Presentation Slide on Priority Queue

1. 1
Presentation on Priority Queue
Presented By:
Md. Noyan Ali
Dept. of CSE, KUET
Presentation Date : May, 2018

2. Definition : A priority queue is a collection of elements such that each elements has
been assigned a priority and such that the order in which elements are deleted and
processed comes from the following rules :
1. An element of higher priority is processed before any element of lower
priority.
2. Two elements with the same priority are processed according to the
order in which they were added to the queue.
A prototype of a priority queue is a timesharing system : programs of high priority
are processed first and programs with the same priority forma a standard queue.
Implementation : There are two ways of representation of Priority Queue –
1. One-way List Representation of Priority Queue &
2. Array Representation of Priority Queue.
Priority Queue

3. INFO
B
D
E
A
C
G
F
PRN
2
4
4
1
2
5
4
LINK
6
4
9
1
3
0
8
5
2 1
2
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5
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7
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AVAIL
START
A 1
G 5 X
F 4
E 4
D 4
C 2
B 2
START
One-Way List Representation of a Priority Queue :

4. INFO
B
X
D
E
A
C
G
F
PRN
2
2
4
4
1
2
5
4
LINK
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3
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9
1
2
0
8
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7 1
2
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AVAIL
START
A 1
G 5 X
F 4
E 4
D 4
C 2
B 2
START
One-Way List Representation of a Priority Queue – Insertion :
X 2
Inserting Element X with Priority 2 :

5. INFO
B
X
D
E
C
G
F
PRN
2
2
4
4
2
5
4
LINK
6
3
4
9
2
0
8
1
5 1
2
3
4
5
6
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8
9
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AVAIL
START
G 5 X
F 4
E 4
D 4
C 2
B 2
START
One-Way List Representation of a Priority Queue – Deletion :
X 2
Deleting an Element :

6. Algorithm for deletion : This algorithm deletes and processes the first element in a
priority queue which appears in a memory as a one-way list -
1. Set ITEM = INFO[START]. [This saves the data in the first node.]
2. Delete first node from the list.
3. Process ITEM.
4. Exit.
Algorithm for addition: This algorithm adds an ITEM with priority number N to a
priority queue which is maintained in a memory as a one-way list -
1. Traverse the one-way list until finding a node X whose priority number
exceeds N. ITEM in front of node X.
2. If no such node i found, insert ITEM as the last element of the list.
Algorithms for One-Way List of PQ Operations :

7. FRONT
2
1
0
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REAR
2
2
0
1
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1 2 3 4 5 6
1 A
2 B C
3
4 D E
5 G
A 1
G 5
F 4
E 4
D 4
C 2
B 2
START
Array Representation of a Priority Queue :

8. FRONT
2
1
0
5
4
REAR
2
3
0
1
4
1
2
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5
1 2 3 4 5 6
1 A
2 B C X
3
4 D E
5 G
A 1
G 5
F 4
E 4
D 4
C 2
B 2
START
Array Representation of a Priority Queue – Insertion :
Inserting Element X with Priority 2 :
C 2

9. FRONT
0
1
0
5
4
REAR
0
3
0
1
4
1
2
3
4
5
1 2 3 4 5 6
1 A
2 B C X
3
4 D E
5 G
G 5 X
F 4
E 4
D 4
C 2
B 2
START
Array Representation of a Priority Queue – Deletion :
Deleting an Element :
C 2

10. Algorithm for deletion: This algorithm deletes and processes the first element in a
priority queue maintained by a two-dimensional array QUEUE -
1. [Find the first nonempty queue.]
Find the smallest k such that FRONT[k] != NULL.
2. Delete and process the front element in row k of queue.
3. Exit.
Algorithm for addition: This algorithm adds an ITEM with priority number M to a
priority queue maintained by a two-dimensional array QUEUE -
1. Insert ITEM as the rear element in row M of QUEUE.
2. Exit.
Algorithms for Array Representation of PQ Operations :

11. • Efficiency for array representation : The array representation of a priority queue is
more time efficient than a one-way list. This is because when adding an element
to a one-way list, one must perform a linear search on the list.
• Efficiency for one-way list representation : The one-way list representation of a
priority queue is more space efficient than the array representation. This is
because in using the array representation, overflow occurs when the number of
elements in any single priority level exceeds the capacity for that level, but in
using one-way list representation , overflow occurs only when the total number of
elements exceeds the total capacity.

12. References :
1. Theory and Problems of DATA STRUCTURE – SEYMOUR LIPSCHUTZ.
2. http://btechsmartclass.com/DS/U3_T6.html. (accessed on 07 May, 2018)

13. Thanks For Watching!