1. CSS 342 Data Structures, Algorithms, and Discrete Mathematics I
Lecture
CHAPTER 13, 14.

2. Agenda
STL
Queues
Lab 5 Intro and peer design requirements

3. STL Sequence Containers: the Big 3 (recap)
Vector
Flexibly sized array
Access any element in constant time (index into array)
Add/Remove from the end of array
Data kept contiguous in memory
Deque
Double ended queue
Can add/look from front or back
Access any element in constant time
Not guaranteed to be contiguous in memory
List
Linked list
Need iterator to traverse
Can add anywhere in list in constant time

4. Recall the stack pop push Last In First Out (LIFO)
We implemented with following structures:
Array
Linked List
(aStack.push(newItem)).pop() is equal to aStack
STL has a stack implementation as a Container Adapter
Container adapter on vector, deque, or list
Default is deque
Functions: empty, size, push, pop, top
pop
push

5. #include <vector>
#include <stack>
#include <deque>
int main( ) {
stack<int> aStack;
stack<int, vector<int>> bStack;
for (int i = 0; i < 3; i++)
bStack.push(i); }
cout << "stack size is: " << bStack.size() << endl;
cout << "top element is: " << bStack.top() << endl;
bStack.pop();
cout << "Popped! " << endl;
return 0;

6. Big-O Challenge for (int i = 1; i <= n; i++) { int j = n;
while (i * i < j)
j--;
MyBigTask(); }

7. Queues

8. Queue First in First Out (FIFO) Can be implemented with
Array
Linked List
(aQ.push(newItem)).pop() != newItem
Uses
Many: powerful data structures for impls.
Often used for short lived (in-memory) or persistent applications (written to disk)
Modeling: Rich field on queueing theory/modeling
Time = 0
Time = 12
Time = 20
Time = 38
CSS342: Queues

9. Queue Specification
STL has a stack implementation as a Container Adapter
Container adapter on vector, deque, or list
Default is deque
Functions: empty, size, push, pop, back, front
pop( ): remove
and get the front item
push() to the back
back(): get the back item
but do not remove it
Front(): get the front item
but do not remove it

10. int main( ) {
queue<int> aQ;
for (int i = 0; i < 5; i++)
aQ.push(i); }
cout << "Queue size is: " << aQ.size() << endl;
cout << "Front element is: " << aQ.front() << endl;
cout << "Back element is: " << aQ.back() << endl;
aQ.pop();
cout << "Popped! " << endl;
return 0;

11. Comparison of Stack and Queue Operations
size, empty, push, pop, =, comparators are all common syntax
Both are adapters which can be built on container classes
Differences:
stack: top
queue: front, back
Semantics: LIFO, FIFO

12. Queue implementation Array Based Pointer based
Can we make a queue from lists?
How can we make a queue Persistent?

13. In-Class Code
Create a queue using an array as an underlying data structure

14. An Array-Based Implementationk 2 4 1
….. 7
…..
front
back 1 2 k
theArray.size( )-1 47 49
….. 4 10 7
front
back 1 2 47 48 49
theArray.size( )-1
1) Shift left on each removal?
2) Shift left when end of queue is reached?
3) Never Shift but maintain moving front and back pointers?
4) Allocate new array when full?

15. Circular-array implementation2 4 1 7 3
theArray.size( )-1
front
back

16. MAX_Q-1 MAX_Q-1 MAX_Q-1 MAX_Q-1 (1) 1 2 3 front (2) 1 2 7 6 front back1 2 3
MAX_Q-1
front
(2) 1 2
MAX_Q-1 7 6
front
back
Initial state
Enqueue(7)
Enqueue(6)
back
count = 0
count = 2 3
(3) 1 2 3
MAX_Q-1 8 9 4
(4) 1 2 3
MAX_Q-1
Enqueue(3)
Enqueue(8)
Enqueue(9)
Dequeue()
Enqueue(2)
Enqueue(4)
back
while (theQ.size()) {
Dequeue() }
back
front
front
count = 0
count = 5

17. Contract (.h) class MyQueue { public: MyQueue(); ~MyQueue();
bool isEmpty() const;
int getCount() const;
int Dequeue() throw (exception);
void Enqueue(const int &val);
private:
vector<int> arr;
int front;
int back;
int count;
void doubleQueueSize(); };

18. int MyQueue::getCount() const{
return count; }
bool MyQueue::isEmpty() const
if (count == 0)
return true;
else
return false;
MyQueue::MyQueue() {
arr.resize(ARRAY_START_SIZE);
front = 0;
back = arr.size() - 1;
count = 0; }
void MyQueue::doubleQueueSize() { }

19. void MyQueue::Enqueue(const int &val){
if (count == arr.size())
doubleQueueSize(); }
back = (back + 1) % arr.size();
arr[back] = val;
count++;
int MyQueue::Dequeue() throw (exception) {
int retVal;
if (count > 0)
retVal = arr[front];
front = (front + 1) % arr.size();
count--;
return retVal; }
else
throw exception("Queue Empty");

20. Lab5 Intro

21. Lab 5 The Jolly Banker Purpose: Two phase project Learn Queues
Learn Binary Search Trees
Practice class design
Two phase project
Design Review: Thursday, 12/1
Lab Due: 12/9.

22. Computer Scientist of the Week
Margaret Hamilton
Developed on-board flight software for Apollo space program
Director of MIT Instruments Lab
Just awarded Presidential Medal of Freedom
Contributions to async software, priority scheduling and priority
Display
Foundations for ultra-reliable software design
Apollo 11 Landing: design helped to successfully land ship amidst
Overloading

23. Computer Scientists who were awarded Presidential Medal of Freedom this week
Grace Hopper
Margaret Hamilton
Robert DeNiro

24. A Pointer-Based Implementation2 4 1 7
NULL
front
back

25. Bell Rang

26. class MyQueue {
public:
MyQueue();
~MyQueue();
bool isEmpty() const;
int getCount() const;
int Dequeue() throw (exception);
void Enqueue(const int &val);
private:
struct Node
int val;
Node *next = NULL; };
Node *front;
Node *back;
int count;

27. bool MyQueue::isEmpty() const{
if (front == NULL)
return true; }
else
return false;
int MyQueue::getCount() const
return count;
MyQueue::MyQueue() {
front = NULL;
back = NULL;
count = 0; }
MyQueue::~MyQueue() {
while (getCount() > 0)
Dequeue(); }

28. void MyQueue::Enqueue(const int &value){
Node *insNode;
insNode = new Node;
insNode->val = value;
if (count == 0)
back = insNode;
front = insNode; }
else
back->next = insNode;
count++;
int MyQueue::Dequeue() throw (exception) {
if (count == 0)
throw exception("Queue is empty"); }
Node *temp = front;
int retVal = temp->val;
front = front->next;
delete temp;
if (count == 1)
back == NULL;
count--;
return retVal;

29. Are we done? Do we need to overload =, copy constructor?
Why or why not?
Any other overloads?

30. Queue usage in OS, Network, Services
Message queues
Scheduling queue
TCP Flow control
Payment processing

31. Unix Message Queues 1 2 Message queue (id = msgid)
struct mymesg {
long mytype;
char mtext[512];
} message_body;
int main( void ) {
int msgid = msgget( 100, IPC_CREAT );
strcpy( message_body.mtext, “hello world\n” );
msgsnd( msgid, &message_body, 512, 0 ); }
struct mymesg {
long mytype;
char mtext[512];
} message_body;
int main( void ) {
int msgid = msgget( 100, IPC_CREAT );
msgrcv( msgid, &message_body, 512, 0, 0 );
cout << message_body.mtext << endl; }
Message queue
(id = msgid) 1 2
Some other process can
enqueue and dequeue a message

32. Multilevel Queue Scheduling
Each queue has its own scheduling algorithm,
foreground (interactive) – RR, 80% CPU time
background (batch) – FCFS, 20% CPU time

33. Multilevel Feedback-Queue Scheduling
A new job enters queue Q0 which is served FCFS. When it gains CPU, job receives 8 milliseconds. If it does not finish in 8 milliseconds, job is moved to queue Q1.
At Q1 job is again served FCFS and receives 16 additional milliseconds. If it still does not complete, it is preempted and moved to queue Q2.

34. Flow Control X X X X Sending application Receiving application packet
Send Socket Buffer
Receive Socket Buffer
send
ack
read blocked
packet X
packet
packet retransmitted
packet dropped
Application is slow to read. X
packet
packet retransmitted
packet dropped X X
write blocked