simple queue data structure presentation .ppt

1. Using a Queue
 We introduce the
We introduce the queue
queue
data type.
data type.
 Several example
Several example
applications of queues
applications of queues
are given in that
are given in that
chapter.
chapter.
 This presentation
This presentation
describes the queue
describes the queue
operations and two ways
operations and two ways
to implement a queue.
to implement a queue.

2. 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

3. The Queue 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

4. The Queue 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

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

6. Overflow vs underflow
 If a program attempts to add an
entry to a queue that is already at
its capacity, this is, of course, an
error.
 This error is called queue overflow.
 If a program attempts to remove
an entry from an empty queue,
that is another kind of error, called
queue underflow

7. 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).
[ 0 ]
[ 0 ] [1]
[1] [ 2 ]
[ 2 ] [ 3 ]
[ 3 ] [ 4 ]
[ 4 ] [ 5 ]
[ 5 ] . . .
. . .
An array of integers
An array of integers
to implement a
to implement a
queue of integers
queue of integers
4 8 6
We don't care what's in
We don't care what's in
this part of the array.
this part of the array.

8. Array Implementation
 The easiest 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).
[ 0 ]
[ 0 ] [1]
[1] [ 2 ]
[ 2 ] [ 3 ]
[ 3 ] [ 4 ]
[ 4 ] [ 5 ]
[ 5 ] . . .
. . .
4 8 6
size
size
3
first
first
0
last
last
2

9. A Dequeue Operation
 When an element leaves the
queue, size is decremented,
and first changes, too.
[ 0 ]
[ 0 ] [1]
[1] [ 2 ]
[ 2 ] [ 3 ]
[ 3 ] [ 4 ]
[ 4 ] [ 5 ]
[ 5 ] . . .
. . .
4 8 6
size
size
2
first
first
1
last
last
2

10. An Enqueue Operation
 When an element enters
the queue, size is
incremented, and last
changes, too.
[ 0 ]
[ 0 ] [1]
[1] [ 2 ]
[ 2 ] [ 3 ]
[ 3 ] [ 4 ]
[ 4 ] [ 5 ]
[ 5 ] . . .
. . .
2
8 6
size
size
3
first
first
1
last
last
3

11. At the End of the Array
 There is special behavior 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]:
[ 0 ]
[ 0 ] [1]
[1] [ 2 ]
[ 2 ] [ 3 ]
[ 3 ] [ 4 ]
[ 4 ] [ 5 ]
[ 5 ]
2 1
6
size
size
3
first
first
3
last
last
5

12. At the End of the Array
 The new element goes at
the front of the array (if that
spot isn’t already used):
[ 0 ]
[ 0 ] [1]
[1] [ 2 ]
[ 2 ] [ 3 ]
[ 3 ] [ 4 ]
[ 4 ] [ 5 ]
[ 5 ]
2 1
6
size
size
4
first
first
3
last
last
0
4

13. Array Implementation
 Easy to implement
 But it has a limited capacity with a fixed
array
 Special behavior is needed when the
rear reaches the end of the array.
[ 0 ]
[ 0 ] [1]
[1] [ 2 ]
[ 2 ] [ 3 ]
[ 3 ] [ 4 ]
[ 4 ] [ 5 ]
[ 5 ] . . .
. . .
4 8 6
size
size
3
first
first
0
last
last
2

14. Linked List Implementation
 A queue can also be
implemented with a linked
list with both a head and a
tail pointer.
10
15
7
null
13
head_ptr
tail_ptr

15. Linked List Implementation
 Which end do you think is
the front of the queue?
Why?
10
15
7
null
13
head_ptr
tail_ptr

16. Linked List Implementation
 The head_ptr points to the
front of the list.
 Because it is harder to
remove items from the tail
of the list.
10
15
7
null
head_ptr
13
tail_ptr
Front
Rear

17. Uses for Queues

18.  Suppose you want a program to read a
word and then write the word.
 One way to accomplish this task is to
read the input one letter at a time and
place each letter in a queue.
 After the word is read, the letters in the
queue are written out.
 Because a queue is a First-In/First-Out
data structure, the letters are written in
the same order in which they were read.
 The pseudocode for this approach to
copying a word is as follows:

20. Recognizing Palindromes
 A palindrome is a string that reads
the same forward and backward;
that is, the letters are the same
whether you read them from right
to left or from left to right.
 For example, the one-word string
“radar” is a palindrome.
 Able was I ere I saw Elba

21.  We can do this by using both a stack and a
queue.
 We will read the line of text into both a stack and
a queue
 The program can simply compare the contents of
the stack and the queue character-by-character
to see if they would produce the same string of
characters.

24. Uses for Queue
nput/output buffering

25. Summary
 Like stacks, queues have many
Like stacks, queues have many
applications.
applications.
 Items enter a queue at the rear and
Items enter a queue at the rear and
leave a queue at the front.
leave a queue at the front.
 Queues can be implemented using
Queues can be implemented using
an array or using a linked list.
an array or using a linked list.

26.  Suppose that locate_ptr is a pointer to a node in
a linked list (and it is not the null pointer). Write a
statement that will make locate_ptr move to the
next node in the list. What does your statement
do if locate_ptr was already pointing to the last
node in the list?
 Write a statement to correctly set the tail pointer
of a list when a new first node has been added
to the list. Assume that head_ptr points to the
new first node.
 Suppose that head_ptr is a head pointer for a
linked list with just one node. What will head_ptr
be after list_head_remove(head_ptr)?

27. Consider the following pseudocode: declare a
stack of characters
while ( there are more characters in the word to
read )
{ read a character push the character on the
stack }
while ( the stack is not empty )
{ write the stack's top character to the screen
pop a character off the stack }
What is written to the screen for the input
"carpets"?

28. Exercice
 Write pseudocode for an algorithm that reads an
even number of characters. The algorithm then
prints the first character, third character, fifth
character, and so on. On a second output line,
the algorithm prints the second character, fourth
character, sixth character, and so on. Use two
queues to store the characters.