Queues

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

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

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

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

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

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)

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

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

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)

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

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