1. Queues Model Operations Pointer Implementations Array Implementation
Stacks and Recursive Procedures
4/21/2019
CS 303 – Queues
Lecture 6

2. ADT Queue - Model Specialization of List FIFO: First In, First Out
Insert at End
Delete at Front
4/21/2019
CS 303 – Queues
Lecture 6

3. Queue - Operations MakeNull(Q) // MakeNull(L)
T/F = Empty(Q) // First(L) == End(L)
Enqueue(Q,x) // Insert(L,End(L),x)
x = Front(Q) // Retrieve(L,First(L))
Dequeue(Q) // Delete(L,First(L))
[Variation: x = Dequeue(Q) Exercise: work out details]
x: ElementType
S: StackType
L: ListType
Any List Implementation will do, but can we do better?
4/21/2019
CS 303 – Queues
Lecture 6

4. Pointer Implementations – First Attempts
Doubly Linked List? [Overkill] Q a1 a2 an
Singly Linked List? [no Header, but still pointer overhead]
Dequeue is EASY, but Enqueue is HARD Q a1 a2 a3
... an
4/21/2019
CS 303 – Queues
Lecture 6

5. A better Pointer ImplementationQ a1 a2 a3
... an
Enqueue and Dequeue are both EASY
4/21/2019
CS 303 – Queues
Lecture 6

6. Array Implementation Circular Array, with zero-based indexingi
i+1
i+n-1
MaxLength-1 a1 a2 an
Front
Rear
Circular Array, with zero-based indexing
Full/Empty conditions are tricky! Leave empty buffer region to
avoid bugs and simplify code.
4/21/2019
CS 303 – Queues
Lecture 6

7. Stacks and Recursive Procedures
Recursion is implemented (by the System) by Pushing state onto a
Stack
Even when this is provided automatically, you must Analyze as if the
stack were explicit.
Sometimes it is useful to manage the stack explicitly
Usually better to write recursive routines and let the System do it
Manual elimination of tail recursion considered harmful!
All bets are off in primitive environments.
4/21/2019
CS 303 – Queues
Lecture 6