错误信息如下：

txt

⚠️  Compiling of exercises/variables/variables4.rs failed! Please try again. Here's the output:
error[E0384]: cannot assign twice to immutable variable `x`
  --> exercises/variables/variables4.rs:11:5
   |
9  |     let x = 3;
   |         -
   |         |
   |         first assignment to `x`
   |         help: consider making this binding mutable: `mut x`
10 |     println!("Number {}", x);
11 |     x = 5; // don't change this line
   |     ^^^^^ cannot assign twice to immutable variable

error: aborting due to previous error

For more information about this error, try `rustc --explain E0384`.

错误信息提示我们，不能对不可变变量二次赋值。

在Rust中，变量绑定默认是不可变的，这里尝试修改x的值，是不被允许的。如果我们想要修改变量x，可以使用mut将其声明为可变的。

rust

fn main() {
    let x = 3; 
    let mut x = 3; 
    println!("Number {}", x);
    x = 5; // don't change this line
    println!("Number {}", x);
}

笔记

变量默认是不可改变的（immutable）
如果要可变，可以使用mut

参考资料

变量与可变性

变量4：exercises/variables/variables4.rs ​

题目 ​

题目解析 ​

参考资料 ​

变量4：exercises/variables/variables4.rs

题目

题目解析

参考资料