Rust Note(3)_Variables I

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Rust 靜態語言, 透過自身的 Ownership System 去管理記憶體,
有別於其他語言有 GC (Garbage Collector) 的方式.

1. Setting variables 變數賦值

使用let關鍵字, 如下:

Sample Code 1-1 變數賦值 let

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fn main() {
    let y : i64 = 10; // 設定變數y型別為整數型64位元, 值為10
    println!("The value of y is {}", y) // 印出y
}

Output:

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$ cargo run
   Compiling rust_project v0.1.0 (D:\000_git_repo_projects\rust_project)
    Finished dev [unoptimized + debuginfo] target(s) in 0.50s
     Running `target\debug\rust_project.exe`
The value of y is 10

2. 變數重複賦值

Compiler 直接拋出錯誤, 不可重複對不可變(不可改變值的變數)的變數賦值
範例 1-2

Sample Code 2-1 重複賦值, compiler報錯

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fn main() {
    let y : i64 = 10; // 設定變數y型別為整數型64位元, 值為10
    println!("The value of y is {}", y) // 印出y
    y = 20;
    println!("The new value of y is {}", y)
}

Output

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error[E0384]: cannot assign twice to immutable variable `y`
 --> src\main.rs:4:5
  |
2 |     let y : i64 = 10; // 設定變數y型別為整數型64位元, 值為10
  |         -
  |         |
  |         first assignment to `y`
  |         help: consider making this binding mutable: `mut y`
3 |     println!("The value of y is {}", y); // 印出y
4 |     y = 20;
  |     ^^^^^^ cannot assign twice to immutable variable

For more information about this error, try `rustc --explain E0384`.
error: could not compile `rust_project` due to previous error

看 terminal 輸出的訊息, 可把變數 y 的型態設定為 mut (mutable) 可變

Sample Code 2-2 指定為可變變數 mut

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fn main() {
    let mut y : i64 = 10; // 設定變數y型別為整數型64位元, 值為10
    println!("The value of y is {}", y); // 印出y
    y = 20;
    println!("The new value of y is {}", y);
}

Output

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$ cargo run
   Compiling rust_project v0.1.0 (D:\000_git_repo_projects\rust_project)
    Finished dev [unoptimized + debuginfo] target(s) in 0.37s
     Running `target\debug\rust_project.exe`
The value of y is 10
The new value of y is 20

告訴 compiler y 這是一個可改變的變數, 這樣就沒問題了.

3. 變數重新賦值

重新賦值 跟 重複賦值 不一樣定義

第一次變數賦值 : 把變數值存到 heap 堆 裡面
第二次重新賦值 : 重新覆蓋在 heap 堆 裡面的值

Sample Code 3-1 重新賦值, 比對上段 重複復職

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fn main() {
    let y : i64 = 10; // 設定變數y型別為整數型64位元, 值為10
    println!("The value of y is {}", y); // 印出y = 10
    let y : i64 = 20; // 重新賦值, 覆蓋過heap堆上的值, 重新取代
    println!("The new value of y is {}", y); // 印出y = 20
}

Output

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$ cargo run
    Finished dev [unoptimized + debuginfo] target(s) in 0.00s
     Running `target\debug\rust_project.exe`
The value of y is 10
The new value of y is 20

compiler 沒有報錯, 因為 y 重新 被賦值

此時如果指定 y 為 mut 型態, compiler 就會出現善意提醒

Sample Code 3-2 提醒mut為非必要

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fn main() {
    let mut y : i64 = 10; // 設定變數y型別為可變整數型64位元, 值為10
    println!("The value of y is {}", y); // 印出y = 10
    let y : i64 = 20; // 重新賦值, 覆蓋過heap堆上的值, 重新取代
    println!("The new value of y is {}", y); // 印出y = 20
}

Output

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warning: variable does not need to be mutable
 --> src\main.rs:2:9
  |
2 |     let mut y : i64 = 10; // 設定變數y型別為可變整數型64位元, 值為10
  |         ----^
  |         |
  |         help: remove this `mut`
  |
  = note: `#[warn(unused_mut)]` on by default

warning: `rust_project` (bin "rust_project") generated 1 warning
    Finished dev [unoptimized + debuginfo] target(s) in 0.39s
     Running `target\debug\rust_project.exe`
The value of y is 10
The new value of y is 20

Compiler 一樣打印出值, 但是會提醒, mut 是不必要的.

4. 常數 (不可變) const

常數 : 意味著 不可變的, 賦值後, 不可做任何修改
宣告常數使用關鍵字 : const <CONST_NAME> : 必須要大寫

Sample Code 4-1 : 提醒大寫

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fn main() {
    const y : i64 = 10; // 設定常數y型別為整數型64位元, 值為10
    println!("The value of y is {}", y); // 印出y = 10
}

Output

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   Compiling rust_project v0.1.0 (D:\000_git_repo_projects\rust_project)
warning: constant `y` should have an upper case name
 --> src\main.rs:2:11
  |
2 |     const y : i64 = 10; // 設定變數y型別為可變整數型64位元, 值為10
  |           ^ help: convert the identifier to upper case (notice the capitalization): `Y`
  |
  = note: `#[warn(non_upper_case_globals)]` on by default

warning: `rust_project` (bin "rust_project") generated 1 warning
    Finished dev [unoptimized + debuginfo] target(s) in 0.39s
     Running `target\debug\rust_project.exe`
The value of y is 10

Compiler 提醒需要大寫, 但仍正常編譯

對常數重新賦值, compiler 報錯

Sample Code 對常數重新賦值, 報錯

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fn main() {
    const Y : i64 = 10; // 設定常數y型別為整數型64位元, 值為10
    println!("The value of y is {}", Y); // 印出y = 10
    let Y : i64 = 20; // 重新賦值 報錯
    println!("{}", Y)
}

Output

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error[E0005]: refutable pattern in local binding: `i64::MIN..=9_i64` and `11_i64..=i64::MAX` not covered
 --> src\main.rs:4:9
  |
2 |     const Y : i64 = 10; // 設定常數y型別為整數型64位元, 值為10
  |     ------------------- constant defined here
3 |     println!("The value of y is {}", Y); // 印出y = 10
4 |     let Y : i64 = 20;
  |         ^
  |         |
  |         interpreted as a constant pattern, not a new variable
  |         help: introduce a variable instead: `y_var`
  |
  = note: the matched value is of type `i64`

For more information about this error, try `rustc --explain E0005`.
error: could not compile `rust_project` due to previous error

const 常數 為不可變動的值


5. 重點整理

  • 宣告設定變數
    • let : 一般變數
    • const : 常數
  • 變數可變
    • mut : 可變, 不套用常數(const)

Rust Note_20211020