1. Queues

2. Introduction A sequential collection of data
Changes occur at both ends, not just one
Queue applications
files waiting to be printed
"jobs" waiting for the CPU or an I/O device
tokens waiting to be processed
signals on a channel (wired or wireless)
Simulations

3. Structural Concept A sequence of data items Operations
Items may be ANY type
integers
chars
arrays
structs
Operations
Items can be removed only at the front
Items can be added only at the end
Only the front item may be viewed

4. FIFO list - only front element is visible
Queue
front
enqueue
dequeue
index
FIFO list - only front element is visible

5. The Queue ADT Operations Construct a queue
Test for queue is empty/full
Enqueue (add new item at end)
Front (retrieve value of item at front)
does not remove item
Dequeue (remove item from front)
next item moves to the front

6. Static-Array Implementation
Operations & constants
Q_MAX specifies item total
enqueue
increments index
use mod operator to "circle back to start"
1st checks for full queue
dequeue
uses mod operator to "circle back to start"
1st checks for empty queue

7. Static-array Implementation Example
typedef Complx QueueElement;
const int CAPACITY = 8;
int myFront, myBack;
QueueElement myQueue[CAPACITY];
Complx X;
enqueue(&myArray,X); -1
myFront
myArray 7 6 5 4 3 2 1
myBack -1
initially empty
(myFront, myBack are negative)

8. Caveats could be too small could be too large (wastes space)
must be careful managing index
going past the "end"
solution (TBD) is dynamic storage
Lab2: build a queue. enqueue, front-output in seq'l order, dequeue

9. Solutions careful mgmt of index values (front, back) bigger queue
circular queue
dynamic array (TBD)

10. Circular Queue Each array element (a node) is 2 items
index of next node
data
Last node has index of first node
Or keep separate int index
subscript of "last" node
1st node = MOD(lastnode+1,CAPACITY)

11. Some More Uses of a Queue
job scheduling
graphic display sequencing
disk I/O access
message processing
spaceship launch sequence
data "buffering"

12. Buffering data produced faster than can be used need to store it
storage is called a "buffer"
any stack or queue can be called a buffer