View Category
Maps
Declaration

Define an empty map

python
map = {}
clojure
(def m {})
fsharp
let map = Map.empty
let map = new Generic.Dictionary<string, string>()
let map = new Hashtable()
fantom
map := [:]
groovy
def map = [:]
Map map = new HashMap();
java
Map map = new HashMap();
erlang
Map = dict:new(),
Map = orddict:new(),
Map = gb_trees:empty(),
Map = ets:new(the_map_name, [set, private, {keypos, 1}]),
perl
# %map = {}
# This was wrong, that would have created a hash with one key
# of the stringified hash reference (HASH(0xNUMBERSHERE)) and a
# value of 'undef', as well as triggering a
# "Reference found where even-sized list expected" with the warnings
# pragma enabled

my %map;

Define an unmodifiable empty map

python
import collections
EmptyDict = collections.namedtuple("EmptyDict", "")
e = EmptyDict()
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
fantom
map := [:].ro
groovy
empty = Collections.EMPTY_MAP
map = [:].asImmutable()
def empty = MapUtils.EMPTY_SORTED_MAP
def empty = ImmutableMap.of()
java
Map empty = Collections.EMPTY_MAP;
SortedMap empty = MapUtils.EMPTY_SORTED_MAP;
erlang

% Erlang data structures are immutable - updating a 'map' sees a modified copy created
Map = dict:new(),
perl
# perl does not provide unmodifiable maps/hashes, but you could use "constant
# functions", if you really need them
# 2011-07-06 Not actually true, see Hash::Util::lock_hash;

sub MAP () { {} }
use Hash::Util qw/lock_hash/;
# two lines
my %hash;
lock_hash(%hash);
# or in one line
lock_hash(my %locked_hash);

Define an initial map

Define the map {circle:1, triangle:3, square:4}
python
shapes = {'circle': 1, 'square': 4, 'triangle': 2}
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)]
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
java
Map shapes = new HashMap();
shapes.put("circle", 1);
shapes.put("triangle", 3);
shapes.put("square", 4);
Map shapes = new HashMap() {{ put("circle",1); put("triangle",3); put("square",4); }}
erlang
Map = dict:from_list([{circle, 1}, {triangle, 3}, {square, 4}]),
Map0 = dict:new(),

% Erlang variables are 'single-assignment' i.e. they cannot be reassigned
Map1 = dict:store(circle, 1, Map0),
Map2 = dict:store(triangle, 3, Map1),
Map3 = dict:store(square, 4, Map2),
Map0 = gb_trees:empty(),

Map1 = gb_trees:enter(circle, 1, Map0),
Map2 = gb_trees:enter(triangle, 3, Map1),
Map3 = gb_trees:enter(square, 4, Map2),
Map = gb_trees:from_orddict(lists:keysort(1, [{circle, 1}, {triangle, 3}, {square, 4}])),
Map = ets:new(the_map_name, [ordered_set, private, {keypos, 1}]),
ets:insert(Map, [{circle, 1}, {triangle, 3}, {square, 4}]),
perl
%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"
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"
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'
java
if (pets.containsKey("mary")) System.out.println("ok");
erlang
dict:is_key(mary, Pets) andalso begin io:format("ok~n"), true end.
IsMember = ets:member(Pets, mary), if (IsMember) -> io:format("ok~n") ; true -> false end.
case gb_trees:lookup(mary, Pets) of none -> false ; _ -> io:format("ok~n") end.
perl
%pets = (joe => 'cat', mary => 'turtle', bill => 'canary');
print 'ok' if ($pets{'mary'});
%pets = (joe => 'cat', mary => 'turtle', bill => 'canary');
print 'ok' if $pets{'mary'};
print 'ok' if $pets{mary};
print 'ok' if exists $pets{mary}

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)
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'
java
String pet = pets.get("joe");
erlang
dict:is_key(joe, Pets) andalso begin io:format("~w~n", [dict:fetch(joe, Pets)]), true end.
case dict:find(joe, Pets) of error -> false ; {ok, Pet} -> io:format("~w~n", [Pet]) end.
IsMember = ets:member(Pets, joe), if (IsMember) -> io:format("~w~n", [ets:lookup_element(Pets, joe, 2)]) ; true -> false end.
case ets:match(Pets, {joe, '$1'}) of [] -> false ; [[Pet]] -> io:format("~w~n", [Pet]) end.
case gb_trees:lookup(joe, Pets) of none -> false ; {value, Pet} -> io:format("~w~n", [Pet]) end.
perl
%pets = (joe => 'cat', mary => 'turtle', bill=>'canary');
print $pets{joe};

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")
fantom
map["rob"] = "dog"
groovy
pets['rob'] = 'dog'
pets.rob = 'dog'
pets.put('rob', 'dog')
java
pets.put("rob", "dog");
erlang
Pets1 = dict:store(rob, dog, Pets0).
ets:insert(Pets, {rob, dog}).
Pets1 = gb_trees:enter(rob, dog, Pets0).
perl
$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"
fantom
pet := map.remove("bill")
echo ("pet=$pet")
groovy
pets = [joe:'cat', mary:'turtle', bill:'canary']
println pets.remove('bill')
java
System.out.println(pets.remove("bill"))
erlang
Pet = dict:fetch(bill, Pets0), Pets1 = dict:erase(bill, Pets0), io:format("~w~n", [Pet]),
Pet = ets:lookup_element(Pets, bill, 2), ets:delete(Pets, bill), io:format("~w~n", [Pet]),
{value, Pet} = gb_trees:lookup(bill, Pets0), Pets1 = gb_trees:delete(bill, Pets0), io:format("~w~n", [Pet]),
perl
print delete $pets{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
python
from collections import defaultdict
h = defaultdict(int)
for k in "abacbb":
h[k] += 1

h = {}
for k in "abacbb":
h[k] = h.setdefault(k, 0) + 1
from collections import Counter
h = Counter("abacbb")
print(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])
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
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 }
java
Map map = new HashMap();
for (Iterator it = list.iterator(); it.hasNext();) {
String s = (String) it.next();
if (!map.containsKey(s)) {
map.put(s, new Integer(1));
} else {
map.put(s, new Integer(((Integer)map.get(s)).intValue() + 1));
}
}
LinkedMap histogram = new LinkedMap();

for (Object letter : list)
histogram.put(letter, !histogram.containsKey(letter) ? 1 : MapUtils.getIntValue(histogram, letter) + 1);
erlang
% Imperative Solution
Histogram = histogram(List),
% Functional (1) Solution
Histogram = histogram(List),
lists:foldl(fun(Elem, OldDict) ->
dict:update_counter(Elem, 1, OldDict)
end,
dict:new(),
[a,b,a,c,b,b])).
perl
foreach(@list) {
$histogram{$_}++;
}
$histogram{$_}++ for @list;

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
python
c = defaultdict(list)
for k in ["one", "two", "four", "three", "five"]:
c[len(k)].append(k)
from itertools import groupby
lst = ["one", "two", "four", "three", "five"]
c = dict((k, list(g)) for k,g in
groupby(sorted(lst, key=lambda x: len(x)), key=lambda x: len(x)))
print(c)
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
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() }
java
SortedMap<Integer, List<String> > map = new TreeMap<Integer, List<String> >(); int key; List<String> vlist;

for (String item : list)
{
key = item.length(); vlist = map.containsKey(key) ? map.get(key) : new ArrayList<String>();
vlist.add(item); map.put(key, vlist);
}
MultiValueMap map = new MultiValueMap();
for (Object item : list) map.put(((String) item).length(), item);
erlang
% Imperative Solution
CatList = categorise(List),
% Functional (1) Solution
CatList = categorise(List),
perl
@list = qw(one two three four five);
push @{$map{length($_)}}, $_ for (@list);