HP Integrity NonStop H-Series - Manual (Page 120)

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©Copyright 1996 Rogue Wave Software
The priority queue Data Abstraction
A priority queue is a data structure useful in problems where you need to rapidly and repeatedly
find and remove the largest element from a collection of values. An everyday example of a
priority queue is the "to do" list of tasks waiting to be performed that most of us maintain to
keep ourselves organized. Some jobs, such as "clean desktop," are not imperative and can be
postponed arbitrarily. Other tasks, such as "finish report by Monday" or "buy flowers for
anniversary," are time-crucial and must be addressed more rapidly. Thus, we sort the tasks
waiting to be accomplished in order of their importance (or perhaps based on a combination of
their critical importance, their long term benefit, and the fun we will have doing them) and
choose the most pressing.
A Queue That is Not a Queue
A more computer-related example of a priority queue is that used by an operating system to
maintain a list of pending processes, where the value associated with each element is the
priority of the job. For example, it may be necessary to respond rapidly to a key pressed at a
terminal, before the data is lost when the next key is pressed. On the other hand, the process of
copying a listing to a queue of output waiting to be handled by a printer is something that can
be postponed for a short period, as long as it is handled eventually. By maintaining processes in
a priority queue, those jobs with urgent priority will be executed prior to any jobs with less
urgent requirements.
Simulation programs use a priority queue of "future events." The simulation maintains a virtual
"clock," and each event has an associated time when the event will take place. In such a
collection, the element with the smallest time value is the next event that should be simulated.
These are only a few instances of the types of problems for which a priority queue is a useful
tool. You probably have, or will, encounter others.
Include Files
Programs that use the priority queue data abstraction should include the file queue, as well as
the include file for the container type (e.g., vector).
# include <queue>
# include <vector>