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Queues in computer science are very similar to queues in real life. A queue in real life would be a line up at a fast food counter or bank tellers machine where people are serviced in the order they arrive.
Therefore, we can say that a queue is a sequential list in which items are inserted into the tail end of the queue and taken from the head. The "tail" is referred to as the REAR and the "head" is the FRONT of a queue. Thus in a queue each element is deleted from an end, labled FRONT, and added at the other end, labled REAR. For this reason, queues are often called First In First Out (FIFO) lists.
The most obvious example is the printer queue. In a multi-user environment, the printer is requested for multiple print jobs, each printing job is inserted at the rear of the queue in the order it is sent to the printer and is then printed on First In First Out basis.
Types of Queues
Queues can be categorised into two types:
Deques(pronounced either deck or deQueue)
The term deques stands for a double-ended queue. In such a queue, elements can be added or removed from both the ends of the queue.
There are two types of deques:
Input restricted deques
In an input-restricted deques, elements can be inserted only from the end of the deques, but can be deleted from both the ends.
Output restricted deques
In an output-restricted deques, elements are inserted from both the ends of the deques, but can be deleted only from one end of the deques.
Often the items added to a queue have a priority associated with them that determines the order in which each element is processed and deleted. Such a kind of queue is called a priority queue.
In priority queues:
An element with a higher priority is processed first.
If two elements share the same priority, then the element that came in first is processed first. Programs with higher priority are processed first. Programs with the same priority form a separate standard queue.
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Prof. Ashay Dharwadker