View Category
Maps
Declaration

Define an empty map

ruby
map = {}
clojure
(def m {})
cpp
Hashtable^ hash = gcnew Hashtable;
Generic::Dictionary<String^, String^>^ dict = gcnew Generic::Dictionary<String^, String^>();
std::map<int, std::string> m;
fantom
map := [:]
groovy
def map = [:]
Map map = new HashMap();

Define an unmodifiable empty map

ruby
map = {}.freeze
clojure
; Clojure maps are immutable
(def m {})
cpp
const std::map<T1,T2> immutable_map_instance_of_type_t1_to_t2;
fantom
map := [:].ro
groovy
empty = Collections.EMPTY_MAP
map = [:].asImmutable()
def empty = MapUtils.EMPTY_SORTED_MAP
def empty = ImmutableMap.of()

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 }
clojure
(def m '{circle 1 triangle 1 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;
fantom
map := ["circle":1, "triangle":2, "square":4]
groovy
shapes = [circle:1, triangle:3, square:4]
// if you require a specific type of map ...
LinkedHashMap shapes1 = [circle:1, triangle:3, square:4]
Properties shapes2 = [circle:1, triangle:3, square:4]
TreeMap shapes3 = [circle:1, triangle:3, square:4]
shapes4 = [circle:1, triangle:3, square:4] as ConcurrentHashMap // as variation
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
clojure
(if (contains? '{joe cat mary turtle bill canary} 'mary)
(println "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;
fantom
map := ["joe":"cat", "mary":"turtle", "bill":"canary"]
if (map.containsKey("mary")) echo("ok")
groovy
pets = [joe:'cat', mary:'turtle', bill:'canary']
if(pets.containsKey('mary')) println 'ok'
pets = [joe:'cat', mary:'turtle', bill:'canary']
if(pets.mary) println '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']
clojure
(def pets '{joe cat mary turtle bill canary})

(println (get pets 'joe))
cpp
if (pets->ContainsKey("joe")) Console::WriteLine(pets["joe"]);
cout << pets["joe"] << endl;
fantom
map := ["joe":"cat", "mary":"turtle", "bill":"canary"]
pet := map["joe"]
echo("pet=$pet")
groovy
pets = [joe:'cat', mary:'turtle', bill:'canary']
assert pets['joe'] == 'cat'
assert pets.joe == 'cat'

Add an entry to a map

Given an empty pets map, add the mapping from "rob" to "dog"
ruby
pets['rob']='dog'
clojure
(assoc {} 'rob 'dog)
cpp
pets->Add("rob", "dog");
pets["rob"] = "dog";
fantom
map["rob"] = "dog"
groovy
pets['rob'] = 'dog'
pets.rob = 'dog'
pets.put('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
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))
cpp
if (pets->ContainsKey("bill"))
{
String^ value = safe_cast<String^>(pets["bill"]); pets->Remove("bill");
Console::WriteLine("{0}", value);
}
fantom
pet := map.remove("bill")
echo ("pet=$pet")
groovy
pets = [joe:'cat', mary:'turtle', bill:'canary']
println pets.remove('bill')
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}
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])
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;
fantom
list := ["a","b","a","c","b","b"]
map := [Str:Int][:]
list.each |Str s, Int i| { if(!map.containsKey(s)) map.add(s,1); else map[s] = ++map[s] }
echo (map)
groovy
histogram = [:]
list.each { item ->
if (!histogram.containsKey(item)) histogram[item] = 0
histogram[item]++
}
histogram = [:]
list.each { histogram[it] = (histogram[it] ?: 0) + 1 }

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 }
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"])
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);
}
fantom
list := ["one", "two", "three", "four", "five"]
map := [Int:List][:]
list.each { List l := map[it.size] ?: [,]; map[it.size] = l.add(it) }
echo(map)
groovy
map = ['one', 'two', 'three', 'four', 'five'].groupBy{ it.size() }