langref.org - ruby, cpp, clojure, and fsharp

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Maps

Declaration

Define an empty map

ruby

map = {}

cpp

Hashtable^ hash = gcnew Hashtable;

Generic::Dictionary<String^, String^>^ dict = gcnew Generic::Dictionary<String^, String^>();

std::map<int, std::string> m;

clojure

(def m {})

fsharp

let map = Map.empty

let map = new Generic.Dictionary<string, string>()

let map = new Hashtable()

Define an unmodifiable empty map

ruby

map = {}.freeze

cpp

const std::map<T1,T2> immutable_map_instance_of_type_t1_to_t2;

clojure

; Clojure maps are immutable
(def m {})

fsharp

// Most native fsharp data structures are immutable - updating a 'map' sees a modified copy created
let map = Map.empty

Define an initial map

Define the map {circle:1, triangle:3, square:4}

ruby

shapes = {'circle'=>1, 'triangle'=>3, 'square'=>4}

shapes = Hash['circle', 1, 'triangle', 3, 'square', 4]

shapes = { :circle => 1, :triangle => 3, :square => 4 }

cpp

Hashtable^ shapes = gcnew Hashtable;

shapes->Add("circle", 1);
shapes->Add("triangle", 3);
shapes->Add("square", 4);

Generic::Dictionary<String^, int>^ shapes = gcnew Generic::Dictionary<String^, int>();

shapes->Add("circle", 1);
shapes->Add("triangle", 3);
shapes->Add("square", 4);

map<string, int> shapes;

shapes["circle"] = 1;
shapes["triangle"] = 3;
shapes["square"] = 4;

clojure

(def m '{circle 1 triangle 1 square 4})

fsharp

let shapes = Map.ofList [("circle", 1); ("triangle", 3); ("square", 4)]

let shapes = Map.empty.Add("circle", 1).Add("triangle", 3).Add("square", 4)

let shapes = new Generic.Dictionary<string, int>()
shapes.Add("circle", 1)
shapes.Add("triangle", 3)
shapes.Add("square", 4)

let shapes = Map [("circle", 1); ("triangle", 3); ("square", 4)]

Access

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"

ruby

puts "ok" if map.has_key?('mary')

puts "ok" if map['mary'] # Only works if map entry can't be nil or false

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;

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"

Retrieve a value from a map

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

ruby

puts map['joe']

cpp

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

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

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)

Add an entry to a map

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

ruby

pets['rob']='dog'

cpp

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

pets["rob"] = "dog";

clojure

(assoc {} 'rob 'dog)

fsharp

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

pets.Add("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"

ruby

puts map.delete :bill

cpp

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

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"

Algorithms

Create a histogram map from a list

Given the list [a,b,a,c,b,b], produce a map {a:2, b:3, c:1} which contains the count of each unique item in the list

ruby

histogram = {}
list.each { |item| histogram[item] = (histogram[item] || 0) +1 }

list = %w{a b a c b b}

histogram = list.each_with_object(Hash.new(0)) do |item, hash|
hash[item] += 1
end

p histogram # => {"a"=>2, "b"=>3, "c"=>1}

list.inject(Hash.new(0)) {|h, item| h[item] += 1; h}

cpp

for each(String^ entry in input) hash[entry] = hash->ContainsKey(entry)
? Convert::ToInt32(hash[entry]->ToString()) + 1 : 1;

for each(String^ entry in input) dict[entry] = dict->ContainsKey(entry) ? dict[entry] + 1 : 1;

map<string,int> hist;
for (auto e: { "a","b","a","c","b","b" })
++hist[e];
for (auto e: hist)
cout << e.first << " : " << e.second << endl;

clojure

(let [l '[a b a c b b]]
(loop [m {}
d (distinct l)]
(let [item (first d)]
(if (zero? (count d))
m
(recur
(assoc m
item
(count
(filter #(= item %) l)))
(rest d))))))

(->> [:a :b :a :c :b :b]
(group-by identity)
(reduce (fn [m e] (assoc m (first e) (count (second e)))) {}))

(reduce conj {} (for [[x xs] (group-by identity "abacbb")] [x (count xs)]))

(frequencies ["a","b","a","c","b","b"])

(frequencies '[a b a c b b])

fsharp

let histogram = (List.foldLeft (fun (acc : Map<char, int>) (e : char) -> if (Map.mem e acc) then (Map.add e ((Map.find e acc) + 1) acc) ; else (Map.add e 1 acc)) (Map.empty) list)

let histogram list =
let rec histogram' list' dict' =
match list' with
| [] -> dict'
| x :: xs ->
match Map.tryFind x dict' with
| Some(Value) -> histogram' xs (Map.add x (Value + 1) dict')
| None -> histogram' xs (Map.add x 1 dict')
histogram' list Map.empty

// ------

let histogram' = histogram list

let histogram = (List.foldLeft (fun (acc : Generic.Dictionary<char, int>) (e : char) -> (if acc.ContainsKey(e) then acc.[e] <- acc.[e] + 1 ; else acc.Add(e, 1)) ; acc) (new Generic.Dictionary<char, int>()) list)

let histogram =
list
|> Seq.groupBy (fun a -> a)
|> Seq.map(fun (key, elements) -> key, Seq.length elements)
|> Map.ofSeq

Categorise a list

Given the list [one, two, three, four, five] produce a map {3:[one, two], 4:[four, five], 5:[three]} which sorts elements into map entries based on their length

ruby

lengths = {}
list.each do |x|
len = x.size
lengths[len] = (lengths[len] || [])
lengths[len] << x
end

lengths = list.group_by {|x| x.size}

list.inject({}) { |h,x| (h[x.size]||=[]) << x; h }

cpp

for each(String^ entry in input)
{
key = entry->Length;
if (!hash->ContainsKey(key)) hash[key] = gcnew ArrayList;
safe_cast<ArrayList^>(hash[key])->Add(entry);
}

clojure

(loop [m {}
l ["one" "two" "three" "four" "five"]]
(if (zero? (count l))
m
(let [item (first l)
key (count item)]
(recur
(assoc m key (cons item (get m key [])))
(rest l)))))

(group-by count ["one" "two" "three" "four" "five"])

fsharp

let catmap = (List.foldLeft (fun (acc : Map<int, List<string> >) (e : string) -> if (Map.mem e.Length acc) then (Map.add e.Length ((Map.find e.Length acc) @ [e]) acc) ; else (Map.add e.Length [e] acc)) (Map.empty) list)

let lengthMap =
["one"; "two"; "three"; "four"; "five"]
|> Seq.groupBy (fun s -> s.Length)
|> Seq.map (fun (length, entries) -> (length, entries |> List.ofSeq))
|> Map.ofSeq