Formerly clojure.contrib.priority-map.
A priority map is very similar to a sorted map, but whereas a sorted map produces a sequence of the entries sorted by key, a priority map produces the entries sorted by value.
In addition to supporting all the functions a sorted map supports, a priority map can also be thought of as a queue of [item priority] pairs. To support usage as a versatile priority queue, priority maps also support conj/peek/pop operations.
Latest stable release is [0.0.7]
Leiningen dependency information:
[org.clojure/data.priority-map "0.0.7"]
Maven dependency information:
<dependency>
<groupId>org.clojure</groupId>
<artifactId>data.priority-map</artifactId>
<version>0.0.7</version>
</dependency>
The standard way to construct a priority map is with priority-map:
user=> (def p (priority-map :a 2 :b 1 :c 3 :d 5 :e 4 :f 3))
#'user/p
user=> p
{:b 1, :a 2, :c 3, :f 3, :e 4, :d 5}
So :b has priority 1, :a has priority 2, and so on. Notice how the priority map prints in an order sorted by its priorities (i.e., the map's values)
We can use assoc to assign a priority to a new item:
user=> (assoc p :g 1)
{:b 1, :g 1, :a 2, :c 3, :f 3, :e 4, :d 5}
or to assign a new priority to an extant item:
user=> (assoc p :c 4)
{:b 1, :a 2, :f 3, :c 4, :e 4, :d 5}
We can remove an item from the priority map:
user=> (dissoc p :e)
{:b 1, :a 2, :c 3, :f 3, :d 5}
An alternative way to add to the priority map is to conj a [item priority] pair:
user=> (conj p [:g 0])
{:g 0, :b 1, :a 2, :c 3, :f 3, :e 4, :d 5}
or use into:
user=> (into p [[:g 0] [:h 1] [:i 2]])
{:g 0, :b 1, :h 1, :a 2, :i 2, :c 3, :f 3, :e 4, :d 5}
Priority maps are countable:
user=> (count p)
6
Like other maps, equivalence is based not on type, but on contents. In other words, just as a sorted-map can be equal to a hash-map, so can a priority-map.
user=> (= p {:b 1, :a 2, :c 3, :f 3, :e 4, :d 5})
true
You can test them for emptiness:
user=> (empty? (priority-map))
true
user=> (empty? p)
false
You can test whether an item is in the priority map:
user=> (contains? p :a)
true
user=> (contains? p :g)
false
It is easy to look up the priority of a given item, using any of the standard map mechanisms:
user=> (get p :a)
2
user=> (get p :g 10)
10
user=> (p :a)
2
user=> (:a p)
2
Priority maps derive much of their utility by providing priority-based seq. Note that no guarantees are made about the order in which items of the same priority appear.
user=> (seq p)
([:b 1] [:a 2] [:c 3] [:f 3] [:e 4] [:d 5])
Because no guarantees are made about the order of same-priority items, note that rseq might not be an exact reverse of the seq. It is only guaranteed to be in descending order.
user=> (rseq p)
([:d 5] [:e 4] [:c 3] [:f 3] [:a 2] [:b 1])
This means first/rest/next/for/map/etc. all operate in priority order.
user=> (first p)
[:b 1]
user=> (rest p)
([:a 2] [:c 3] [:f 3] [:e 4] [:d 5])
Priority maps support metadata:
user=> (meta (with-meta p {:extra :info}))
{:extra :info}
But perhaps most importantly, priority maps can also function as priority queues. peek, like first, gives you the first [item priority] pair in the collection. pop removes the first [item priority] from the collection. (Note that unlike rest, which returns a seq, pop returns a priority map).
user=> (peek p)
[:b 1]
user=> (pop p)
{:a 2, :c 3, :f 3, :e 4, :d 5}
It is also possible to use a custom comparator:
user=> (priority-map-by > :a 1 :b 2 :c 3)
{:c 3, :b 2, :a 1}
Sometimes, it is desirable to have a map where the values contain more information than just the priority. For example, let's say you want a map like:
{:a [2 :apple], :b [1 :banana], :c [3 :carrot]}
and you want to sort the map by the numeric priority found in the pair.
A common mistake is to try to solve this with a custom comparator:
(priority-map
(fn [[priority1 _] [priority2 _]] (< priority1 priority2))
:a [2 :apple], :b [1 :banana], :c [3 :carrot])
This will not work! In Clojure, like Java, all comparators must be total orders,
meaning that you can't have a "tie" unless the objects you are comparing are
in fact equal. The above comparator breaks that rule because
[2 :apple]
and [2 :apricot]
tie, but are not equal.
The correct way to construct such a priority map is by specifying a keyfn, which is used
to compute or extract the true priority from the priority map's vals. (Note: It might seem a little odd
that the priority-extraction function is called a keyfn, even though it is applied to the
map's values. This terminology is based on the docstring of clojure.core/sort-by, which
uses keyfn
for the function which computes the sort keys.)
In the above example,
user=> (priority-map-keyfn first :a [2 :apple], :b [1 :banana], :c [3 :carrot])
{:b [1 :banana], :a [2 :apple], :c [3 :carrot]}
You can also combine a keyfn with a comparator that operates on the extracted priorities:
user=> (priority-map-keyfn-by first > :a [2 :apple], :b [1 :banana], :c [3 :carrot])
{:c [3 :carrot], :a [2 :apple], :b [1 :banana]}
Copyright (C) 2013 Mark Engelberg
Distributed under the Eclipse Public License, the same as Clojure.