Queue and Stack

1
Queue and Stack

• Abstract
• Queue
• Container where elements are added to one end and
  removed from the other.
• Stack
• Container where items are added and removed from
  one end

2
Queue and Stack

• Queue First In First Out (FIFO)

Out
In
Stack Last in First Out (LIFO)
In
Out
3
Queue and StackPublic Methods

• Queue
• Add
• Remove
• isEmpty
• isFull

• Stack
• Push
• Pop
• isEmpty
• isFull

4
Queue and StackImplementation

• Stack
• private Object items
• private int top
• Queue
• private Object items
• private int front, rear, count

5
Queue Implementation

• Make the array circular
• Variables front, rear, count

front points to where the first element is
rear points to where next element goes
6
itemsrear obj rear (rear 1)
items.length count
obj itemsfront front (front 1)
items.length count --
7
public class Queue private Object items
private int front, rear, count public
Queue( int size ) items new
Objectsize front rear count 0
public boolean isEmpty() return
count 0 public boolean isFull ()
return count gt items.length
... // class Queue
8
public class Queue private Object items
private int front, rear, count ...
public void add ( Object x )
itemsrear x rear (rear 1)
items.length count public
Object remove() Object x
itemsfront front (front 1)
items.length count -- return x
// class Queue
9
Queue Class

• front rear count 0

front
0
rear
1
4
3
2
10
Queue Class

• Add A

Add B
front
front
0
0
A
A
1
4
B
1
4
3
rear
2
3
2
rear
11
Queue Class

• Add C, D

Add E
front
front
0
rear
0
A
A
1
4
E
B
1
4
B
C
D
D
C
3
2
3
rear
2
12
Queue Class

• After a while

Remove twice
0
rear
0
rear
1
4
x0
1
4
x2
front
front
3
2
3
2
13
Method numItems

• As Implementer
• public int numItems()
• return count

14
Method numItems

• As User
• public int numItems(Queue q, int qSize)
• (
• Queue tq new Queue(qSize)
• int num 0
• while ( !q.isEmpty() )
• tq.add( q.remove() )
• num
• while ( !tq.isEmpty() )
• q.add( tq.remove() )
• return num
• Is the 2nd while loop necessary?

15
Method numItems

• As User
• public int numItems(Queue q, int qSize)
• (
• Queue tq new Queue(qSize)
• int num 0
• while ( !q.isEmpty() )
• tq.add( q.remove() )
• num
• q tq
• return num
• Pass by Value!

16
Method numItems

• As User
• public int numItems(Queue q, int qSize)
• (
• Queue tq new Queue(qSize)
• int num 0
• while ( !q.isEmpty() )
• tq.add( q.remove() )
• num
• while ( !tq.isEmpty() )
• q.add( tq.remove() )
• return num
• The 2nd while loop is necessary!
• Can you do it without tq?

17
Method numItems

• As User
• public int numItems(Queue q, int qSize)
• (
• Object tq new ObjectqSize
• int num 0
• while ( !q.isEmpty() )
• tqnum q.remove()
• for (int index 0 index lt num index )
• q.add( tqindex )
• return num

18
Schedule

• Prog3 Grace time
• 9 pm, Thursday
• Prog4 Due Time
• 11 pm, Oct 28
• 1ReadOnly folder
• StackMain.txt
• TvectorMain.txt
• Friday
• No class!