1. Queue ADT
(Abstract Data Type) N … 3 2 1

2. What is a Queue?
A queue is an ordered collection, or list, of items that can only be modified by removing items at the beginning or adding items to the end
Items are processed on a first come/first serve basis
There is no access to the middle of the list (no “cutting in line”)
Real life examples:
Waiting on a grocery store check-out line (actual physical line)
Waiting in a reception room or for a table in a restaurant (“virtual” line)

3. Queues in the abstract Sometimes referred to as a “FIFO”
Stands for “first-in/first-out”
A stack would be a “LIFO”
Some definitions:
Enqueue: add an item to the end
Dequeue: remove an item from the beginning

4. Abstract Data Type (ADT) for a Queue
QueueADT requires that a program implementing a queue contain certain minimum core capabilities:
Elements must be maintained in an ordered list
Facilities must be provided to both remove first item in list (dequeue) and add new items to the end of the list (enqueue)
Individual elements in the queue can be any valid data type (or collection of data types)
Other possible capabilities:
Query the queue for contents, length, …
Clear or empty the queue
Limit or expand the size of the queue

5. Queue Representations
Using an Array
Using a Linked List 1 2 3 4 …
N-1 1 2 … N
null

6. Array Implementation of Queue
Use an array to store data elements
Advantages:
Easy to access
Natural ordering
Size Limitation
Array is fixed size
Number of entries is limited unless special measures are taken 1
N-1
first
last 2

7. Queue Array Implementation with wrap-around
Starting index issue
Problem: As items are dequeued, empty cells show up at the beginning of the array
Solution: implement a wrap- around capability
When the last element is filled, start over at the beginning
normal configuration 1 2
first
last
with wrap-around 1 2
first
last

8. Array Implementation of Queue in C++
qarray.c
Simple implementation of QueueADT using C++
Stores user provided integers in a queue
Features:
Fixed size array
Implements wrap-around feature
Basic functionality: enqueue, dequeue, and display methods
main.c - simple wrapper for testing

9. Linked List Implementation of Queue
Use a linked list to store data elements
Advantages:
Virtually unlimited number of elements
No “wrap-around” worries
Potential issues
More complex coding
Need to manage pointers and data containers
On-the-fly memory allocation/deallocation
More memory required (data plus pointers)
Most likely slower than array implementation 1 2 … N
null

10. Queue Linked List Implementation in C++
qllist.c
Simple implementation of QueueADT using C++
Stores user provided integers in a queue
Features:
Unlimited queue size
Basic functionality: enqueue, dequeue, and display methods
main.c - simple wrapper for testing (same as for array implementation)

11. Final Considerations What about more complex data (not just integers)?
The integers can be pointers to other data containers (classes, structs …)
Generalize by making a template
Which implementation is better? Answer is: “it depends…”
If a fixed size queue is acceptable, probably an array
For example: large quantities of predictable, streaming data that requires buffering
(audio or video streaming from the internet)
If managing random, or unpredictable, data, a linked list might be better
Particularly if speed or memory is less of an issue
Lists of names, addresses …