Computer Science 1 (Studio) 07B-Queues

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Computer Science 1 (Studio) 07B-Queues
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What is a Data Structure?

• In computer science, a data structure is a way of
  storing data so that it can be used efficiently
  by an algorithm
• Youve already been exposed to several data
  structures in Java
• Classes are collections of attributes and
  behaviors which represent an object
• Arrays are a contiguous collection of similar
  primitives/objects
• The choice of data structure in a program is of
  primary importance because it effects difficulty
  of implementation and performance

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What is a Queue?

• A queue is a data structure which has the unique
  characteristic of being a First In First Out
  collection
• Another characteristic of a queue is that, in
  theory, it has infinite capacity
• Regardless of how many elements are currently
  present in the queue, another can always be added

Queue
front
back
C
G
D
A
B
H
F
Queue
E
J
A
B
C
D
E
F
G
H
I
Queue
A
B
C
D
E
F
G
H
I
J
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What is a Queue?

• Examples of real life queues
• Amusement park line
• Concert ticket line
• Bursars office line
• Examples of queues in computer applications
• Processes or users waiting for a system resource
• A printing queue for spooling print jobs
• A queue of mp3s to download off Kazaa before the
  RIAA shuts it down

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Queue Operations - Enque

• A queue supports two fundamental operations for
  storing and retrieving data elements
• The process of adding an element to the end of a
  queue is known as enqueing

front
back
A
B
Enque B
A
B
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Queue Operations - Deque

• The process of removing an element from the front
  of the queue is known as dequeing
• The visual effect of dequeing is that the front
  element disappears and all other elements in the
  queue shift forward a slot

front
back
A
B
Deque A
B
A
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Queue Operations

• In practice, most queues are implemented with a
  maximum size
• A queue of size 10 which currently has 9 elements
  stored in it
• We can ask a queue what its size is
• We can ask a queue how many elements it has in it

B
C
D
E
F
G
H
I
J
front
back
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Queue Operations

• We can also ask a queue if it is full or empty
• Describe a scenario where you want to know if the
  queue is currently full
• Describe another scenario where you want to know
  if the queue is currently empty

B
C
D
E
F
G
H
I
J
front
back
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Queue Operations

• We should also be able to ask a queue to print
  out its current contents (useful for debugging)

B
C
D
E
F
G
H
I
J
front
back
Queue from Front to Back B, C, D, E, F, G, H, I,
J
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Other Queue Operations

• Give me a reverse queue
• Make a copy of a queue

B
C
D
E
F
G
H
I
J
J
I
H
G
F
E
D
C
B
front
back
front
back
B
C
D
E
F
G
H
I
J
B
C
D
E
F
G
H
I
J
front
back
front
back
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Other Queue Operations

• Are these two queues equal?
• What is the front element in the queue?
• Were peeking at it, not actually dequeing it

B
C
D
E
F
G
H
I
J
J
I
H
G
F
E
D
C
B
front
back
front
back
Equal queues false
B
C
D
E
F
G
H
I
J
Front Element B
front
back
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Other Queue Operations

• What is the back element in the queue?
• Does the queue contain a specific element?

B
C
D
E
F
G
H
I
J
Back Element J
front
back
Queue contains F true Queue contains X false
B
C
D
E
F
G
H
I
J
front
back
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Queue Caveats

• A queue contains elements of the same type
• A queue does not support random access
• We cant get at the fourth element from the front
  of the queue and change it directly

B
C
D
E
F
G
H
I
J
front
back
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Double Ended Queues

• A double ended queue (deque) adds removal from
  the back, and insertion at the front

insert front
remove back
B
C
D
E
F
G
H
I
J
remove front
insert back
front
back
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Other Queues

• A priority queue constantly organizes the
  elements in the queue based on increasing
  priority
• A circular queue avoids shifting by repositioning
  the front and back locations

Add 66
Remove 55
Add 55
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Understanding How To Implement a Queue

• Were going to implement our first queue using an
  array as the underlying collection
• The queue will be circular so that we dont have
  to shift elements around
• Lets look at the mechanics of this before we
  dive into the implementation
• Consider a queue whose maximum size is 5

0
1
2
3
4
size 5
elements 0
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Understanding How To Implement a Queue

• Based on this information, how can we tell when
  the queue is full or empty?
• How can we tell what the size of the queue is?
• How can we tell how many elements are in the
  queue?

0
1
2
3
4
size 5
elements 0
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Understanding How To Implement a Queue

• In order to track the front and the back of the
  queue as elements get enqued and dequed, we will
  rely on integer indexes
• Initially the back of the queue is at index -1
  and the front of the queue is at index 0

0
1
2
3
4
front 0
size 5
elements 0
back -1
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Understanding How To Implement a Queue

• Whenever an element is enqueued, the back index
  is incremented
• The element is then added into the array at the
  back index

Enque 10
0
1
2
3
4
10
front 0
size 5
elements 1
back 0
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Understanding How To Implement a Queue
Enque 20
0
1
2
3
4
10
20
front 0
size 5
elements 2
back 1
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Understanding How To Implement a Queue
Enque 30
0
1
2
3
4
10
20
30
front 0
size 5
elements 3
back 2
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Understanding How To Implement a Queue

• Whenever an element is dequed, the element at the
  front index is returned
• The front index is then incremented

Deque 10
0
1
2
3
4
20
30
front 1
size 5
elements 2
back 2
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Understanding How To Implement a Queue
Deque 20
0
1
2
3
4
30
front 2
size 5
elements 1
back 2
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Understanding How To Implement a Queue

• The queue is now empty again

Deque 30
0
1
2
3
4
front 3
size 5
elements 0
back 2
25
Understanding How To Implement a Queue

• Continue enqueueing

Enque 40
0
1
2
3
4
40
front 3
size 5
elements 1
back 3
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Understanding How To Implement a Queue
Enque 50
0
1
2
3
4
40
50
front 3
size 5
elements 2
back 4
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Understanding How To Implement a Queue

• On the next enque, the back index needs to wrap
  around to the start

Enqueue 60
0
1
2
3
4
60
50
40
front 3
size 5
elements 3
back 0
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Understanding How To Implement a Queue
Enque 70
0
1
2
3
4
60
50
40
70
front 3
size 5
elements 4
back 1
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Understanding How To Implement a Queue
Deque 40
0
1
2
3
4
60
50
70
front 4
size 5
elements 3
back 1
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Understanding How To Implement a Queue

• On the next deque, the front index needs to wrap
  around

Deque 50
0
1
2
3
4
60
70
front 0
size 5
elements 2
back 1
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Implementing a Queue - Javadoc

• Look at the javadoc for the queue class we are
  implementing
• http//www.cs.rit.edu/cs1s/week7/IntQueue.html
• The queue will only work with integers and it
  will be called IntQueue
• The size of the queue will be based on how
  IntQueue is constructed
• // If no size is specified, the default size is
  25
• IntQueue q1 new IntQueue()
• // Here the size is 10
• IntQueue q2 new IntQueue(10)

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Implementing a Queue First Step

• In Class Activity
• Start by creating a class, IntQueue.java
• Using the javadoc, make a compilable skeleton
• Remember to return dummy values for methods that
  require them
• Write the code for the two constructors
• You will have to create several private data
  members to achieve this
• An integer array reference variable (the actual
  collection)
• The number of elements currently in the queue
• The front index
• The back index
• A constant for the default size of the queue
  (default 25)
• You dont need to save the size of the array.
  Once the constructor creates the array, you can
  tell its size by accesing the arrays length

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Implementing a Queue First Step

• The end result of your code so far should be to
  create the following diagram in memory
• IntQueue myQ new Queue (5)

0
1
2
3
4
front 0
size 5
elements 0
back -1
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Implementing a Queue Accesors

• In Class Activity
• The next step is to implement the accessors and
  simple methods
• Implement getNumberOfElements()
• Implement getSize()
• Implement isFull()
• Implement isEmpty()
• Once this is complete, write another program,
  TestQueue.java, which tests the creation and
  accessors for IntQueue

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Implementing a Queue - Enque

• The final steps for implementing the queue are
  writing the methods for enqueing and dequeing
  elements
• Both of these methods require a way for the
  indexes to wrap around when they reach the end of
  the array

Before Enqueue 60
0
1
2
3
4
50
40
front 3
size 5
elements 2
back 4
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Implementing a Queue - Enque

• In this example, how can I change the back index
  to go from 4 to 0?
• Hint you always know the size of the array
• Write down generic algorithm here

After Enque 60
0
1
2
3
4
50
40
60
front 3
size 5
elements 2
back 0
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Implementing a Queue - Enque

• Remember this
• Enqueue works with the back index
• Dequeue works with the front index
• With enque, the back index must be incremented
  first before adding the element into the array

After Enque 60
Before Enque 60
0
1
2
3
4
0
1
2
3
4
40
50
40
50
60
front 3
front 3
back 4
back 0
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Implementing a Queue - Deque

• With deque, the element must be added into the
  array before incrementing the front index

Before deque 20
After deque 20
0
1
2
3
4
0
1
2
3
4
20
30
30
front 1
front 2
back 2
back 2
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Implementing a Queue - Preconditions

• Theres two preconditions we must worry about
  before we can write enqueue and dequeue
• What should happen if we try to enque an element
  and the array is already full?
• Write our solution here
• What should happen if we try to deque an element
  and the array is empty?
• Write our solution here

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Implementing the Queue - Print

• For debugging purposes, we need a way to print
  out the contents of the queue
• Consider this example
• We need to create a loop variable which starts at
  the front index and moves until all the elements
  in the queue have been printed

0
1
2
3
4
50
40
60
front 3
size 5
elements 2
back 0
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Implementing a Queue - Print

• Here we want to print out indexes 3, 4 and 5
• myQueue3
• myQueue4
• myQueue0
• Write down the algorithm here

0
1
2
3
4
40
50
60
front 3
size 5
elements 2
back 0
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Implementing a Queue Enqueue Print

• In Class Activity
• Write the enqueue method
• Be sure to include the precondition discussed
  earlier
• Remember that the back index must be incremented
  before adding the element into the array
• Dont forget to change the number of elements in
  the queue if the element was added
• Next write the printQue method so the elements
  that are enqueued can be printout out
• Finally, in your TestQueue class, write a test
  program which adds elements to the queue and
  prints them out
• Make sure you check that adding elements to a
  full queue doesnt blow up

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Implementing a Queue - Deque

• In Class Activity
• The final activity is to write the deque method
• Be sure to handle the precondition where the
  queue is empty (return 0)
• Remember that deque uses the front index
• The element must be accessed in the array using
  the front index first (before incrementing the
  front index)
• Write some tests in your TestQueue class which
  tests that you can correctly deque elements
• Make sure you can handle the case where the queue
  is empty and a deque is requested

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Revision History

• Revision History
• v1.00, 10/21/2004 1142 AM, sps
• Initial revision.
• -- v2.00, 10/24/2005, chr
• Minor correction