Class 42: Priority Queues

Held: Monday, 14 November 2005

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.

Overview:

• Priority queue basics.
• Applying priority queues.
• Implementing priorty queues.

Priority queue basics

• 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, we use the compareTo method associated with those objects.
  • We can also choose a Comparator to use when we create the priority queue.

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.
• Priority queues also provide a natural easy sorting algorithm:
  • Put all the values into the priority queue.
  • Remove them one-by-one.

Implementing priorty queues

• I intentionally left this as an open-ended question.
• We'll need to think about the advantages and disadvantages of each implementation that you come up with.