A map is a special kind of set, it associates keys with values. A map is created similarly as a set.
let capitals =
Map.empty.
Add("Canada", "Ottawa").
Add("China", "Beijing").
Add("Russia", "Moscow").
Add("USA", "Washington D.C.");
The FSharp.Collections.Map module contains a variety of useful methods for working with maps.
| Function | Description |
|---|---|
| Add(key, value) | Returns a new map with the binding added to the given map. If a binding with the given key already exists in the input map, the existing binding is replaced by the new binding in the resulting map. |
| Change(key, f) | Returns a new map with the value stored under key changed according to f. |
| ContainsKey(key) | Tests if an element is in the domain of the map. |
| Count | The number of bindings on the map. |
| IsEmpty | Returns true if there are no bindings in the map. |
| [key] | Lookup an element in the map. Raise KeyNotFoundException if no binding exists in the map. |
| Remove(key) | Removes an element from the domain of the map. No exception is raised if the element is not present. |
| TryFind(key) | Lookup an element in the map, returning a Some value if the element is in the map domain and None if not. |
| TryGetValue(key, value) | Lookup an element in the map, assigning to value if the element is in the map domain and returning false if not. |
The following example uses the .[key] to access elements in the map.
let capital = capitals.["Canada"];
Console.WriteLine("The capital of Canada is {0}", capital)
You can also convert the list to a map using the following syntax.
let capitals1 =
[("Canada", "Ottawa"); ("China", "Beijing");
("Denmark", "Copenhagen"); ("Egypt", "Cairo");
("France", "Paris"); ("Germany", "Berlin");
("Japan", "Tokyo"); ("Russia", "Moscow");
("Spain", "Madrid"); ("Sweden", "Stockholm");
("Taiwan", "Taipei"); ("USA", "Washington D.C.")]
|> Map.ofList
You can also use the TryFind method to get the value of a particular key.
let found = capitals1.TryFind "France"
match found with
| Some x -> printfn "Found %s." x
| None -> printfn "Did not find the specified value."