Presentation is loading. Please wait.

Presentation is loading. Please wait.

Queues Printer queues Several jobs submitted to printer Jobs form a queue Jobs processed in same order as they were received.

Similar presentations


Presentation on theme: "Queues Printer queues Several jobs submitted to printer Jobs form a queue Jobs processed in same order as they were received."— Presentation transcript:

1

2

3 Queues Printer queues Several jobs submitted to printer Jobs form a queue Jobs processed in same order as they were received

4 Properties of queues First In First Out (FIFO) Data added at one end only (the Tail of the queue) Data removed at other end only (the Head of the queue)

5 Queue operations Initialise queue Add item (to tail of queue) Remove item (from head of queue) Check if queue empty Check if queue full

6 Queues using arrays Use array to store queue elements Define a data item Head which identifies the item at Head of queue Define a data item Tail which identifies the first empty location after last item in queue Tail identifies location where next item is placed in queue

7 Queues using arrays (first try) Head Tail Empty queue (Tail == Head)

8 Queues using arrays (first try) Head Tail Add item T at location Tail T

9 Queues using arrays (first try) Head Tail Add item H at location Tail TH

10 Queues using arrays (first try) Head Tail Add item I at location Tail THI

11 Queues using arrays (first try) Head Tail Remove item T from Head HI

12 Queues using arrays (first try) Head Tail Continue until Tail == ArraySize Queue full? AQUEUE

13 Queues using arrays (first try) Head Tail Must shift queue contents back to start of array - inefficient! AQUEUE

14 Circular Queue Use a circular queue Consider (perceive?) the array as a circular structure (i.e. as if the last element of the array is connected/joined to the first element of the array) The benefit of this insight is that we never have to shift data

15 A circular array MaxSize - 1 MaxSize - 2 MaxSize - 3

16 A queue using a circular array MaxSize - 1 Tail Head Empty queue Tail == Head 0

17 A queue using a circular array T MaxSize - 1 Tail Head Add T at Tail Tail = (Tail + 1) % MaxSize 0

18 A queue using a circular array T H MaxSize - 1 Tail Head Add H at Tail Tail = (Tail +1) % MaxSize 0

19 A queue using a circular array T H MaxSize - 1 Tail Head Add I at Tail Tail = (Tail +1) % MaxSize 0 I

20 A queue using a circular array H MaxSize - 1 Tail Head Remove T from Head Head = (Head +1) % MaxSize 0 I

21 A queue using a circular array MaxSize - 1 Tail Head Continue until Tail == MaxSize E U E U Q A

22 A queue using a circular array MaxSize - 1 Tail Head 0 E U E U Q A Add Z at Tail Tail = (Tail +1) % MaxSize i.e. [(MaxSize - 1) + 1] % MaxSize = MaxSize % MaxSize = 0 Z

23 Empty and Full Queue Tests Empty queue condition: Head = = Tail Full queue condition: (Tail + 1) % MaxSize = = Head

24 Queue ADT in Java Constructor isempty isfull Join Leave

25 Constructor public QueueOfInts() {Queue = new int[10] ; Head = Tail = 0 ; } public QueueOfInts(int capacity) {Queue = new int[capacity] ; Head = Tail = 0 ; }

26 Test for empty queue public boolean isempty() { return Head == Tail) } }

27 Test for full queue public boolean isfull() { if ((Tail + 1) % Queue.length == Head) { return true; } else { return false; }

28 Adding an element public void Join(int val) {Queue[Tail] = val ; Tail = (Tail + 1) % Queue.length; }

29 Remove an element public int Leave() {int Removed ; Removed = Queue[Head]; Head = (Head + 1) % Queue.length ; return Removed ; }


Download ppt "Queues Printer queues Several jobs submitted to printer Jobs form a queue Jobs processed in same order as they were received."

Similar presentations


Ads by Google