Queue

Abstract Data Type
Stack & Queue is an example of ADT
An array is not ADT.

Using a Queue
Several example
applications of queues are
given in that Slides.
This presentation describes
the queue operations and
two ways to implement a
queue.
Data Structures
and Other Objects
Using C++

What is Queue?
A QUEUE IS A CONTAINER IN WHICH INSERTIONS
ARE MADE ONLY AT THE BACK DELETIONS,
RETRIEVALS, AND MODIFICATIONS ARE MADE
ONLY AT THE FRONT.

The Queue Operations
A queue is like a line of
people waiting for a
bank teller. The queue
has a front and a rear.
$ $
Front
Rear

EnQueue Operations
New people must enter the queue at
the rear. The C++ queue class calls
this a push, although it is usually
called an enqueue operation.
$ $
Front
Rear

DeQueue Operations
 When an item is taken from the queue,
it always comes from the front. The C++
queue calls this a pop, although it is
usually called a dequeue operation.
$ $
Front
Rear

Array Implementation
A queue can be implemented with an array, as
shown here. For example, this queue contains
the integers 4 (at the front), 8 and 6 (at the rear).
[[ 00 ]] [[11]] [[ 22 ]] [[ 33 ]] [[ 44 ]] [[ 55 ]] .. .. ..
4 8 6
AAnn aarrrraayy ooff iinntteeggeerrss ttoo
iimmpplleemmeenntt aa qquueeuuee ooff
iinntteeggeerrss
WWee ddoonn''tt ccaarree wwhhaatt''ss iinn
tthhiiss ppaarrtt ooff tthhee aarrrraayy..

Array Implementation
The efficient implementation also keeps
track of the number of items in the
queue and the index of the first element
(at the front of the queue), the last
element (at the rear).
[[ 00 ]] [[11]] [[ 22 ]] [[ 33 ]] [[ 44 ]] [[ 55 ]] .. .. ..
4 8 6
3 ssiizzee
0 ffiirrsstt
2 llaasstt

A Dequeue Operation
When an element leaves the queue,
size is decremented, and first
changes, too.
2 ssiizzee
1 ffiirrsstt
2 llaasstt
[[ 00 ]] [[11]] [[ 22 ]] [[ 33 ]] [[ 44 ]] [[ 55 ]] .. .. ..
4 8 6

An Enqueue Operation
When an element enters the queue,
size is incremented, and last changes,
too.
3 ssiizzee
1 ffiirrsstt
3 llaasstt
[[ 00 ]] [[11]] [[ 22 ]] [[ 33 ]] [[ 44 ]] [[ 55 ]] .. .. ..
8 6 2

At the End of the Array
There is special behaviour at the end
of the array. For example, suppose we
want to add a new element to this
queue, where the last index is [5]:
2
3 ssiizzee
3 ffiirrsstt
5 llaasstt
[[ 00 ]] [[11]] [[ 22 ]] [[ 33 ]] [[ 44 ]] [[ 55 ]]
5 6 1

At the End of the Array
The new element goes at the front of
the array (if that spot isn’t already
used):
4 ssiizzee
3 ffiirrsstt
0 llaasstt
[[ 00 ]] [[11]] [[ 22 ]] [[ 33 ]] [[ 44 ]] [[ 55 ]]
4 2 6 1

Array Queue Reviews
 Easy to implement.
 But it has a limited capacity with a fixed array
 Special behaviour is needed when the rear
reaches the end of the array.
[[ 00 ]] [[11]] [[ 22 ]] [[ 33 ]] [[ 44 ]] [[ 55 ]] .. .. ..
4 8 6
3 ssiizzee
0 ffiirrsstt
2 llaasstt

Linked List Implementation
A queue can also be
implemented with a linked
list with both a head and a
tail pointer.
null
10
15
7
13
Head-ptr
Tail-ptr

Which end do you think is the
front of the queue? Why ?
10
15
7
13
Head-ptr
Tail-ptr
null

The head-ptr points to the
front of the list.
Because it is harder to
remove items from the tail
of the list.
Rear
Front
10
15
7
13
Head-ptr
Tail-ptr
null

The Queue Class
template <class Item>
class queue<Item>
{
public:
queue( );
void push(const Item& entry);
void pop( );
bool empty( ) const;
Item front( ) const;
}
 The C++ standard
template library has a
queue template class.
 The template parameter
is the type of the items
that can be put in the
queue.

Summary
QQuueeuueess hhaavvee mmaannyy aapppplliiccaattiioonnss..
 IItteemmss eenntteerr aa qquueeuuee aatt tthhee rreeaarr aanndd
lleeaavvee aa qquueeuuee aatt tthhee ffrroonntt..
QQuueeuueess ccaann bbee iimmpplleemmeenntteedd uussiinngg aann
aarrrraayy oorr uussiinngg aa lliinnkkeedd lliisstt..

Daily Life Examples
There's a queue of questions
that you've asked in exam, all
waiting for you to accept
answers for them.
Waiting in a queue in any
bank for paying bills, depositing
cash OR cheques.

Queue

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Queue

Editor's Notes