View Category
Maps
Declaration

Define an empty map

php
$map = array();
$map = array("" => "");
clojure
(def m {})

Define an unmodifiable empty map

php
/* It's not possible to define an array as a constant
* Instead we can use the serialize-function */

$fruits = array("apple", "banana", "orange");
define("FRUITS", serialize($fruits));
echo FRUITS; // a:3:{i:0;s:5:"apple";i:1;s:6:"banana";i:2;s:6:"orange";}
$my_fruits = unserialize(FRUITS); // and normal array again
clojure
; Clojure maps are immutable
(def m {})

Define an initial map

Define the map {circle:1, triangle:3, square:4}
php
$map = array("circle" => 1, "triangle" => 3, "square" => 4);
clojure
(def m '{circle 1 triangle 1 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"
php
if (array_key_exists("mary", $pets)) {
echo "ok";
}
if (isset($pets["mary"])) { // only works if $pets["mary"] can't be false or 0
echo "ok";
}
clojure
(if (contains? '{joe cat mary turtle bill canary} '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")
php
echo $pets["joe"];
clojure
(def pets '{joe cat mary turtle bill canary})

(println (get pets 'joe))

Add an entry to a map

Given an empty pets map, add the mapping from "rob" to "dog"
php
$pets["rob"] = "dog";
clojure
(assoc {} '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"
php
$last = array_pop($pets); // removes last key and assign it to the variable
echo $last;
$last = $pets[count($pets)-1];
echo $last;
unset($last);
$bill = $pets["bill"];
echo $bill;
unset($bill);
print_r(array_pop($pets));
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))
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
php
$list = array("a","b","a","c","b","b");
$map = array_count_values($list);
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])

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
php
foreach ($array as $m) {
$l = strlen($m);
$map[$l][] = $m;
}
arsort($map);
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"])