Comp 245 Data Structures. Queues

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1 Cmp 245 Data Structures Queues

2 Intrductin t the Queue ADT It is a FIFO (first-in, first-ut) structure Access t the Queue can take place at tw lcatins: 1) the Frnt fr data remval and 2) the Rear fr data strage N access t elements ther than the Frnt and Rear

3 Abstract View f a Queue

4 A RRRRRbust Queue Full The Full() functin will be used in cnjunctin with Enqueue Empty The Empty() functin will be used in cnjunctin with Dequeue

5 Queue Applicatin Drive-In Teller Simulatin Cmputers are excellent fr simulating reallife situatins. Queuing (Line) simulatin sftware is available t bserve a key behavir WAIT TIME fr a custmer. Running this simulatin can help a business determine hw many lines t have pen at certain times during the day in rder t achieve an acceptable wait time fr custmers. Als, this helps tellers (servicers) t be prductive.

6 Queue Applicatin Drive-In Teller Simulatin The Drive-In Teller Simulatin will mdel a single-server, single-queue situatin. We must btain the fllwing infrmatin befre we run the simulatin Length f simulatin Arrival rate prbability f a custmer Expected service time The utput frm the simulatin will be average wait time fr a custmer.

7 Queue Applicatin Drive-In Teller Simulatin Queuing Equatins AR = Arrival Prbability, ST = Service Time 1) (AR)(ST) < 1 (STABLE) 2) (AR)(ST) > 1 (UNSTABLE) 3) (AR)(ST) = 1 (FLUCUATING)

8 Queue Applicatin Drive-In Teller Simulatin Hw d we represent a clck? A cunter variable Hw d we determine if a custmer has arrived in line? A randm number generatr will dictate an enqueue. Hw d we represent a teller? A cunter variable Hw d we represent a custmer? A timestamp Hw d we determine hw lng a custmer has waited? Subtract timestamp frm current time n clck Hw des a custmer get ut f line t be serviced? When the teller becmes available and the queue is nt empty a dequeue is executed. Hw d we determine average wait time fr a custmer? Ttal wait time accumulatin/ttal custmers served

9 Implementing a Queue ADT Array Based There are three different types f array based Queues: Packing Circular Free-Space The three differ in The number f cntrl fields needed t maintain the queue Hw they enqueue and dequeue Hw they determine if the queue is empty r full

10 Implementing a Queue ADT Array Based - Packing Needs an array(a) and a cunt(c) t implement. Enq data will be added t the queue at A[C] Deq data will be remved frm the queue at A[0]; IMPORTANT: after a Deq, array must be packed! Full (C == MAX) Empty- (C == 0)

11 Packing Queue Example

12 Implementing a Queue ADT Array Based - Circular Needs an Array(A), Cunt(C), Frnt(F) and a Rear(R) t implement. Enq will add data t A[R], after the Enq, R must be incremented r wrapped. That is R++ r R = 0. Cunt must be incremented. Deq will remve data frm A[F], after the Deq, F must be incremented r wrapped. That is F++ r F = 0. Cunt must be decremented. Empty (C = = 0) Full (C == MAX)

13 Circular Queue Example Part I

14 Circular Queue Example Part II

15 Implementing a Queue ADT Array Based Free Space Needs an Array(A), Frnt(F) and a Rear(R) t implement. Initially set F and R t the last slt in the array. Enq Yu will increment r wrap R first and then add data t A[R] Deq Yu will increment r wrap F first and then remve data frm A[F] Empty (F = = R) Full (F == R+1 r F == wrap(r)) THIS PROTECTS EMPTY CONDITION

16 Free Space Queue Example Part I

17 Free Space Queue Example Part II

18 Static Queue Review Packing Enq A[C] Deq A[0] Full (C==Max) Empty (C==0) Circular Enq A[R++] Deq A[F++] Full (C==Max) Empty (C==0) Free Space Enq A[++R] Deq A[++F] Full (F==R+1) Empty (F==R)

19 Implementing a Queue ADT Linked List Based Requires tw pinters: 1) Frnt and 2) Rear There is a special cnditin n Enq: Enq when the Queue is empty. There is a special cnditin n Deq: Deq when the Queue has nly ne element.

20 Enq with a Linked List

21 Deq with a Linked List

22 Queue Implementatin Review Array Based Packing (cunter) Circular (cunter, frnt, rear) Free Space (frnt, rear) Linked List Based Need nly a frnt and rear pinter Must handle special cases fr enq and deq

23 C++ STL Queue Cntainer Class The STL queue is a type f cntainer specifically designed t perate in a FIFO cntext (first-in firstut), where elements are inserted int ne end f the cntainer and extracted frm the ther. Here is an example f STL queue usage: queuestl.cpp

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