Today's Agenda
• Priority Queues and Heaps
Priority Queues and Heaps
Motivation
• Recall the FindMin operation : Find the smallest ( or highest priority) item quickly,
• Some Applications:
– Operating system needs to schedule jobs according to priority instead of
FIFO
– Find student with highest grade, employee with highest salary etc.
Priority Queues
• Think of a priority queue as a kind of bag that holds priorities. You can put one in,
and you can take out the current highest priority. (Priorities can be any Comparable
values; in our examples, we'll just use numbers.)
• A priority queue is different from a "normal" queue, because instead of being a
"first-in-first-out" data structure, values come out in order by priority.
• A priority queue might be used, for example, to handle the jobs sent to the Computer
Science Department's printer: Jobs sent by the department chair should be printed
first, then jobs sent by professors, then those sent by graduate students, and finally
those sent by undergraduates. The values put into the priority queue would be the
priority of the sender (e.g., using 4 for the chair, 3 for professors, 2 for grad
students, and 1 for undergrads), and the associated information would be the
document to print. Each time the printer is free, the job with the highest priority
would be removed from the print queue, and printed. (Note that it is OK to have
multiple jobs with the same priority; if there is more than one job with the same
highest priority when the printer is free, then any one of them can be selected.)
The operations that need to be provided for a priority queue are shown in the following
table, assuming that just priorities (no associated information) are to be stored in a priority
queue.
Priority Queue ADT
• Priority Queue should efficiently do
– PriorityQ()
• Create an empty priority queue
– FindMin()
• Returns minimum value but does not delete it
– DeleteMin()
