Priority queue

Priority queue is a draft programming task. It is not yet considered ready to be promoted as a complete task, for reasons that should be found in its talk page.

A priority queue is somewhat similar to a queue, with an important distinction: each item is added to a priority queue with a priority level, and will be later removed from the queue with the highest priority element first. That is, the items are (conceptually) stored in the queue in priority order instead of in insertion order.

Task: Create a priority queue. The queue must support at least two operations:

1. Insertion. An element is added to the queue with a priority (a numeric value).
2. Top item removal. Deletes the element or one of the elements with the current top priority and return it.

Optionally, other operations may be defined, such as peeking (find what current top priority/top element is), merging (combining two priority queues into one), etc.

To test your implementation, insert a number of elements into the queue, each with some random priority. Then dequeue them sequentially; now the elements should be sorted by priority. You can use the following task/priority items as input data:

Priority    Task
  3        Clear drains
  4        Feed cat
  5        Make tea
  1        Solve RC tasks
  2        Tax return

The implementation should try to be efficient. A typical implementation has O(log n) insertion and extraction time, where n is the number of items in the queue. You may choose to impose certain limits such as small range of allowed priority levels, limited capacity, etc. If so, discuss the reasons behind it.

J

Implementation:

<lang j>coclass 'priorityQueue'

PRI=: QUE=:

insert=:4 :0

 p=. PRI,x
 q=. QUE,y
 assert. p -:&$ q
 assert. 1 = #$q
 ord=: \: p
 QUE=: ord { q
 PRI=: ord { p
 i.0 0

)

topN=:3 :0

 assert y<:#PRI
 r=. y{.QUE
 PRI=: y}.PRI
 QUE=: y}.QUE
 r

)</lang>

Efficiency is obtained by batching requests. Size of batch for insert is determined by size of arguments. Size of batch for topN is its right argument.

Example:

<lang j> Q=: conew'priorityQueue'

  3 4 5 1 2 insert__Q 'clear drains';'feed cat';'make tea';'solve rc task';'tax return'
  >topN__Q 1

make tea

  >topN__Q 4

feed cat clear drains tax return solve rc task</lang>

Python

Python has the class queue.PriorityQueue in its standard library.

The data structures in the "queue" module are synchronized multi-producer, multi-consumer queues for multi-threaded use. <lang python>>>> import queue >>> help(queue.PriorityQueue) Help on class PriorityQueue in module queue:

class PriorityQueue(Queue)

|  Variant of Queue that retrieves open entries in priority order (lowest first).
|  
|  Entries are typically tuples of the form:  (priority number, data).
|  
|  Method resolution order:
|      PriorityQueue
|      Queue
|      builtins.object
|  
|  Methods inherited from Queue:
|  
|  __init__(self, maxsize=0)
|  
|  empty(self)
|      Return True if the queue is empty, False otherwise (not reliable!).
|      
|      This method is likely to be removed at some point.  Use qsize() == 0
|      as a direct substitute, but be aware that either approach risks a race
|      condition where a queue can grow before the result of empty() or
|      qsize() can be used.
|      
|      To create code that needs to wait for all queued tasks to be
|      completed, the preferred technique is to use the join() method.
|  
|  full(self)
|      Return True if the queue is full, False otherwise (not reliable!).
|      
|      This method is likely to be removed at some point.  Use qsize() >= n
|      as a direct substitute, but be aware that either approach risks a race
|      condition where a queue can shrink before the result of full() or
|      qsize() can be used.
|  
|  get(self, block=True, timeout=None)
|      Remove and return an item from the queue.
|      
|      If optional args 'block' is true and 'timeout' is None (the default),
|      block if necessary until an item is available. If 'timeout' is
|      a positive number, it blocks at most 'timeout' seconds and raises
|      the Empty exception if no item was available within that time.
|      Otherwise ('block' is false), return an item if one is immediately
|      available, else raise the Empty exception ('timeout' is ignored
|      in that case).
|  
|  get_nowait(self)
|      Remove and return an item from the queue without blocking.
|      
|      Only get an item if one is immediately available. Otherwise
|      raise the Empty exception.
|  
|  join(self)
|      Blocks until all items in the Queue have been gotten and processed.
|      
|      The count of unfinished tasks goes up whenever an item is added to the
|      queue. The count goes down whenever a consumer thread calls task_done()
|      to indicate the item was retrieved and all work on it is complete.
|      
|      When the count of unfinished tasks drops to zero, join() unblocks.
|  
|  put(self, item, block=True, timeout=None)
|      Put an item into the queue.
|      
|      If optional args 'block' is true and 'timeout' is None (the default),
|      block if necessary until a free slot is available. If 'timeout' is
|      a positive number, it blocks at most 'timeout' seconds and raises
|      the Full exception if no free slot was available within that time.
|      Otherwise ('block' is false), put an item on the queue if a free slot
|      is immediately available, else raise the Full exception ('timeout'
|      is ignored in that case).
|  
|  put_nowait(self, item)
|      Put an item into the queue without blocking.
|      
|      Only enqueue the item if a free slot is immediately available.
|      Otherwise raise the Full exception.
|  
|  qsize(self)
|      Return the approximate size of the queue (not reliable!).
|  
|  task_done(self)
|      Indicate that a formerly enqueued task is complete.
|      
|      Used by Queue consumer threads.  For each get() used to fetch a task,
|      a subsequent call to task_done() tells the queue that the processing
|      on the task is complete.
|      
|      If a join() is currently blocking, it will resume when all items
|      have been processed (meaning that a task_done() call was received
|      for every item that had been put() into the queue).
|      
|      Raises a ValueError if called more times than there were items
|      placed in the queue.
|  
|  ----------------------------------------------------------------------
|  Data descriptors inherited from Queue:
|  
|  __dict__
|      dictionary for instance variables (if defined)
|  
|  __weakref__
|      list of weak references to the object (if defined)

>>> </lang>

Tcl

Library: Tcllib (Package: struct::prioqueue)

<lang tcl>package require struct::prioqueue

set pq [struct::prioqueue] foreach {priority task} {

   3 "Clear drains"
   4 "Feed cat"
   5 "Make tea"
   1 "Solve RC tasks"
   2 "Tax return"

} {

   # Insert into the priority queue
   $pq put $task $priority

}

1. Drain the queue, in priority-sorted order

while {[$pq size]} {

   # Remove the front-most item from the priority queue
   puts [$pq get]

}</lang> Which produces this output:

Make tea
Feed cat
Clear drains
Tax return
Solve RC tasks