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Queues

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Queues

Chapter 4

- It is a data structure in which the elements are added at one end, called the rear, and deleted from the other end, called the front or first.
- A queue is simply a waiting line that grows or shrinks by adding or taking elements form it.
- Unlike stack, it’s a structure in which both ends are used.
- A queue is an FIFO structure.
- One possible queue implementation is an array(circular array).
- A more natural queue implementation is a doubly linked list.

- Operations to manage the queue are:
- Clear()
- isEmpty()
- enqueue(el)
- dequeue()
- firstEl()

- InitializeQueue:
- Initializes the queue to an empty state

- DestroyQueue:
- Removes all the elements from the queue, leaving the queue empty

- IsEmptyQueue:
- Checks whether the queue is empty. If the queue is empty, it returns the value true; otherwise, it returns the value false

- IsFullQueue:
- Checks whether the queue is full. If the queue is full, it returns the value true; otherwise, it returns the value false

- Front:
- Returns the front (first) element of the queue; the queue must exist

- Back:
- Returns the last (rear) element of the queue; the queue must exist

- AddQueue:
- Adds a new element to the rear of the queue; the queue must exist and must not be full.

- DeleteQueue:
- Removes the front element of the queue; the queue must exist and must not be empty.

- A queue is a linear list in which data can be inserted at one end, called rear, and deleted from the other end, called the front. It is a first in-first out (FIFO) data structure.
- no search,
- no adding in arbitrary positions,
- no sorting,
- no access to anything beyond the front and rear elements.

- Enqueue:
- inserts an element at the rear of the queue.

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Data

Enqueue

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front

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operation

- Dequeue:
- deletes element at the front of the queue.

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Dequeue

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- Queue Front:
- Examines the element at the front of the queue.

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Data

Queue Front

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- Queue Rear:
- Examines the element at the rear of the queue.

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Queue Rear

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operation

- For a linked list implementation of a queue, we use two types of structures: a head and a node.

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Conceptual queue

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Physical queue

queueHead

front<node pointer>count<integer>

rear<node pointer>

end queueHead

node

data<datatype>

next<node pointer>

end node

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count

front

rear

Queue head structure

data

next

node structure

- Create Queue

algorithm createQueue

- queue.front = null
- queue.rear = null
- queue.count = 0
end createQueue

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Queue head structure

Enqueue

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Case 1: insert into null queue

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Case 2: insert into queue

algorithm enqueue

Insert (push) data into a queue.

PostdataIn has been inserted

Return true if successful, false if overflow

- If (queue full)
- return false

- end if
- allocate (newptr)
- newptr->data = dataIn
- newptr->next = null pointer
- if (queue.count = zero)
- queue.front = newPtr

- else
- queue.rear->next = newPtr

- end if
- queue.rear = newptr
- queue.count = queue.count + 1
- return true
- end enqueue

- Enqueue

Dequeue

count

front

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Queue

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1

0

plum

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(Recycled)

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Case 1: delete only item in queue

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Before

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Case 2: delete item at front of queue

algorithm dequeue

This algorithm deletes a node from a queue.

Postdata at front of queue returned to user through item and front element deleted and recycled

Return true if successful, false if overflow

- If (queue.count is 0)
- return false

- end if
- Item= queue.front->data
- deleteLoc = queue.front
- if (queue.count is 1)
- queue.rear = null pointer

- end if
- queue.front = queue.front->next
- queue.count = queue.count – 1
- recycle (deleteLoc)
- return true
- end dequeue

- Dequeue

algorithm QueueFront

This algorithm receives the data at the front of the queue without changing the queue contents.

Postdata passed back to caller

Return true if successful, false if underflow

- if (queue.count is 0)
- return false

- end if
- dataout = queue.front->data
- return true
- end QueueFront

- Queue Front

- Empty Queue

algorithm emptyQueue

This algorithm checks to see if a queue is empty.

Return true if empty, false if queue has data

- return (if queue.countequal 0)
- end emptyQueue

algorithm fullQueue

This algorithm checks to see if a queue is full. The queue is full if memory cannot be allocated for another node.

Return true if full, false if there is room for another node

- allocate (tempPtr)
- If (allocation successful)
- recycle(tempPtr)
- return false

- else
- return true

- end if
- end fullQueue

- Full Queue

algorithm Queuecount

Returns the number of elements currently in queue.

- return queue.count
- end Queuecount

algorithm destroyQueue

This algorithm deletes all data from a queue.

Post all data have been deleted and recycled

- ptr= queue.front
- Loop (ptrnot null)
- deletePtr= ptr
- ptr= ptr->next
- recycle (deletePtr)

- end loop
- queue.front = null
- queue.rear = null
- queue.count = 0
- return
- end destroyQueue

- A priority queue is an ADT with an inserting accessing protocol: only the highest-priority element can be accessed.
- It is arranged to support access to the highest priority.
- A priority queue stores collection of entries. Each entry is a (key, value) pair.
- Applications:
- Hospital Emergency Rooms
- Stock market

- Keys in a priority queue can be arbitrary objects on which an order is defined.
- Two distinct items in a priority queue can have the same key.

- Main methods of the Priority Queue ADT
- insert(k, x) inserts an entry with key k and value x
- removeMin() removes and returns the entry with smallest key

- Additional methods
- minKey(k, x) returns, but does not remove, an entry with smallest key
- minElement() returns, but does not remove, the element of an item with smallest key
- size()
- isEmpty()