Data Structures

1
Data Structures
Andreas Savva

• Chapter 3
• Queues

2
Queues

• A data structure modeled after a line of people
  waiting to be served.
• The first entry which is inserted is the first
  one that will be removed. (First In First Out
  FIFO)

3
Application of Queuesin our everyday life
4
Application of QueuesMost common than Stacks

• Waiting for a printer
• Access to disk storage
• In a time-sharing system use of the CPU
• Along with stacks, queues are one of the simplest
  data structures

5
Basic Operations for Queues

• Create a queue, leaving it empty.
• Clear the queue leaving it empty.
• Test whether the queue is Empty.
• Test whether the queue is Full.
• Return the Size of the queue.
• Retrieve the first entry of the queue, provided
  the queue is not empty.
• Serve remove the first entry from the queue,
  provided the queue is not empty.
• Retrieve and Serve retrieves and remove the
  first entry from the queue, provided the queue is
  not empty.
• Append a new entry at the end of the queue,
  provided that the queue is not full.
• Print all the entries of the queue.

6
The Class Queue
class Queue
methods Queue (constructor) clear
empty full size retrieve serve
retrieve_and_serve append print Data
members
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Stack implementation - Array

• typedef double Queue_entry
• enum Error_code success,overflow,underflow
• const int max 100
• class Queue
• public
• Queue()
• void clear()
• bool empty() const
• bool full() const
• int size() const
• Error_code retrieve(Queue_entry ) const
• Error_code serve()
• Error_code retrieve_and_serve(Queue_entry )
• Error_code append(const Queue_entry )
• void print() const
• private
• int count
• Queue_entry entrymax

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Data Structure - Array

• front is at position 0
• rear is at position n-1

front
rear
One item
front
rear
n items
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Create Queue
We use a constructor to initialize a new created
queue as empty.

• QueueQueue()
• // Pre None.
• // Post Initialize Queue to be empty.

entry
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Clear
void Queueclear() // Pre None. // Post All
entries in the Queue have been // removed it is
now empty.
entry
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Empty
bool Queueempty() const // Pre None. //
Post Return true if the Queue is
empty, // otherwise false is returned.
entry
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Full
bool Queuefull() const // Pre None. //
Post Returns true if the Queue is
full, // otherwise false is returned.
entry
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Size
int Queuesize() const // Pre None. //
Post Return the number of entries in the
Queue.
entry
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Retrieve
Error_code Queueretrieve(Queue_entry item)
const // Pre None. // Post If the Queue is not
empty, the front of the queue is recorded // as
item. Otherwise an Error_code of underflow is
return.
entry
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Serve or Dequeue
Error_code Queueserve() // Pre None. //
Post If the Queue is not empty, the front of the
Queue is removed // Otherwise an Error_code of
underflow is returned.
count
entry
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Retrieve and Serve
Error_code Queueretrieve_and_serve(Queue_entry
item) // Pre None. // Post If the Queue is not
empty, the front of the queue is removed
and // recorded as item. Otherwise an Error_code
of underflow is return.
item
entry
A
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Append or Enqueue
Error_code Queueappend(const Queue_entry
item) // Pre None. // Post If the Queue is not
full, item is added at the end of the
Queue. // If the Queue is full, an Error_code of
overflow is returned // and the Queue is left
unchanged.
entry

n1
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Print
void Queueprint() const // Pre None. //
Post Display the entries of the Queue.
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Data Structure Circular Array
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Queue containing one item
front
rear
Empty Queue
-1
front
rear
Queue with one empty position
front
rear
Full Queue
front
rear
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The class Queue Circular Array

• class Queue
• public
• Queue()
• void clear()
• bool empty() const
• bool full() const
• int size() const
• Error_code retrieve(Queue_entry ) const
• Error_code serve()
• Error_code retrieve_and_serve(Queue_entry )
• Error_code append(const Queue_entry )
• void print() const
• private
• int next(int n) const
• int front, rear
• Queue_entry entrymax

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Circular Queue
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Create Queue

• QueueQueue()

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Clear
void Queueclear()
rear-1
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Empty
bool Queueempty() const
Not Empty
Empty
rear-1
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Next position
rear
int Queuenext(int n) const



front next(front) rear next(rear) entryrear

front
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Full
bool Queuefull() const

NOT FULL
FULL

28
Size
int Queuesize() const
max
rear front 1
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Retrieve
Error_code Queueretrieve(Queue_entry item)
const
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Serveor Dequeue

Error_code Queueserve()
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RetrieveandServe

Error_code Queueretrieve_and_serve(Queue_entry
item)
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AppendorEnqueue
rear

Error_code Queueappend(const Queue_entry
item)
33
Print
void Queueprint() const