F#映射(Maps)

在F#中，字典(Maps)是一种特殊的集合，将值(value)与键(key)相关联。

创建字典

通过使用Map.empty创建空Map并使用添加功能添加项目来创建Map。以下示例演示了这一点-

(* Create an empty Map *)
let students =
   Map.empty. (* Creating an empty Map *)
      Add("Zara Ali", "1501").
      Add("Rishita Gupta", "1502").
      Add("Robin Sahoo", "1503").
      Add("Gillian Megan", "1504");;
printfn "Map- students: %A" students

(* Convert a list to Map *)
let capitals =
   [ "Argentina", "Buenos Aires";
      "France ", "Paris";
      "Chili", "Santiago";
      "Malaysia", " Kuala Lumpur";
      "Switzerland", "Bern" ]
   |> Map.ofList;;
printfn "Map capitals : %A" capitals

Map - students: map
[("Gillian Megan", "1504"); ("Rishita Gupta", "1502"); ("Robin Sahoo", "1503
");
("Zara Ali", "1501")]
Map capitals : map
[("Argentina", "Buenos Aires"); ("Chili", "Santiago"); ("France ", "Paris");
("Malaysia", " Kuala Lumpur"); ("Switzerland", "Bern")]

您可以使用键访问Map中的各个元素。

(* Create an empty Map *)
let students =
   Map.empty. (* Creating an empty Map *)
      Add("Zara Ali", "1501").
      Add("Rishita Gupta", "1502").
      Add("Robin Sahoo", "1503").
      Add("Gillian Megan", "1504");;
printfn "Map- students: %A" students

(*Accessing an element using key *)
printfn "%A" students.["Zara Ali"]

Map - students: map
[("Gillian Megan", "1504"); ("Rishita Gupta", "1502"); ("Robin Sahoo", "1503
");
("Zara Ali", "1501")]
"1501"

基本操作

下表显示了Map上的基本操作-

方法	说明
Add	返回新Map,并将绑定添加到给定Map。
ContainsKey	测试元素是否在Map的域中。
Count	Map中的绑定数。
IsEmpty	如果Map中没有绑定,则返回true。
Item	在Map中查找元素。如果Map中没有绑定,则引发KeyNotFoundException。
Remove	从Map的域中删除一个元素。如果元素不存在,则不会引发异常。
TryFind	在Map中查找元素,如果元素在Map的域中,则返回 Some 值,否则返回 None 。

以下示例演示了上述某些功能的用法-

(* Create an empty Map *)
let students =
   Map.empty. (* Creating an empty Map *)
      Add("Zara Ali", "1501").
      Add("Rishita Gupta", "1502").
      Add("Robin Sahoo", "1503").
      Add("Gillian Megan", "1504").
      Add("Shraddha Dubey", "1505").
      Add("Novonil Sarker", "1506").
      Add("Joan Paul", "1507");;
printfn "Map- students: %A" students
printfn "Map- number of students: %d" students.Count

(* finding the registration number of a student*)
let found = students.TryFind "Rishita Gupta"
match found with
| Some x -> printfn "Found %s." x
| None -> printfn "Did not find the specified value."

Map - students: map
[("Gillian Megan", "1504"); ("Joan Paul", "1507"); ("Novonil Sarker", "1506"
);
("Rishita Gupta", "1502"); ("Robin Sahoo", "1503");
("Shraddha Dubey", "1505"); ("Zara Ali", "1501")]
Map - number of students: 7
Found 1502.