langref.org - python, clojure, fsharp, and cpp - Maps

Check if a key exists in a map

Given a map pets {joe:cat,mary:turtle,bill:canary} print "ok" if an pet exists for "mary"

python

pets = dict(joe='cat', mary='turtle', bill='canary')
if ("mary" in pets) print "ok"

clojure

(if (contains? '{joe cat mary turtle bill canary} 'mary)
(println "ok"))

fsharp

if (Map.mem "mary" pets) then printfn "ok"

if pets.ContainsKey("mary") then printfn "ok"

cpp

if (pets->ContainsKey("mary")) Console::WriteLine("ok");

if (pets.find("mary") != pets.end()){
std::cout << "ok" << std::endl;
}

if (pets.count("mary") > 0)
cout << "ok" << endl;

Retrieve a value from a map

Given a map pets {joe:cat,mary:turtle,bill:canary} print the pet for "joe" ("cat")

python

print pets['joe']

clojure

(def pets '{joe cat mary turtle bill canary})

(println (get pets 'joe))

fsharp

if (Map.mem "joe" pets) then printfn "%s" (Map.find "joe" pets)

if (pets |> Map.exists (fun key _ -> key = "joe")) then printfn "%s" (Map.find "joe" pets)

let key = "joe"
match (pets |> Map.tryfind key) with
| Some(value) -> printfn "%s" value
| None -> printfn "Key %s not found" key

if pets.ContainsKey("joe") then printfn "%s" pets.["joe"]

if pets.ContainsKey("joe") then printfn "%s" (pets.["joe"] :?> string)

cpp

if (pets->ContainsKey("joe")) Console::WriteLine(pets["joe"]);

cout << pets["joe"] << endl;

Add an entry to a map

Given an empty pets map, add the mapping from "rob" to "dog"

python

pets['rob'] = 'dog'

clojure

(assoc {} 'rob 'dog)

fsharp

pets <- (Map.add "rob" "dog" pets)

pets.Add("rob", "dog")

cpp

pets->Add("rob", "dog");

pets["rob"] = "dog";

Remove an entry from a map

Given a map pets {joe:cat,mary:turtle,bill:canary} remove the mapping for "bill" and print "canary"

python

print pets.pop('bill')

clojure

; Maps are immutable
; The following expression will return a new map without the 'bill key
(let [pets '{joe cat mary turtle bill canary}]
(println (get pets 'bill))
(dissoc pets 'bill))

fsharp

let key = "bill"
match (pets |> Map.tryFind key) with
| Some(value) -> pets <- (Map.remove key pets) ; printfn "%s : %s removed" key value
| None -> printfn "Key %s not found" key

let key = "bill"
let entry = if (pets.ContainsKey(key)) then Some(pets.[key]) ; else None
pets.Remove(key)

match entry with
| Some(value) -> printfn "%s" value
| None -> printfn "key not found"

cpp

if (pets->ContainsKey("bill"))
{
String^ value = safe_cast<String^>(pets["bill"]); pets->Remove("bill");
Console::WriteLine("{0}", value);
}