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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.

Releases and Dependency Information

Latest stable release is [0.0.7]

Leiningen dependency information:

[org.clojure/data.priority-map "0.0.7"]

Maven dependency information:



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
{: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)

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})

You can test them for emptiness:

user=> (empty? (priority-map))

user=> (empty? p)

You can test whether an item is in the priority map:

user=> (contains? p :a)

user=> (contains? p :g)

It is easy to look up the priority of a given item, using any of the standard map mechanisms:

user=> (get p :a)

user=> (get p :g 10)

user=> (p :a)

user=> (:a p)

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:

  (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.