F＃ - 可变数据（ Mutable Data）

F＃中的变量是immutable,这意味着一旦变量绑定到某个值，就无法更改。 它们实际上被编译为静态只读属性。

以下示例演示了这一点。

例子 (Example)

let x = 10
let y = 20
let z = x + y
printfn "x: %i" x
printfn "y: %i" y
printfn "z: %i" z
let x = 15
let y = 20
let z = x + y
printfn "x: %i" x
printfn "y: %i" y
printfn "z: %i" z

编译并执行该程序时，它显示以下错误消息 -

Duplicate definition of value 'x'
Duplicate definition of value 'Y'
Duplicate definition of value 'Z'

可变变量

有时您需要更改存储在变量中的值。 为了指定在程序的后续部分中声明和赋值变量的值可能发生变化，F＃提供了mutable关键字。 您可以使用此关键字声明和分配可变变量，您将更改其值。

mutable关键字允许您在可变变量中声明和赋值。

您可以使用let关键字为可变变量分配一些初始值。 但是，要为其分配新的后续值，您需要使用《-运算符”。

例如，

let mutable x = 10
x <- 15

以下示例将清除概念 -

例子 (Example)

let mutable x = 10
let y = 20
let mutable z = x + y
printfn "Original Values:"
printfn "x: %i" x
printfn "y: %i" y
printfn "z: %i" z
printfn "Let us change the value of x"
printfn "Value of z will change too."
x <- 15
z <- x + y
printfn "New Values:"
printfn "x: %i" x
printfn "y: %i" y
printfn "z: %i" z

编译并执行程序时，它会产生以下输出 -

Original Values:
x: 10
y: 20
z: 30
Let us change the value of x
Value of z will change too.
New Values:
x: 15
y: 20
z: 35

可变数据的使用

通常需要可变数据并用于数据处理，特别是对于记录数据结构。 以下示例演示了这一点 -

open System
type studentData =
   { ID : int;
      mutable IsRegistered : bool;
      mutable RegisteredText : string; }
let getStudent id =
   { ID = id;
      IsRegistered = false;
      RegisteredText = null; }
let registerStudents (students : studentData list) =
   students |> List.iter(fun st ->
      st.IsRegistered <- true
      st.RegisteredText <- sprintf "Registered %s" (DateTime.Now.ToString("hh:mm:ss"))
      Threading.Thread.Sleep(1000) (* Putting thread to sleep for 1 second to simulate processing overhead. *))
let printData (students : studentData list) =
   students |> List.iter (fun x -> printfn "%A" x)
let main() =
   let students = List.init 3 getStudent
   printfn "Before Process:"
   printData students
   printfn "After process:"
   registerStudents students
   printData students
   Console.ReadKey(true) |> ignore
main()

编译并执行程序时，它会产生以下输出 -

Before Process:
{ID = 0;
IsRegistered = false;
RegisteredText = null;}
{ID = 1;
IsRegistered = false;
RegisteredText = null;}
{ID = 2;
IsRegistered = false;
RegisteredText = null;}
After process:
{ID = 0;
IsRegistered = true;
RegisteredText = "Registered 05:39:15";}
{ID = 1;
IsRegistered = true;
RegisteredText = "Registered 05:39:16";}
{ID = 2;
IsRegistered = true;
RegisteredText = "Registered 05:39:17";}