Smu - Mca - Mc0066 Assign 1

August 2010

Master of Computer Application (MCA) – Semester 2

MC0066 – OOPS using C++ – 4 Credits (Book ID: B0681 & B0715)

Assignment Set – 1 (40 Marks)

Book ID: B0681

1. Describe the steps in compiling and executing a C++ program with programmatic illustration.

Ans.There are three steps in executing a c++ program: Compiling, Linking and Running the program. The c++ programs have to be typed in a compiler. All the programs discussed in the book will be compiled on turbo c++ compiler. The turbo c++ compiler comes with an editor to type and edit c++ program. After typing the program the file is saved with an extension .cpp. This is known as source code. The source code has to be converted to an object code which is understandable by the machine. This process is known as compiling the program. You can compile your program by selecting compile from compile menu or press Alt+f9. After compiling a file with the same name as source code file but with extension .obj. is created.

Second step is linking the program which creates an executable file .exe (filename same as source code) after linking the object code and the library files (cs.lib) required for the program. In a simple program, linking process may involve one object file and one library file. However in a project, there may be several smaller programs. The object codes of these programs and the library files are linked to create a single executable file. Third and the last step is running the executable file where the statements in the program will be executed one by one.

When you execute the program, the compiler displays the output of the program and comes back to the program editor. To view the output and wait for user to press any key to return to the editor, type getch() as the last statement in the program. Getch() is an inbuilt predefined library function which inputs a character from the user through standard input. However you should include another header file named conio.h to use this function. Conio.h contains the necessary declarations for using this function. The include statement will be similar to iostream.h.

Compiling and Linking

During compilation, if there are any errors that will be listing by the compiler. The errors may be any one of the following

1. Syntax error

August 2010




This error occurs due to mistake in writing the syntax of a c++ statement or wrong use of reserved words, improper variable names, using variables without declaration etc. Examples are : missing semi colon or paranthesis, type integer for int datatype etc. Appropriate error message and the statement number will be displayed. You can see the statement and make correction to the program file, save and recompile it.

2. Logical error

This error occurs due to the flaw in the logic. This will not be identified by the compiler. However it can be traced using the debug tool in the editor. First identify the variable which you suspect creating the error and add them to watch list by selecting Debug ->Watches->Add watch. Write the variable name in the watch expression. After adding all the variables required to the watch list, go to the statement from where you want to observe. If you are not sure, you can go to the first statement of the program. Then select Debug ->Toggle Breakpoint (or press ctrl + f8). A red line will appear on the statement. Then Run the program by selecting Ctrl + f9 or Run option from run menu. The execution will halt at the statement where you had added the breakpoint. The watch variables and their values at that point of time will be displayed in the bottom in the watch window. Press F8 to execute the next statement till you reach the end of the program. In this way you can watch closely the values in the watch variables after execution of each and every statement in the program. If you want to exit before execution of the last statement press Ctrl + Break. To remove the breakpoint in the program go to the statement where you have added breakpoint select Debug ->Toggle Breakpoint (or press ctrl + f8). Select Debug -> watch ->remove watches to remove the variables in the watch list. This tool helps in knowing the values taken by the variable at each and every step. You can compare the expected value with the actual value to identify the error.

3. Linker error

This error occur when the files during linking are missing or mispelt

4. Runtime error

This error occurs if the programs encounters division by zero, accessing a null pointer etc during execution of the program

2. Describe the theory with programming examples the selection control statements in C++.

Ans.

If statementSyntax : if (expression or condition)

August 2010




{ statement 1;statement 2;}else { statement 3;statement 4;}

The expression or condition is any expression built using relational operators which either yields true or false condition. If no relational operators are used for comparison, then the expression will be evaluated and zero is taken as false and non zero value is taken as true. If the condition is true, statement1 and statement2 is executed otherwise statement 3 and statement 4 is executed. Else part in the if statement is optional. If there is no else part, then the next statement after the if statement is exceuted, if the condition is false. If there is only one statement to be executed in the if part or in the else part, braces can be omitted.

Following example program implements the if statement.

// evenodd.cpp# include <iostream.h># include <conio.h>void main(){int num;cout<<”Please enter a number”<<endl;cin>>num;if ((num%2) == 0)cout<<num <<” is a even number”;elsecout<<num <<” is a odd number”;getch();}

The above program accepts a number from the user and divides it by 2 and if the remainder (remainder is obtained by modulus operator) is zero, it displays the number is even, otherwise as odd. We make use of the relational operator == to compare whether remainder is equal to zero or not.

Nested If statement

If statement can be nested in another if statement to check multiple conditions.

August 2010




If (condition1){ if (condition 2){ statement1;Statement2;}else if (condition3){statement3;}}else statement4;

The flowchart of the above example is shown below

Multiple conditions can be checked using logical && operator(AND) and || operator (OR).

If ((condition1) && (condition2))statement1;else statement2;

In the above example statement1 will be executed if both the condition1 and condition2 are true and in all other cases statement2 will be executed.

If ((condition1 || (condition2))statement1;elsestatement2;

http://resources.smude.edu.in/slm/wp-content/uploads/2009/06/clip-image00248.gif

August 2010




In the above example statement1 will be executed if either condition1 or condition2 are true and even if both are true. Statement2 will be executed if both the conditions are false. The following program demonstrates the use of && operator and nested if statement.

//Large.cpp# include <iostream.h>void main(){ int a,b,c;cout<<”Please enter three numbers”;cin>>a>>b>>c; if ((a>b) && (b>c))cout<<a<< “ is the largest number”;else if ((b>a) && (b>c))cout<<b<< “ is the largest number”;else if ((c>a) && (c>b))cout<<c<< “ is the largest number”;}

The above program accepts three numbers from the user and displays which is the largest number among the three.( assumption is that all the numbers are unique, the program has to be modified if you would like to allow same number twice)

Switch statement

Nested ifs can be confusing if the if statement is deeply nested. One alternative to nested if is the switch statement which can be used to increase clarity in case of checking the different values of the same variable and execute statements accordingly.

Syntax :

Switch (variablename){ case value1: statement1;break;case value2: statement2;break;case value3: statement3;break;default: statement4;}

If the variable in the switch statement is equal to value1 then statement1 is executed, if it is equal to value2 then statement2 is executed, if it is value3 then statement3 is executed.

August 2010




If the variable value is not in any of the cases listed then the default case statement or statement4 is executed. The default case specification is optional, however keeping it is a good practice. It can also be used for displaying any error message. Each case can have any number of statements. However every case should have a break statement as the last statement. Break statement takes the control out of the switch statement. The absence of the break statement can cause execution of statements in the next case. No break is necessary for the last case. In the above example, default case does not contain a break statement.

The flowchart for the switch statement is shown below

The following program implements the switch statement

position.cpp

# include<iostream.h>void main(){ char pos;int x=15, y=15;cout << “ you are currently located at” <<x<<” “<<y<<endl;cout>>”please choose the letter to move l for left, r for right, u for up and d for down” <<endl;cin>>pos;switch (pos){ case ‘l’: x–;break;case ‘r’: x++;break;case ‘u’: y++;break;case ‘d’: y–;

http://resources.smude.edu.in/slm/wp-content/uploads/2009/07/clip-image00417.jpg

August 2010




break;default: cout<<”You selected a wrong option”;}cout<<“ you are now located at” <<x<<” “<<y;}

The above program asks the user to enter l,r,u,d for allowing him to move left,right,up and down respectively. The position is initialised to 15 and 15 which are x and y coordinates of his position. Depending upon the what user has selected the the x and y co-ordinates are incremented or decremented by one(x++ is same as x=x+1). If the user types a letter other than l,r,u,d, he gets an error message. Since the switch variable is a character, l,u,r,d and enclosed within single quote.

++ and — operator can be used as postfix or as prefix operator which has no effect if used as an independent statement. However if it used as part of an expression, the prefix operator will be operated and then the expression will be evaluated whereas the postfix operated will be evaluated later.

For example in the statement x= a+ (b++), a will be added to b and then stored in x and then the value of b will be incremented. If the same expression is written as x=a+(++b), the the b will be incremented and then added to a and stored in x.

3. Given a RxC Matrix, A, i.e. R rows and C columns we define a Saddle-Point as Saddle_Pt (A(i,j)) = A(i,j) is the minimum of Row i and the maximum of Col j. e.g.

1 2 3 4 5 6 7 8 9 -- 7 is Saddle_Pt. at position (3,1)

Write a program in C++ to check and print for saddle points in a matrix.

Ans.

#include<iostream.h>#include<conio.h>void main(){clrscr();int a[3][3],i,j,k,sp,minr,pos,flag=1;cout<<"Enter the contents of the array ";

August 2010




for(i=0;i<3;i++){ for(j=0;j<3;j++) cin>>a[i][j];}cout<<"The matrix representation of array is: ";for(i=0;i<3;i++){ cout<<""; for(j=0;j<3;j++) cout<<a[i][j]<<" ";}cout<<endl;

for(i=0;i<3;i++){flag=1;sp=a[i][0],pos=0;

for(j=1;j<3;j++){ if(a[i][j]<sp) { sp=a[i][j]; pos=j; }}

for(k=0;k<3;k++){ if(a[k][pos]<sp) { flag=0; break; }}

August 2010




if(flag==1)cout<<"The saddle point of row "<<i+1<<" is "<<sp<<endl;

}getch();}

4. Describe and Demonstrate the concept of Pass by Value and Pass By Reference using appropriate programming examples of your own.

Ans. Pass by Value

Consider a pair of C++ functions defined in Program. The function One calls the function Two. In general, every function call includes a (possibly empty) list of arguments. The arguments specified in a function call are called actual parameters . In this case, there is only one actual parameter y. void Two(int x){ x=2; cout << r << endl;}

void One(){ int y=1; Two(y); cout << y << endl;}Program: Example of Pass-By-Value Parameter PassingThe method by which the parameter is passed to a function is determined by the function definition. In this case, the function Two is defined as accepting a single argument of type int called x. The arguments which appear in a function definition are called formal parameters . If the type of a formal parameter is not a reference , then the parameter passing method is pass-by-value. The semantics of pass-by-value work like this: The effect of the formal parameter definition is to create a local variable of the specified type in the given function. E.g., the function Two has

August 2010




a local variable of type int called x. When the function is called, the values (r-values) of the actual parameters are used to initialize the formal parameters before the body of the function is executed.

Since the formal parameters give rise to local variables, if a new value is assigned to a formal parameter, that value has no effect on the actual parameters. Therefore, the output obtained produced by the function One defined in Program is: 21

Pass by Reference

Consider the pair of C++ functions defined in Program . The only difference between this code and the code given in Program is the definition of the formal parameter of the function Two: In this case, the parameter x is declared to be a reference to an int. In general, if the type of a formal parameter is a reference, then the parameter passing method is pass-by-reference . void Two(int& x){ x=2; cout << r << endl;}void One(){ int y=1; Two(y); cout << y << endl;}Program: Example of Pass-By-Reference Parameter PassingA reference formal parameter is not a variable. When a function is called that has a reference formal parameter, the effect of the call is to associate the reference with the corresponding actual parameter. I.e., the reference becomes an alternative name for the corresponding actual parameter. Consequently, this means that the actual parameter passed by reference must be variable.

A reference formal parameter can be used in the called function everywhere that a variable can be used. In particular, if the reference formal parameter is used where a r-value is required, it is the r-value of actual parameter that is obtained. Similarly, if the reference parameter is used where an l-value is required, it is the l-value of actual parameter that is obtained. Therefore, the output obtained produced by the function one defined in Program is: 2

August 2010




2

Book ID: B0715

5. Describe the theory of Derivation and Inheritance.

Ans.Derivation

Inheritance is implemented in C++ through the mechanism of derivation. Derivation allows you to derive a class, called a derived class, from another class, called a base class.

Derived class syntax

Derived class syntax >>-derived_class--:---------------------------------------------> .-,---------------------------------------------------------.-------- V |>----+----------------------------+--qualified_class_specifier-+->< +-virtual--+-----------+-----+ | +-public----+ | | +-private---+ | | '-protected-' | '-+-public----+--+---------+-' +-private---+ '-virtual-' '-protected-'

In the declaration of a derived class, you list the base classes of the derived class. The derived class inherits its members from these base classes.

The qualified_class_specifier must be a class that has been previously declared in a class declaration.

An access specifier is one of public, private, or protected.

The virtual keyword can be used to declare virtual base classes.

The following example shows the declaration of the derived class D and the base classes V, B1, and B2. The class B1 is both a base class and a derived class because it is derived from class V and is a base class for D:

August 2010




class V { /* ... */ };class B1 : virtual public V { /* ... */ };class B2 { /* ... */ };

class D : public B1, private B2 { /* ... */ };

Classes that are declared but not defined are not allowed in base lists.

For example: class X;// errorclass Y: public X { };

The compiler will not allow the declaration of class Y because X has not been defined.

When you derive a class, the derived class inherits class members of the base class. You can refer to inherited members (base class members) as if they were members of the derived class. For example:

class Base {public: int a,b;};class Derived : public Base {public: int c;};

int main() { Derived d; d.a = 1; // Base::a d.b = 2; // Base::b d.c = 3; // Derived::c}

The derived class can also add new class members and redefine existing base class members. In the above example, the two inherited members, a and b, of the derived class d, in addition to the derived class member c, are assigned values. If you redefine base class members in the derived class, you can still refer to the base class members by using the :: (scope resolution) operator. For example:

August 2010




#include <iostream>using namespace std;

class Base {public: char* name; void display() { cout << name << endl; }};

class Derived: public Base {public: char* name; void display() { cout << name << ", " << Base::name << endl; }};

int main() { Derived d; d.name = "Derived Class"; d.Base::name = "Base Class";

// call Derived::display() d.display();

// call Base::display() d.Base::display();}

The following is the output of the above example:

Derived Class, Base ClassBase Class

You can manipulate a derived class object as if it were a base class object. You can use a pointer or a reference to a derived class object in place of a pointer or reference to its base class. For example, you can pass a pointer or reference to a derived class object D to a function expecting a pointer or reference to the base class of D. You do not need to use an explicit cast to achieve this; a standard conversion is performed. You can implicitly convert a

August 2010




pointer to a derived class to point to an accessible unambiguous base class. You can also implicitly convert a reference to a derived class to a reference to a base class.

The following example demonstrates a standard conversion from a pointer to a derived class to a pointer to a base class:


class Base {public: char* name; void display() { cout << name << endl; }};

class Derived: public Base {public: char* name; void display() { cout << name << ", " << Base::name << endl; }};

int main() { Derived d; d.name = "Derived Class"; d.Base::name = "Base Class";

Derived* dptr = &d;

// standard conversion from Derived* to Base* Base* bptr = dptr;

// call Base::display() bptr->display();

}

The following is the output of the above example:

August 2010




Base Class

The statement Base* bptr = dptr converts a pointer of type Derived to a pointer of type Base.

The reverse case is not allowed. You cannot implicitly convert a pointer or a reference to a base class object to a pointer or reference to a derived class. For example, the compiler will not allow the following code if the classes Base and Class are defined as in the above example:

int main() { Base b; b.name = "Base class";

Derived* dptr = &b;}

The compiler will not allow the statement Derived* dptr = &b because the statement is trying to implicitly convert a pointer of type Base to a pointer of type Derived.

If a member of a derived class and a member of a base class have the same name, the base class member is hidden in the derived class. If a member of a derived class has the same name as a base class, the base class name is hidden in the derived class.

Inheritance

Inheritance is a mechanism of reusing and extending existing classes without modifying them, thus producing hierarchical relationships between them.

Inheritance is almost like embedding an object into a class. Suppose that you declare an object x of class A in the class definition of B. As a result, class B will have access to all the public data members and member functions of class A. However, in class B, you have to access the data members and member functions of class A through object x. The following example demonstrates this:


class A { int data;public: void f(int arg) { data = arg; } int g() { return data; }};

August 2010




class B {public: A x;};

int main() { B obj; obj.x.f(20); cout << obj.x.g() << endl;// cout << obj.g() << endl;}In the main function, object obj accesses function A::f() through its data member B::x with the statement obj.x.f(20). Object obj accesses A::g() in a similar manner with the statement obj.x.g(). The compiler would not allow the statement obj.g() because g() is a member function of class A, not class B.

The inheritance mechanism lets you use a statement like obj.g() in the above example. In order for that statement to be legal, g() must be a member function of class B.

Inheritance lets you include the names and definitions of another class's members as part of a new class. The class whose members you want to include in your new class is called a base class. Your new class is derived from the base class. The new class contains a sub object of the type of the base class. The following example is the same as the previous example except it uses the inheritance mechanism to give class B access to the members of class A:


class A { int data;public: void f(int arg) { data = arg; } int g() { return data; }};

class B : public A { };

int main() { B obj; obj.f(20); cout << obj.g() << endl;

August 2010




}Class A is a base class of class B. The names and definitions of the members of class A are included in the definition of class B; class B inherits the members of class A. Class B is derived from class A. Class B contains a subobject of type A.

You can also add new data members and member functions to the derived class. You can modify the implementation of existing member functions or data by overriding base class member functions or data in the newly derived class.

You may derive classes from other derived classes, thereby creating another level of inheritance. The following example demonstrates this:

struct A { };struct B : A { };struct C : B { };

Class B is a derived class of A, but is also a base class of C. The number of levels of inheritance is only limited by resources.

Multiple inheritance allows you to create a derived class that inherits properties from more than one base class. Because a derived class inherits members from all its base classes, ambiguities can result. For example, if two base classes have a member with the same name, the derived class cannot implicitly differentiate between the two members. Note that, when you are using multiple inheritance, the access to names of base classes may be ambiguous.

A direct base class is a base class that appears directly as a base specifier in the declaration of its derived class.

An indirect base class is a base class that does not appear directly in the declaration of the derived class but is available to the derived class through one of its base classes. For a given class, all base classes that are not direct base classes are indirect base classes. The following example demonstrates direct and indirect base classes:

class A { public: int x;};class B : public A { public: int y;};class C : public B { };

August 2010




Class B is a direct base class of C. Class A is a direct base class of B. Class A is an indirect base class of C. (Class C has x and y as its data members.)

Polymorphic functions are functions that can be applied to objects of more than one type. In C++, polymorphic functions are implemented in two ways:

•Overloaded functions are statically bound at compile time.•C++ provides virtual functions. A virtual function is a function that can be called for a number of different user-defined types that are related through derivation. Virtual functions are bound dynamically at run time

6. Describe the Friend functions and friend classes with programming examples.

Ans.Friend function

When a data is declared as private inside a class, then it is not accessible from outside the class. A function that is not a member or an external class will not be able to access the private data. A programmer may have a situation where he or she would need to access private data from non-member functions and external classes. For handling such cases, the concept of Friend functions is a useful tool.

A friend function is used for accessing the non-public members of a class. A class can allow non-member functions and other classes to access its own private data, by making them friends. Thus, a friend function is an ordinary function or a member of another class.

The friend function is written as any other normal function, except the function declaration of these functions is preceded with the keyword friend. The friend function must have the class to which it is declared as friend passed to it in argument.

Some important points to note while using friend functions in C++:

The keyword friend is placed only in the function declaration of the friend function and not in the function definition.

It is possible to declare a function as friend in any number of classes.

When a class is declared as a friend, the friend class has access to the private data of the class that made this a friend.

August 2010




A friend function, even though it is not a member function, would have the rights to access the private members of the class.

It is possible to declare the friend function as either private or public.

The function can be invoked without the use of an object. The friend function has its argument as objects, seen in example below.

#include class exforsys{private:int a,b;public:void test(){a=100;b=200;}friend int compute(exforsys e1)

//Friend Function Declaration with keyword friend and with the object of class exforsys to which it is friend passed to it};

int compute(exforsys e1){//Friend Function Definition which has access to private datareturn int(e1.a+e2.b)-5;}

main(){exforsys e;e.test();cout<<"The result is:"< //Calling of Friend Function with object as argument.}

The output of the above program is

The result is:295

August 2010




The function compute() is a non-member function of the class exforsys. In order to make this function have access to the private data a and b of class exforsys , it is created as a friend function for the class exforsys. As a first step, the function compute() is declared as friend in the class exforsys as:

friend int compute (exforsys e1)

Friend Class

C++ provides the friend keyword to do just this. Inside a class, you can indicate that other classes (or simply functions) will have direct access to protected and private members of the class. When granting access to a class, you must specify that the access is granted for a class using the class keyword: friend class aClass;

Note that friend declarations can go in either the public, private, or protected section of a class--it doesn't matter where they appear. In particular, specifying a friend in the section marked protected doesn't prevent the friend from also accessing private fields. Here is a more concrete example of declaring a friend:

class Node { private: int data; int key; // ...

friend class BinaryTree; // class BinaryTree can now access data directly};

Now, Node does not need to provide any means of accessing the data stored in the tree. The BinaryTree class that will use the data is the only class that will ever need access to the data or key. (The BinaryTree class needs to use the key to order the tree, and it will be the gateway through which other classes can access data stored in any particular node.) Now in the BinaryTree class, you can treat the key and data fields as though they were public:

class BinaryTree{ private: Node *root;

int find(int key);

August 2010




};int BinaryTree::find(int key){ // check root for NULL... if(root->key == key) { // no need to go through an accessor function return root->data; } // perform rest of find}

7. Illustrate with suitable examples various file handling methods in C++.

Ans.

Opening a File – Different Methods

So far we have seen just one way to open a file, either for reading, either for writing. But it can be opened another way too. So far, you should be aware of this method:

ifstream OpenFile(“cpp-home.txt”);

Well, this is not the only way. As mentioned before, the above code creates an object from class ifstream, and passes the name of the file to be opened to its constructor. But in fact, there are several overloaded constructors, which can take more than one parameter. Also, there is function open() that can do the same job. Here is an example of the above code, but using the open() function:

ifstream OpenFile;

OpenFile.open(“cpp-home.txt”);

Other use of open() is for example if you open a file, then close it, and using the same file handle open another file. This way, you will need the open() function.

Consider the following code example:

#include <fstream.h>void read(ifstream &T) { //pass the file stream to the function//the method to read a file

August 2010




char ch;while(!T.eof()) {T.get(ch);cout << ch;}cout << endl << "——–" << endl;}void main() {ifstream T("file1.txt");read(T);T.close();T.open("file2.txt");read(T);T.close();}

So, as long as file1.txt and file2.txt exists and has some text into, you will see it.

ifstream OpenFile(char *filename, int open_mode);

You should know that filename is the name of the file (a string). What is new here is the open_mode. The value of open_mode defines how to a file can be opened. Here is a table of the open modes:

ios::in Open file to read

ios::out Open file to write

ios::app All the date you write, is put at the end of the file. It calls ios::out

ios::ate All the date you write, is put at the end of the file. It does not call ios::out

ios::trunc Deletes all previous content in the file. (empties the file)

ios::nocreate If the file does not exists, opening it with the open() function gets impossible.

ios::noreplace If the file exists, trying to open it with the open() function, returns an error.

ios::binary Opens the file in binary mode.

August 2010




All these values are int constants from an enumerated type. But for making your life easier, you can use them as you see them in the table. Here is an example on how to use the open modes:

#include <fstream.h>void main() {ofstream SaveFile("file1.txt", ios::ate);SaveFile << "That’s new!n";SaveFile.close();}

As you see in the table, using ios::ate will write at the end of the file. If it wasn’t used, the file would have been overwritten. So, if file1.txt has this text:

Hi! This is test from www.cpp-home.com!

Running the above code, will add “That’s new!” to it, so it will look this way:

Hi! This is test from www.cpp-home.com!That’s new!

If you want to set more than one open mode, just use the OR operator (|). This way:

ios::ate | ios::binary

Using different open modes helps make file handling an easy job. Having

the liberty to choose a combination of these, in a sane way, comes in very handy in using streams effectively, and to the requirements of the project.

Moving on to something more intriguing and important; we can create a file stream handle, which you can use to read/write file, in the same time. Here is how it works:

fstream File(“cpp-home.txt”, ios::in | ios::out);

In fact, that is only the declaration. The code line above creates a file stream handle, named File. As you know, this is an object from class fstream. When using fstream, you should specify ios::in and ios::out as open modes. This way, you can read from the file, and write in it, in the same time, without creating new file handles. Well, of course, you can only read or write. Here is the code example:

#include <fstream.h>void main() {fstream File("test.txt", ios::in | ios::out);

August 2010




File << "Hi!"; //put “Hi!” in the filestatic char str[10]; //when using static, the array is automatically//initialized, and very cell NULLedFile.seekg(ios::beg); //get back to the beginning of the file//this function is explained a bit laterFile >> str;cout << str << endl;File.close(); }

Let us now understand the above program:

fstream File(“test.txt”, ios::in | ios::out);

This line, creates an object from class fstream. At the time of execution, the program opens the file test.txt in read/write mode. This means, that you can read from the file, and put data into it, at the same time.

File << “Hi!”;

I am sure the reader is aware of this and hence the explanation for this statement is redundant.

static char str[10];

This makes a char array with 10 cells. The word static initializes the array when at the time of creation.

File.seekg(ios::beg);

To understand this statement, let us go back and recollect some basics. We have seen this before:

while(!OpenFile.eof()) {OpenFile.get(ch);cout << ch;}

This is a while loop, that will loop until you reach the end of the file. But how does the loop know if the end of the file is reached? The answer is; when you read the file, there is something like an inside-pointer (current reading/writing position) that shows where you are up to, with the reading (and writing, too). Every time you call OpenFile.get(ch) it returns the

August 2010




current character to the ch variable, and moves the inside-pointer one character after that, so that the next time this function is called, it will return the next character. And this repeats, until you reach the end of the file.

Going back to the code line; the function seekg() will put the inside-pointer to a specific place (specified by you). One can use:

· ios::beg – to put it in the beginning of the file

· ios::end – to put it at the end of the file

Or you can also set the number of characters to go back or after. For example, if you want to go 5 characters back, you should write:

File.seekg(-5);

If you want to go 40 characters after, just write:

File.seekg(40); It is imperative to mention that the seekg() function is overloaded, and it can take two parameters, too. The other version is this one:

File.seekg(-5, ios::end);

In this example, you will be able to read the last 4 characters of the text, because:

· You go to the end (ios::end)

· You go 5 characters before the end (-5)

Why you will read 4 but not 5 characters? One character is lost, because the last thing in the file is neither a character nor white space. It is just position (i.e., end of file).

Why this function was used the program above. After putting “Hi!” in the file, the inside-pointer was set after it, i.e., at the end of the file. And as we want to read the file, there is nothing that can be read at the end. Hence, we have to put the inside-pointer at the beginning. And that is exactly what this function does.

File >> str;

I believe this line reminds us of cin >>. In fact, it has much to do with it. This line reads one word from the file, and puts it into the specified array. For example, if the file has this text:

August 2010




Hi! Do you know me?

Using File >> str, will put just “Hi!” to the str array. And, as what we put in the file was “Hi!” we don’t need to do a while loop, that takes more time to code. That’s why this technique was used. By the way, in the while loop for reading, that was used so far, the program reads the file, char by char. But you can read it word by word, this way:

char str[30]; //the word can’t be more than 30 characters longwhile(!OpenFile.eof()){OpenFile >> str;cout << str;}

You can also read it line by line, this way:char line[100]; //a whole line will be stored herewhile(!OpenFile.eof()) {OpenFile.getline(line,100); //where 100 is the size of the arraycout << line << endl;

}

8. Explain the concept of class templates in C++ with some real time programming examples.

Ans.A class template definition looks like a regular class definition, except it is prefixed by the keyword template. For example, here is the definition of a class template for a Stack.template <class T>class Stack { public: Stack(int = 10); ~Stack() { delete [] stackPtr ; } int push(const T&); int pop(T&); int isEmpty()const { return top == -1; } int isFull() const { return top == size – 1; } private: int size; // number of elements on Stack. int top; T* stackPtr;};

August 2010




T is a type parameter and it can be any type. For example, Stack<Token>, where Token is a user defined class. T does not have to be a class type as implied by the keyword class. For example, Stack<int> and Stack<Message*> are valid instantiations, even though int and Message* are not "classes".

Implementing Class Template Member Functions

Implementing template member functions is somewhat different compared to the regular class member functions. The declarations and definitions of the class template member functions should all be in the same header file. The declarations and definitions need to be in the same header file.

Consider the following:

//B.Htemplate <class t>class b { public: b(); ~b();};

//B.CPP#include "B.H"template <class t>b<t>::b() {}template <class t>b<t>::~b() {}

//MAIN.CPP#include "B.H"void main() { b<int> bi; b <float> bf;}

When compiling B.cpp, the compiler has both the declarations and the definitions available. At this point the compiler does not need to generate any definitions for template classes, since there are no instantiations. When the compiler compiles main.cpp, there are two instantiations: template class B<int> and B<float>. At this point the compiler has the declarations but no definitions.

While implementing class template member functions, the definitions are prefixed by the keyword template. Here is the complete implementation of class template Stack:

//stack.h#pragma oncetemplate <class T>class Stack { public: Stack(int = 10); ~Stack() { delete [] stackPtr; } int push(const T&); int pop(T&); // pop an element off the stack int isEmpty()const { return top == -1; }

August 2010




int isFull() const { return top == size – 1; } private: int size; // Number of elements on Stack int top; T* stackPtr;};//constructor with the default size 10template <class T>Stack<T>::Stack(int s) { size = s > 0 && s < 1000 ? s : 10; top = -1; // initialize stack stackPtr = new T[size];}// push an element onto the Stack template <class T>int Stack<T>::push(const T& item) { if (!isFull()) { stackPtr[++top] = item; return 1; // push successful } return 0; // push unsuccessful}// pop an element off the Stacktemplate <class T> int Stack<T>::pop(T& popValue) { if (!isEmpty()) { popValue = stackPtr[top--]; return 1; // pop successful } return 0; // pop unsuccessful}

Using a class template

Using a class template is easy. Create the required classes by plugging in the actual type for the type parameters. This process is commonly known as "Instantiating a class". Here is a sample driver class that uses the Stack class template.

#include <iostream>#include "stack.h"

August 2010




using namespace std;void main() { typedef Stack<float> FloatStack; typedef Stack<int> IntStack; FloatStack fs(5); float f = 1.1; cout << "Pushing elements onto fs" << endl; while (fs.push(f)) { cout << f << ‘ ‘; f += 1.1; } cout << endl << "Stack Full." << endl << endl << "Popping elements from fs" << endl; while (fs.pop(f)) cout << f << ‘ ‘; cout << endl << "Stack Empty" << endl; cout << endl; IntStack is; int i = 1.1; cout << "Pushing elements onto is" << endl; while (is.push(i)) { cout << i << ‘ ‘; i += 1; } cout << endl << "Stack Full" << endl << endl << "Popping elements from is" << endl; while (is.pop(i)) cout << i << ‘ ‘; cout << endl << "Stack Empty" << endl;}

Output:

Pushing elements onto fs1.1 2.2 3.3 4.4 5.5 Stack Full.Popping elements from fs5.5 4.4 3.3 2.2 1.1 Stack EmptyPushing elements onto is

August 2010




1 2 3 4 5 6 7 8 9 10 Stack FullPopping elements from is10 9 8 7 6 5 4 3 2 1

Stack Empty

In the above example we defined a class template Stack. In the driver program we instantiated a Stack of float (FloatStack) and a Stack of int(IntStack). Once the template classes are instantiated you can instantiate objects of that type (for example, fs and is.)

There are two advantages:

· typedef’s are very useful when "templates of templates" come into usage. For example, when instantiating an STL vector of int’s, you could use:

typedef vector<int, allocator<int> > INTVECTOR;

· If the template definition changes, simply change the typedef definition. For example, currently the definition of template class vector requires a second parameter.

typedef vector<int, allocator<int> > INTVECTOR; INTVECTOR vi1;In a future version, the second parameter may not be required, for example, typedef vector<int> INTVECTOR; INTVECTOR vi1;

Smu - Mca - Mc0066 Assign 1

Documents

Transcript of Smu - Mca - Mc0066 Assign 1