CS 101: Computer Programming and Utilizationcs101/2019.1/lectures/Lecture19.pdfLecture 19:more...

Autumn 2019 CS101@CSE IIT Bombay

CS 101:Computer Programming and

Utilization

Puruwith

CS101 TAs and Staff

Course webpage: https://www.cse.iitb.ac.in/~cs101/

Lecture 19: more Structures, Classes and Objects

http://www.cse.iitb.ac.in/~cs101/


Object Oriented Programming

A methodology for designing programs

2


On Design

• Whenever you design something complex, it is useful to have a plan

• Example: Plan for designing a building:

− Understand the requirements− Understand the constraints: budget, land area− Plan how many floors to have− What should be on each floor

• A plan/methodology is also useful when designing (large) projects and similarly while designing programs

3


Object Oriented Programming

• Understand what is required and write clear specifications (needed in all methodologies)

• Identify the entities involved in the problemE.g., in a library management program: books, patrons

• Identify the information associated with each entity− Fixed information: name of the book− Variable information (state): who has borrowed the book at

present

• Organize code so that the entities and their actions/inter relationships are explicitly represented in the code− Information associated with entities: structure variables− Relationships/actions of entities: functions

4


The C++ structure

• Member variables− Basic facility provided in C++ to conveniently gather together

information associated with an entity− Inherited from the C language

• Member functions− New feature introduced in C++− Actions/operations that effect the entity

− User defined data type with variables and functions

5


Defining a structure type

General formstruct structure-type{

member1-type member1-name;member2-type member2-name;...

}; // Don’t forget the semicolon!

Examplestruct Book{

char title[50];double price;

};

A structure-type is a user-defined data type, just as int, char, doubleare primitive data typesStructure-type name and member names can be any identifiers

6


Nested structures (structure is a data type!)

struct Point{ double x,y;

};

struct Disk{ Point center; // contains Pointdouble radius;

};

Disk d;d.radius = 10;d.center = {15, 20}; // sets the x {member of center member of d

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Parameter Passing by Valuestruct Point{double x, y;};

Point midpoint(Point a, Point b){Point mp;mp.x = (a.x + b.x)/2;mp.y = (a.y + b.y)/2;return mp;

}

int main(){Point p={10,20}, q={50,60};Point r = midpoint(p,q);cout << r.x << endl;cout << midpoint(p,q).x << endl;

}

8


Parameter Passing by Reference

struct Point{double x, y;};

Point midpoint( const Point &a, const Point &b){Point mp;mp.x = (a.x + b.x)/2;mp.y = (a.y + b.y)/2;return mp;

}

int main(){Point p={10,20}, q={50,60};Point r = midpoint(p,q);cout << r.x << endl;

}

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Using struct V3V3 sum(const V3 &a, const V3 &b){

V3 v;v.x = a.x + b.x; v.y = a.y + b.y; v.z = a.z + b.z;return v;

}

V3 scale(const V3 &a, double f){V3 v;v.x = a.x * f; v.y = a.y * f; v.z = a.z * f;return v;

}

double length(const V3 &v){return sqrt(v.x*v.x + v.y*v.y + v.z*v.z);

}

10


Member functions

• It is not enough to just define a struct to hold vectors, usually we will also define functions which work on structures/entiries

• In C++, you can make the functions a part of the struct definition itself. Such functions are called member functions.

• By collecting together relevant functions into the definition of the struct, the code becomes better organized (object oriented!)

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The Complete Definition of V3struct V3{

double x, y, z;double length const(){

return sqrt(x*x + y*y + z*z);}V3 sum const(V3 b){

V3 v;v.x = x+b.x; v.y=y+b.y; v.z=z+b.z;return v;

}V3 scale const(double f){

V3 v;v.x = x*f; v.y = y*f; v.z = z*f;return v;

}}

12


One More Example: Taxi Dispatch

• Problem statement: Clients arrive and have to be assigned to (earliest waiting) taxies

• An important part of the solution was a blackboard on which we wrote down the ids of the waiting taxies

• How would we implement this using OOP?– Create a struct to represent each entity:

– customer, taxi, blackboard?

13


The Queue structureconst int N=100;

struct Queue{int elements[N], nwaiting,front;bool insert(int v){

…}

bool remove(int &v){…

}};

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Member Function Insert

• A value can be inserted only if the queue has space• The value must be inserted into the next empty index in the queue• The number of waiting elements in the queue is updated• Return value indicates whether operation was successful

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Member Function Insert

• A value can be inserted only if the queue has space• The value must be inserted into the next empty index in the queue• The number of waiting elements in the queue is updated• Return value indicates whether operation was successful

struct Queue{…bool insert(int v){

if(nWaiting >= N) return false;elements[(front + nWaiting)%N] = v; nWaiting++;return true;

}};

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Member Function remove

• A value can be removed only if the queue is non-empty• The value must be removed from the front of the queue. The front

should be incremented mod N• The number of waiting elements in the queue is decremented by 1• Return value indicates whether operation was successful

struct Queue{…bool remove(int &v){

if(nWaiting < 1) return false;v=elements[front]; front=(front+1)%N; nWaiting--;return true;

}};

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Main Program

int main(){Queue q;q.front = q.nWaiting = 0;while(true){char c; cin >> c;if(c == ‘d’){int driver; cin >> driver;if(!q.insert(driver)) cout <<“Q is full\n”;

}else if(c == ‘c’){int driver;if(!q.remove(driver)) cout <<“No taxiavailable”;else cout <<“Assigning <<driver<< endl;

}}

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Remarks

• The member functions only contain the logic of how to

manage the queue

• The main program only contains the logic of dealing with

taxis and customers

• The new program has become simpler compared to the earlier

version, where the above two were mixed up together

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Structures vs Arrays

• User defined data type, collection of variables of multiple types

• Members can be accessed using the “.” operator

• Structure name denotes the super variable consisting of the entire collection of contained variables

• Structures can be copied using assignments. Also copied when passed by value, or returned from a function

• Member functions can be written to represent actions of the entities represented by the structure

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• Collection of variables of single data type

• Members can be accessed using the “[ ]” operator

• Array name denotes pointer to first element of array

• Array elements need to be explicity copied

• Array elements can be accessed by an expression whose value can be computed at run time


Objects As Software Components

• A software component can be built around a struct

• Just as a hardware component is useful for building big hardware

systems,

so is a software component for building large software systems

• A software component must be convenient to use, and also safe,

i.e., help in preventing programming errors

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Packaged software components

• Hardware devices that you buy from the market are packaged, and made safe to use

– Fridge, television : no danger of getting an electric shock.– A “control panel” is provided on the device. A user does not

have to change capacitor values to change the channel on a television

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Packaged software components

• Analogous idea for software:– Make functionality associated with a struct available to the

user only through member functions (control panel)– Do not allow the user to directly access the data members

inside a struct. (Just as a user cannot touch the circuitry) The user does not need to know what goes on inside

• If you build a better fridge but keep the control panel the same as the previous model, the user does not need to relearn how to use the new fridge– If you build a better version of the struct, but keep the

member functions the same, the programs that use the struct need not change

– Reusable, modular, abstract!

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The modern version of a struct

• Can behave like a packaged component

• Designer of the struct provides member functions

• Designer of the struct decides what happens during execution of

standard operations

• Once structs are designed in this manner, using them becomes convenient and less error-prone

• Structs endowed with above features are more commonly called

objects

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The modern version of a struct

• Designer of the struct decides what happens during execution of

standard operations such as:

– Creation of the object

– Assignment

– Passing the object to a function

– Returning the object from a function

– Destroying the object when it is not needed

25


Structures, Classes and Objects

• Constructors• Copy Constructors• Destructors• Operator overloading• Overloading the assignment operator• Access control• Classes• Graphics and input/output classes

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The Queue Struct in Taxi Dispatch

const int N=100;struct queue{int elements[N],

nWaiting,front;bool insert(int v){…}

book remove(int &v){…

}};

• Once the queue is created, we expect it to be used only through the member functions, insert and remove

• Ideally, we do not expect/want elements, nWaiting, front to be directly accessed

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Main Program Using Queueint main(){Queue q;q.front = q.nWaiting = 0;

while(true){char c; cin >> c;if(c == ‘d’){int driver; cin >> driver;if(!q.insert(driver)) cout <<“Q is full\n”;

}else if(c == ‘c’){int driver;if(!q.remove(driver)) . . .

• Main program does use qthrough operations insert and remove

• However, at the beginning, q.front and q.nWaiting are directly manipulated

• Against the philosophy of software packaging!

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The Constructor member function

• In C++, the programmer can define a special member function called a constructor which will always be called when an instance of the struct is created

• A constructor has the same name as the struct, and has no return type

• Why useful?

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The Constructor member function

• When q is created in the main program, the constructor is called automatically

struct Queue{int elements[N], front,

nWaiting;Queue(){ // constructornWaiting = 0;front = 0;

}// other member functions

};int main(){Queue q;// no need to set// q.nWaiting, q.front// to 0.

}30


Constructors In General

struct A{…A(parameters){…}

};

int main(){A x(arguments);

}

• Constructor can take arguments• The creation of the object x in

main can be thought of as happening in two steps– Memory is allocated for x– The constructor is called on x

with the given arguments

• Many constructors possible, provided they have different signatures

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Another example: Constructor for V3struct V3{double x,y,z;V3(){x = y = z = 0;

}V3(double a){x = y = z = a;

}};

int main();V3 v1(5), v2;

}

• When defining v1, an argument is given

• So the constructor taking a single argument is called. Thus each component of v1 is set to 5

• When defining v2, no argument is given.

• So the constructor taking no arguments gets called. Thus each component of v2 is set to 0

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Remarks• If and only if programmer does not define a constructor, will C++

define a constructor which takes no arguments, and does nothing– If a constructor taking arguments is defined, you implicitly tell

C++ that you want programmers to give arguments. – if some programmer does not give arguments, C++ will flag it as

an error– If you want both kinds of initialization, define both kinds of

constructor• A constructor that does not take arguments (defined programmer

or by C++) is called a default constructor• If you define an array of struct, each element is initialized using

the default constructor

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The Copy Constructor

• Suppose an object is passed by value to a function– It must be copied to the variable denoted by the parameter

• Suppose an object is returned by a function– The value returned must be copied to a temporary variable in

the calling program

• By default the copying operations are implemented by copying each member of one object to the corresponding member of the other object– this default behaviour can be changed by defining a copy

constructor

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Example

struct Queue{int elements[N], nWaiting, front;

Queue(const Queue &source){ // Copy constructorfront = source.front;nWaiting = source.nWaiting;

for(int i=front, j=0; j<nWaiting; j++){elements[i] = source.elements[i];i = (i+1) % N;

}

};

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Copy Constructor in the Example

• The copy constructor must take a single reference argument: the object which is to be copied

• Note that the argument to the copy constructor must be a reference, otherwise the copy constructor will have to be called to copy the argument!

• This is will result in an unending recursion• Member elements are not copied fully. Only the useful part of it is

copied– More efficient

• More interesting use later

struct Queue{int elements[N], nWaiting, front;Queue(const Queue &source){ // Copy constructor

……}

};

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Tracking the use of constructorsstruct Queue{int copyID;Queue(){

cout << "copyId=" << copyId;}Queue(const Queue &source){copyId = source.copyId;cout << "copyId=" << copyId;

}};

Queue updateQueue(Queue q){q.copyId = 3;return q;

}int main(){Queue q;q.copyId = 1;

Queue r(q);r.copyId = 2;

Queue z=updateQueue(r);}

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What will be printed?


Destructors

• When control goes out of a block in which a variable is defined,

that variable is destroyed

– Memory allocated for that variable is reclaimed

• You can define a destructor function, which will get executed

before the memory is reclaimed

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Destructor Example

• If a queue that you have defined goes out of scope, it will be destroyed

• If the queue contains elements at the time of destruction, it is likely an error

• So you may want to print a message warning the user• It is usually an error to call the destructor explicitly. It will be called

automatically when an object is to be destroyed. It should not get called twice.

• More interesting uses of the destructor will be considered in later chapters.

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Destructor Example

struct Queue{int elements[N], nWaiting, front;. . .

~Queue(){ //Destructorif(nWaiting>0) cout << “Warning:” <<“ non-empty queue being destroyed.” << endl;

}

};

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Operator Overloading

• In Mathematics, arithmetic operators are used with numbers, but also other objects such as

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Operator Overloading

• In Mathematics, arithmetic operators are used with numbers, but also other objects such as vectors

• Something like this is also possible in C++!

• An expression such as x @ y where @ is any “infix” operator is considered by C++ to be equivalent to x.operator@(y) in which operator@ is a member function

• If the member function operator@ is defined, then that is called to execute x @ y

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Example: Arithmetic on V3 objects

struct V3{double x, y, z;V3(double a, double b, double c){x=a; y=b; z=c;

}

V3 operator+(V3 b){ // adding two V3sreturn V3(x+b.x, y+b.y, z+b.z); // constructor call

}

V3 operator*(double f){ // multiplying a V3 by freturn V3(x*f, y*f, z*f); // constructor call

}};

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Using V3 Arithmetic

int main(){

V3 u(1,2,3), a(4,5,6), s;

double t=10;

s = u*t + a*t*t*0.5;

cout << s.x <<‘ ‘<< s.y <<‘ ‘

<< s.z << endl;

}

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Remarks

• Expression involving vectors can be made to look very much like what you studied in Physics

• Other operators can also be overloaded, including unary operators (see the book)

• Overload operators only if they have a natural interpretation for the struct in question

• Otherwise you will confuse the reader of your program

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Pointers to Structures

• Disk d1={{2,3},4}, *dptr;

• *dptr is defined to have type Disk, so dptr is a pointer to a variable

of type Disk

• Normal pointer operations are allowed on structure pointers

• dptr = &d1;

• (*dptr).radius = 5; //changes the radius of d1

• Operator ->– (*x).y is same as x->y

• dptr->radius = 5; // same effect as above

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Pointers as Structure Members

struct Disk2{double radius;Point *centerptr;

}

Point p={10,20};Disk2 d;d.centerptr = &p;cout << d.centerptr->x << endl; // will print 10.

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The this Pointer• So far, we have not provided a way to refer to the receiver itself inside the

definition of a member function.

• Within the body of a member function, the keyword this points to the receiver i.e., the struct on which the member function has been invoked.

• Trivial use: write this->member instead of member directly

struct V3{double x, y, z;double length(){

return sqrt(this->x * this->x+ this->y * this->y + this->z * this->z);

}}

• More interesting use later.

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Overloading The Assignment Operator

• Normally if you assign one struct to another, each member of the rhs is copied to the corresponding member of the lhs

• You can change this behaviour by defining member function operator= for the struct

• A return type must be defined if you wish to allow chained assignments, i.e., v1 = v2 = v3; which means v1 = (v2 = v3);– The operation must return a reference to the left hand side

object

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Examplestruct Queue{...

Queue& operator=(Queue &rhs){front = rhs.front;nWaiting = rhs.nWaiting;

for(int i=0; i<nWaiting; i++){elements[i] = rhs.elements[i];i = (i+1) % N;

}return *this;

}

};// only the relevant elements are copied

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Access Control

• It is possible to restrict access to members or member functions of a struct

• Members declared public: no restriction

• Members declared private: Can be accessed only inside the definition of the struct

• Typical strategy:

Declare all data members to be private, and

some subset of function members to be public

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Access Control Example

struct Queue{private:int elements[N], nWaiting, front;

public:Queue(){ … } bool insert(int v){ ..

}bool remove(int &v){ ..

}};

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Remarks

• public:, private: : access specifiers

• An access specifier applies to all members defined following it, until another specifier is given

• Thus elements, nWaiting, front are private, while Queue(), insert, remove are public

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Remarks

• The default versions of the constructor, copy constructor, destructor, assignment operator are public

• If you specify any of these as private, then they cannot be invoked outside of the struct definition

• Thus if you make the copy constructor of a struct X private, then you will get an error if you try to pass a struct of type X by value

• Thus, as a designer of a struct, you can exercise great control over how the struct gets used

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Classes

• A class is essentially the same as a struct, except:– Any members/member functions in a struct are public by

default– Any members/member functions in a class are private by

default

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Classes

• Example: a Queue class:

class Queue{int elements[N], nWaiting, front;

public:Queue(){…}bool remove(int &v){…}bool insert(int v){…}

};

• The members - elements, nWaiting and front will be private.

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Example

struct V3{double x,y,z;V3(double v){x = y = z = v;

}double X(){return x;

}};

struct V3{double x,y,z;V3(double v);double X();

};

//implementationsV3::V3(double v){x = y = z = v;

}double V3::X(){return x;

}

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Concluding Remarks

• The notion of a packaged software component is important. • Making data members private: hiding the implementation from the

user• Making some member functions public: providing an interface

using which the object can be used• Separation of the concerns of the developer and the user• Idea similar to what we discussed in connection with ordinary

functions– The specification of the function must be clearly written down

(analogous to interface)– The user should not worry about how the function does its

work (analogous to hiding data members)

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Input Output Classes

• cin, cout : objects of class istream, ostream resp. predefined in C++• <<, >> : operators defined for the objects of these classes• ifstream: another class like istream• You create an object of class ifstream and associate it with a file on

your computer• Now you can read from that file by invoking the >> operator!• ofstream: a class like ostream, to be used for writing to files• Must include header file <fstream> to uses ifstream and ofstream

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Example of file i/o

#include <fstream>#include <simplecpp>int main(){ifstream infile(“f1.txt”);// constructor call. object infile is created and associated// with f1.txt, which must be present in the current directoryofstream outfile(“f2.txt”);// constructor call. Object outfile is created and associated // with f2.txt, which will get created in the current directory

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Example of file i/o

repeat(10){int v;infile >> v;outfile << v;

}// f1.txt must begin with 10 numbers. These will be read and// written to file f2.txt

}

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“String theory”

• Iterative computations are demonstrated well on arrays

• strings … luckily the system manages the array space for us

• Can assign and append to strings• Can read a position: cout << message[px]• Can write a position: message[px] = ‘q’• That’s all we need for now

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Printing a string in reverse

string message;getline(cin, message);int mx = message.size()-1;while (mx >= 0) {

cout << message[mx];--mx;

}• mx updated in a completely predictable way• Ideal candidate to write as for loop

Character at position mx in

string message

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Finding needles in a haystack

• Given two strings, needles and haystack• needles has no repeated characters• haystack may repeat characters• How many characters in needles appear in

haystack at least once?• needles = “bat”, haystack = “tabla” à 3• needles = “tab”, haystack = “bottle” à 2

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One needle in a haystack

• Subproblem: given one character ch and a string find if ch appears in string at least once

char ch; // suitably initializedstring haystack; // suitably initializedint ans = 0; // will change to 1 if foundfor (int hx = 0; hx < haystack.size(); ++hx) {

if (ch == haystack[hx]) {++ans;break; // quit on first match

}}

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Many needles: nested loop

main() {string needles, haystack;getline(cin, needles); getline(cin, haystack);int ans = 0;for (int nx=0; nx < needles.size(); ++nx) {

char ch = needles[nx];for (int hx = 0; hx < haystack.size(); ++hx) {

if (ch == haystack[hx]) {++ans;break; // quit on first match

}} // ends haystack loop

} // ends needles loop}

Generalize to work in case needles can also

have repeated characters

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Duplicate needles

• needles = “bat”, haystack = “tabla” à 3• needles = “tab”, haystack = “bottle” à 2• needles = “bata”, haystack = “tabla” à 3• Two approaches

– Dedup needles before executing earlier code (reducing to known problem)

– Dedup needles “on the fly” (inside the nx loop)Exercise: If the input strings have n and h characters, at most how much time does

the needle-in-haystack search code take?

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Generalize to arbitrary lengths

• “Hello” < “Help” but “Hello” > “Hell”• Scan both strings from the beginning• If differing character found, same as before• If a string ends, it is “less” than the otherint ans=0, ax=0, bx=0, an=as.size(), bn=bs.size();for (; ans==0 && ax < an && bx < bn; ++ax, ++bx) {

if ( (ans = as[ax] – bs[bx]) != 0) break;}if (ans == 0) {

ans = an – bn;}

This results in an arbitrary integer as the return value in case of unequal input string lengths, but the sign of

ans is what is important68


break

while (true) {ans += base/fac;base *= x;fac *= (++ix);if (base/fac < epsilon) {

break;}cout << (base/fac) << endl;

}

Terminates immediately enclosing while loop

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CS 101: Computer Programming and Utilizationcs101/2019.1/lectures/Lecture19.pdfLecture 19:more...

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