Class 31: Priority Queues

Held: Wednesday, March 15, 2006

Summary: Today we investigate priority queues, linear structures in which the policy is highest-priority first, which usually translates into smallest first.

Related Pages:

• EBoard.
• Sample Code.

Assignments

• Read The TAO of Java: Heaps (ready Thursday).

Notes:

• No homework over break.

Overview:

• Philosopy of Priority Queues.
• Aplications of Priority Queues.
• Methods for Priority Queues.
• Implementing Priority Queues.

Philosophy of Priority Queues

• Priority queues are the third basic linear structure.
• The policy for priority queues is get returns the highest-priority object.
• How do we determine priority? Typically, we use the smallest or largest element.
  • If the objects in our priority queue are all Comparable to each other, we use the compareTo method associated with those objects.
  • We can also choose a Comparator to use when we create the priority queue and use its compare method.
  • Some of you suggested having put take an additional parameter, the priority. I'd rather not change the signature of that method.
    • The alternative strategy is to wrap the value, as in

      public class Prioritized<T>
      {
        T value;
        int priority;
        ...
      } // class Prioritized&tl;T>
      

Applications of Priority Queues

• Priority queues have natural applications in task-processing systems, since they ensure that the most important tasks get accomplished first.
  • Of course, they are also inherently unfair to low-priority tasks.
• Many folks use some form of priority queue for their to-do lists.
• Priority queues also provide a natural easy sorting algorithm:
  • Put all the values into the priority queue.
  • Remove them one-by-one.

Priority Queue Methods

• Priority queues are simply linear structures, so they have the basic linear structure operations.
• That is,
  • void put(T val)
  • T get()
  • boolean isEmpty()

Implementing priorty queues

• Basic strategy grid:
  • Use linked nodes vs. use an array
  • Order values on put vs. order values on get
• Other strategies:
  • A vector of vectors, with each position in the outer vector corresponding to a numeric priority (and all values of the same priority being in the vector at that space).
• Suggesters:
  • Linked Nodes, Sort on put: Emily
  • Linked Nodes, Sort on get: Emily
  • Array: Sort on put: Aaron, Emily, Tony
  • Array: Sort on get: Emily
  • Vector of Vectors: Ian BR
• Non-suggesters:
  • Ben, Betsy, Christine, D.L., Hak, Ian A, Kennan, Lindsay, Nii, Sam, Scott
• We're going to choose one and implement it live.