Pointers and Functions

1. Pointers are very much used in function and declaration.
2. Sometimes with the use of pointer , a complex function can be easily represented and accessed.
3. The use of pointers in a function definition may be classified into two groups.
     Call by Value
     Call by Reference

Call By Value:-

 1.In a call by value , whenever a portion of program invokes a function with a formal arguments , then control is transferred from main function to calling function and value of actual argument is copied to argument of function.
 2. While calling a function , when copy of formal argument is sent to arguments of the called function then it is known as call by value.
 3. In a call by value, when a control is transferred back from called function to the calling function of the program then altered values are not transferred back.

WAP for implementation of 'call by value' concept.

#include<conio.h>
#Include<iostream.h>
void main( )
{
int x,y;
clrscr( );
x = 12;
y = 13;
cout<<"\n\n\tValue of x--->"<<x<<"\t"<<"Value of y--->"<<y;
funct(x,y);
cout<<"\n\n\tValue of x--->"<<x<<"\t"<<"Value of y--->"<<y;
getch( ); 
}
void funct(int a, int b)
{
a = a*a;
b = b*b;
}

Output:-
Value of x--->12     Value of y--->13
Value of x--->12     Value of y--->13


Call By Reference:-

 1. When a function is called by a portion of a program , the address of actual arguments are copied on to formal arguments though they may be referred by different variable name.
 2. While calling a function, when the address or reference of actual arguments are sent to the called function then it is known as call by reference.
 3. The content of that address can be accessed freely either within a function or within a calling routine.
 4. In a call by reference when the address of actual arguments are sent then these address are stored within the pointer arguments of called function.
 5. Thus the use of pointers as function arguments permits the corresponding data item to be altered globally from within the function.
 6. Any change that is made to the data item will be recognized in both functions and calling portion of the program.

WAP to implement the concept of call by reference.

#include<conio.h>
#include<iostream.h>
void main( )
{
int x,y;
clrscr( );
x = 12;
y = 13;
cout<<"\n\n\tValue of x--->"<<x<<"\t"<<"Value of y--->"<<y;
funct(&x,&y);
cout<<"\n\n\tValue of x--->"<<x<<"\t"<<"Value of y--->"<<y;
getch( ); 
}
void funct(int *p1, int *p2)
{
*p1 = *p1 **p1;
*p2 = *p2**p2;
}

Output:-
Value of x is--->12           Value of y is--->13
Value of x is--->144          Value of y is--->169

WAP to exchange the values of two variable using call by value.

#include<conio.h>
#Include<iostream.h>
void exchange(int a, int b)
{
int temp;
temp=a;
a=b;
b=temp;
cout<<"\n\n\tAfter Exchange Value of A is--->"<<a<<"\tValue of B is--->"<<b;
}
void main( )
{
int a=12,b=14;
clrscr( );
cout<<"\n\n\tBefore Exchange Value of A is--->"<<a<<"\tValue of B is--->"<<b;
exchange(a,b);
getch( );
}

Output:-

 Before Exchange Value of A is--->12       Value of B is--->14
 After Exchange Value of A is--->14          Value of B is--->12

WAP to exchange the value of two variables using call by reference

#include<conio.h>
#include<iostream.h>
void exchange(int *a, int *b)
{
int *temp;
*temp=*a;
*a=*b;
*b=*temp;
}
void main( );
{
int a=12,b=14;
clrscr( );
cout<<"\n\n\tBefore Exchange Value of A is--->"<<a<<"\tValue of B is--->"<<b;
exchange(&a,&b); 
cout<<"\n\n\tAfter Exchange Value of A is--->"<<a<<"\tValue of B is--->"<<b;
getch( );
} 

Output:-
 Before Exchange Value of A is--->12       Value of B is--->14
 After Exchange Value of A is--->14          Value of B is--->12

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Pointers

A pointer is a variable which holds the memory address of another variable.

Advantage of pointer :
 1. It supports dynamic allocation and de allocation of memory segments.
 2. It allows creation of data structure such as linked list, stacks, queues, trees and graphs.
 3. With the help of pointer , variable can be swapped without physically moving them.
 4. It allows to pass variable , arrays , function , strings an structure as a function argument.
 5. Pointer allows to return structure variable from functions.

A pointer variable of two parts:
 1. The Pointer operator
 2. The Address operator

Pointer Operator :
 1. A pointer variable can be represented by a combination of *(Asterisk) with a variable.
 2. If a variable of integer data type is declared with *(Asterisk) then if means the variable is of type 'pointer to integer'.
 3. int *ptr
     When ptr is a pointer variable which holds the address of an integer data type variable.
 4. All pointer variable must be declared before it is used in a program like any other variable.
 5. When a pointer variable is declared , the variable name must be preceded by an asterisk(*).
 6 The general format of declaration is
     Datatype *pointervariable;
     Ex : char *cptr;
            float *fptr;
            double *dptr;

The Address Operator :
 1. An address operator can be represented by the combination by the combination of '&'(ampersand) with the variable.
 2. The '&' is a unary operator the returns the memory address of its operand.
 3. ptr = &x;
      The meaning of this statement is 'ptr' is a pointer variable which receives the address of a variable 'x'.


WAP to display the content of a pointer

#include<conio.h>
#include<iostream.h>
void main( )
{
 int x;
int *ptr;
clrscr( );
cout<<"\n\tEnter Value of x";
cin>>x;
ptr=&x;
cout<<"\n\ntx--->"<<x"\t"<<"ptr is--->"<<ptr;
cout<<"\n\n\tx--->"<<x"\t"<<"*ptr is--->"<<*ptr;
getch( );
}

Output :-
  Enter Value of x15
 x--->15  ptr is--->0⤬8fc1fff4
 x--->  *ptr is--->15

WAP to assign the pointer variable to another pointer and display the content of both the pointer variable.

#include<conio.h>
#include<iostream.h>
void main( )
{
int x=12;
int *ptr1, *ptr2;
clrscr( );
ptr1=&x;
ptr2=ptr1;
cout<<"\n\n\tValue of x is--->"<<x;
cout<<"\n\n\tContent at ptr1 is--->"<<*ptr1;
cout<<"\n\n\tContent at ptr2 is--->"<<*ptr2;
getch( );
}

Output :-
 Value of x is--->12
 Content at ptr1 is--->12
 Content at ptr2 is--->12

WAP to display the content of the pointer variable using arithmetic operation.

#include<conio.h>
#include<iostream.h>
void main( )
{
int x,y;
int *ptr;
x=12;
ptr=&&x;
cout<<"\n\n\tValue of x is--->"<<x<<"\t"<<"Value at pointer is--->"<<*ptr;
y=*ptr+1;
cout<<"\n\n\tValue of y is--->"<<y<<"\t"<<"Value at pointer is--->"<<*ptr;
getch( );
}

Output :-
 Value of x is--->12                  Value at pointer is--->12
 Value of y is--->13                  Value at pointer is--->12

WAp to display the memory address of a variable using pointer before incrementation and decrementation and after incrementation and decrementation.

#include<conio.h>
#include<iostream.h>
void main( )
{
int x=12;
int *ptr1, *ptr2;
clrscr( );
ptr1=&x;
ptr2=&x;
cout<<"\n\n\tMemory Address before incrementation--->"<<ptr1<<endl;
cout<<"\n\n\tMemory Address before decrementation--->"<<ptr1<<endl;
ptr1++;
ptr2--;
cout<<"\n\n\tMemory Address after  incrementation--->"<<ptr1<<endl;
cout<<"\n\n\tMemory Address after decrementation--->"<<ptr1<<endl;
getch( );
}

Output :-
   Memory Address before incrementation--->0⤫8fd2fff4
   Memory Address before decrementation--->0⤫8fd2fff4
   Memory Address after incrementation--->0⤫8fd2fff6
   Memory Address before incrementation--->0⤫8fd2fff2

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Arrays

The C language provides a capability that enables the user to define a set of ordered data items as an array.

Suppose we had a set of grades that we wished to read into the computer and suppose we wished to perform some operations on these grades, we will quickly realize that we cannot perform such an operation until each and every grade has been entered since it would be quite a tedious task to declare each and every student grade as a variable especially since there may be a very large number.
    In C we can define variable called grades, which represents not a single value of grade but a entire set of grades. Each element of the set can then be referenced by means of a member called as index number of subscript.

Declaration of Arrays
  Like any other variable arrays must be declared before they are used. The general form of declaration is :
   type variables-name[50];
 The type specifies the type of the elements that will be contained in the arrays, such as int, float or char and the size indicates the maximum number of elements that cane stored inside the array. For example :
   float height[50];
 Declares the height to be an array containing 50 real elements. Any subscripts  0 to 49 are valid. In C the array elements index or subscript begins with number zero.
  So height [0] refers to the first element of the array. (For this reason, it is easier to think of it as referring to element number zero, rather than as referring to the first element).


Initialization of Arrays
  We can initialize the elements in the array in the same way as the ordinary variables when they are declared. The general form of initialization of arrays is :
   type array_name[size] = {list of values};
The values in the list care separated by commas, for example the statement
  int number[3] = {0,0,0};
Will declare the array size as a array of size 3 and will assign zero to each element.
   How to access the array element can be understood by the following program :
/* Program to count the number of positive and negative numbers*/

#include<stdio.h>
void main( )
{
int a[50], n, count_neg=0, count_pos=0,I;
printf("Enter the size of the arrays\n");
scanf("%d",&n);
printf("Enter the elements of the array\n");
for(I=0;I<n;I++)
scanf("%d",&a[I]);
for(I=0;I<n;I++)
{
if(a[I] < 0)
count_neg++;
else
count_pos++;
}
printf("There are %d negative numbers in the array\n",count_neg);
printf("There are %d positive numbers in the array\n",count_pos);
}

Multidimensional Arrays
  Often there is a need to store and manipulate two dimensional data structure such as matrices and tables. Here the array has two subscripts. One subscripts denotes the row and the other the column.
    The declaration of two dimension arrays is as follows :
       data_type array_name[row_size][column_size];
        int m[10][20];
Here m is declared as a matrix having 10 rows (numbered from 0 to 9) and 20 columns (numbered 0 through 19). The first element of the matrix is m[0][0] and the last row last column is m[9][19].


Elements of Multidimension Arrays
  A 2 dimensional array marks[4][3] is shown below. The first element is given by marks[0][0] contains 35.5 and second element is marks[0][1] and contains 40.5 and so on.

Initialization of Multidimensional Arrays

 Like the one dimension arrays, 2 dimension arrays may be initialized by following their declaration with a list of initial values enclosed in braces.

Example :
   int table[2][3] = {0,0,01,1,1};
Initializes the elements of first row to zero and second row to 1. The initialization is done row by row. The above statement can be equivalently written as
  int table[2][3] = { {0,0,0},{1,1,1}};
/* Example : Program to add two matrices & store the results in the  3rd matrix */

#include<stdio.h>
#include<conio.h>
void main( )
{
int a[10][10], b[10][10], c[10][10], i, j, m, n, p, q;
clrscr( )
{
printf("enter the order of the matrix\n");
scanf("%d%d",&p, &q);
if(m= =p && n = = q)
{
printf("Matrix can be added\n");
printf("enter the elements of the matrix a");
for(i=0;i<m;i++)
for(j=0;j<n;j++)
scanf("%d",&a[i][j]);
printf("enter the elements of the matrix b");
for(i=0;i<m;i++)
for(j=0;j<n;j++)
c[i][j]=a[i][j]+b[i][j];
printf("The matrix c is ");
 for(i=0;i<m;i++)
{
 for(j=0;j<n;j++)
printf("%d"\t,c[i][j]);
printf("\n");
}
}
getch( );
}

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Union

Unions like structure contain members whose individual data types may differ from one another. However the members that compose a union all share the same storage area within the computers memory where as each member within a structure is assigned its own unique storage area. Thus unions are used to observe memory. They are useful for application involving multiple members. Where values need not be assigned to all the members at any one time. Like structure union can be declared using the keyword union as follows :
  union item
  {
   int m;
   float p;
   char c;
   }
   code;

This declares a variable code to type union item. The union contains three members each with a different data type. However we can use only one of them at a time. This is because if only one location is allocated for union variable irrespective of time. The compiler allocates a piece of storage that is large enough to access a union member we can use the same syntax that we use to access structure members. That is-
 code.m
 code.p
 code.c
are all valid member variables.

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Structure

Arrays are used to store large set of data and manipulate them but the disadvantage is that all the elements stored in an array are to be of the same data type. If we need to use a collection of different data type items it is not possible using an array. When we require using a collection of different data item of different data types we can use a structure. Structure is a method of packing data of different types. A structure is a convenient method of handling a group of related data items of different data types.
 structure definition:
 struct tag_name
 {
 data type member 1;
 data type member 2;
 ..................;
 ...................;
 };

Example :
  struct lib_books
  {
  char title[20];
  char author[15];
  int pages;
  float price;
  };

The keyword struct declares a structure to hold the details of four fields namely title, author pages and price. These are members of the structures.Each member may belong to different or same data type. The tag name can be used to define objects that have the tag names structure. The structure we just declared is not a variable by itself but a template for the structure.
         We can declare structure variables using the tag name anywhere in the program.

For example the statement,
  struct lib_books book1,book2,book3;
  declares book1,book2,book3 as variables of type struct lib_books each declaration has four elements of the structure lib_books.The complete structure declaration might look like this

struct lib_books
{
char title[20];
char author[15];
int pages;
float price;
};
struct lib_books, book1, book2, book3;

Giving values to members
 As mentioned earlier the members themselves are not variables they should be linked to structure variable in order to make them meaningful members. The link between a member and a variable is established using the member operator'.' which is known as dot operator or period operator.
   For example :
    Book!.price
Is the variable representing the price of book1 and can be treated like any other ordinary variable. We can use scanf statement to assign values like

scanf("%s", book1.file);
scanf("%d",&book1.pages);
    OR
we can assign variables to the members of book1
strcpy(book1.title, "basic");
strcpy(book1.author, "Irawen");
book1.pages = 250;
book1.price = 28.50;

 

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The case Control Structure

Switch Statement
 The switch statement cause a particular group of statements to be chosen from several available groups. The selection is based upon the current value of an expression that is include within the switch statement.

The form of switch statement is
 switch (integer expression)
 {
  case constant 1:
  do this;
  case constant 2 :
  do this;
  case constant 3 :
  do this;
  default:
  do this;
  }

Example:-
int main ( )
{
int num = 2;
switch (num +2)
{ 
case 1:
printf("case 1 : Value is : %d", num);
case 2:
printf("Case 1: Value is : %d", num);
case 3 :
printf("Default : Value is : %d", num);
default :
printf("Default : Value is : %d", num);
}
return 0;
}

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The Break and Continue Statement

The Break Statement :-
   The keyword break allows us to jump out of a loop instantly without waiting to get back to the conditional test. When the keyword break is encountered inside any C loop, automatically passes to the first statement after the loop. For e.g. the following program is to determine whether a number is prime or not

 Logic : To test a number is prime or not, is to divide it successively by all numbers from 2 to one less than itself. If the remainder of any of the divisions is zero, the number is not a prime.

Following program implements this logic
  main( )
  {
   int num i;
   printf("Enter a number");
   scanf("%d",&num);
   i = 2;
   while (i <= num-1)
   {
     if(num% i = = 0)
      {
        printf("Not a prime number");
        break;
      }
     i++;
   }
   if(i == num)
   printf("Prime number");
 }

The Continue Statement:-
   The keyword continue allows us to take the control to the beginning of the loop bypassing the statements inside the loop which have not yet been executed. When the keyword continue is encountered inside any C loop control automatically passes to the beginning of the loop.

 For example
 main( )
 {
  int i,j;
  for(i=1; i<=2; i++)
  {
    for(j=1; j<=2; j++)
     {
       if(i = = j)
       continue;
       printf("%d%d", i,j);
      }
    }
  }

The output of the above program would be.........
 12
 21 

When the value of i equal to that of j, the continue statement takes the control to the for loop (inner) bypassing rest of the statement pending execution in the for loop (inner).

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Loop Control Structures

These are three methods by way of which we can repeat a part of a program in C programming.
1. Using a for Statement
2. Using a while Statement
3. Using a do-while Statement

The While Loop (pre-test Loop)
   The general form of while is as shown below :
     initialize loop counter ;
     while (test loop counter using a condition)
     { 
         do this;
         and this;
         increment loop counter;
     }
The parentheses after the while contains so long as this condition remains true all statements within the body of the while loop keep getting executed repeatedly for e.g.
  /* calculate simple interest for 3 sets of p, n and r */
     main( )
     {
      int p, n, count;
      float r, si;
      count = 1;
      while(count <= 3)
       {
        printf("Enter values of p, n and r");
        scanf("%d%d%f", &p,&n,&r);
        si = p*n*r/100;
        printf("simple interest = Rs. %f" ,si);
        count = count +1;
       }
     }

The while construct consists of a block of code and condition. The condition is evaluated, and if the condition is true, the code within the block is executed. This repeats until the condition becomes false. Because while loops check the condition before the block is executed, the control structure is often also known as a pre-test loop. Compare with the do while loop, which tests the condition after the loop has executed.
 For example, in the C programming language (as well as Java  and C++, which use the same syntax in this case), the code fragment
  x = 0;
  while <x < 5)
     {
         printf("x = %d\n", x);
         x++
     }
first checks whether x is less than 5, which it is, so then the {loop body} is entered, where the printf function is run and x is incremented by 1. After completing all the statements in the loop body, the condition, (x < 5), is checks again, and the loop is executed again, the process repeating until the variable x has the value 5.
  Note that it is possible, and in some cases desirable, for the condition to always evaluate to true, creating an infinite loop. When such a loop is created intentionally, there is usually another control structure (such as a break statement) that controls termination of the loop.


The do-while Loop (Post-test loop)
  The do-while loop takes the following form
   do
   {
     this;
     and this;
     and this;
     and this;
   } while ( this condition is true);

There is a minor difference between the working of while and do-while loops. The difference is the place where the condition is tested. The while tests the condition before executing any of the statements within the while loop. As against this the do-while tests the condition after having executed the statements within the loop. 
        
                           The do while construct consists of a block of code and a condition. First, the code within the block is executed, and then the condition is evaluated. If the condition is true the code within the block is executed again. This repeats until the condition becomes false. Because do while loops check the condition after the block is executed, the control structure is often also known as a post-test loop. Contrast with the while loop, which tests the condition before the code within the block is executed
   
         It is possible, and in some cases desirable, for the condition to always evaluate to true, creating an infinite loop. When such a loop is created intentionally, there is usually another control structure(such as a break statement) that allows termination of the loop.
            Some languages may use a different naming convention for this type of loop. For example, the Pascal language has a "repeat until" loop, which continues to run until the control expression is true (and then terminates) - whereas a "do-while" loop runs while the control expression is true (and terminates once the expression becomes false).

The for loop
  The general form of for statement is as under :
    for(initialize counter; test counter, increment counter)
    {
      do this;
      and this;
      and this;
    }

Now let us write the simple interest problem using the for loop .

 /* Calculate simple interest for 3 sets of p, n and r */
  main( )
  {
    int p,n,count;
    float r, si;
    for(count=1; count <=3; count = count + 1)
     { 
       printf("Enter values of p, n and r");
       scanf("%d%d%f",&p, &n, &r);
       si = p*n*r / 100;
       printf("Simple interest = Rs. %f" , si);
      }
    }
  

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Control Structure in C

The decision control structure in C can be implemented using
 1. The if Statement 
 2. The if - else Statement
 3. The Nested if - else Statement

The if Statement

  The general form of it statement looks like this :
   
     if(this condition is true)
            execute this statement;

The if statement by itself will execute a single statement or a group of statement when the condition following if is true.
  
The simple example of a if statement is :
    if(varName = = 20)
      printf("Value of the variable is 20");

We can use the block to specify the statement to pre executed if the given condition is true.
 if(varName = = 20)
 {
    printf("Value of the variable is 20");
    printf("Print what ever you want !!!");
 }

The if - else Statement

  The if statement by itself will execute a single statement or a group of statements when the condition following if is true. It does nothing when the condition is false. If the condition is false then a group of statements can be executed using else statement.
The following program illustrates this
 /* Calculation of gross salary */
   main( )
   {
    float bs, gs, da, hra;
    printf("Enter basic salary");
    scanf("%f", &bs);
    if(bs<1500)
    { 
          hra = bs * 10/100;
          da = bs * 90/100;
     }
    else
     {
           hra = 500;
           da = bs * 98/100;
      }
      gs = bs+hra+da;
      printf("gross salary = Rs. /.f" , gs);
     }

The Nested if - else Statement

   It we write an entire if - else construct within the body of the if statement or the body of an else statement. This is called nesting of if . For example

    if(condition)
    {
         if(condition)
          {
             do this;
          }
          else
          {
            do this;
            and this;
          }
         else
             do this;
    }

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Data Types in C Language

A programming language is proposed to help programmer to process certain kinds of data and to provide useful output. The task of data processing is accomplished by executing series of commands called program. A program usually contains different types of data types (integer, float, character etc.) and need to store the values being used in the program. C language is rich of data types. A C programmer has to employ proper data type as per his requirements.

   C has different data types for different types of data and can be broadly classified as:
      1. Primary data types
      2. Secondary data types

Primary data types consists following data types.  

Data Types in C

Integer type :-
  Integers are whole numbers with a range of values, range of values are machine dependent. Generally an integer occupies 2 bytes memory space and its value range limited to -32768 to +32768 (that is, -2 to the power 15 to + 2 to the power 15 -1). A signed integer use one bit for  storing sign and rest 15 bits for number.

 To control the range of numbers and storage space, C has three classes of integer storage namely short int, and long in. All three data types have signed and unsigned forms. A short int requires half the amount of storage than normal integer. Unlike signed integer, unsigned integers are always positive and use all the bits for the magnitude of the number. Therefore  the range of an unsigned will be  form 0 to 65535. The long integers are used to declare a longer range of value and it occupies 4 bytes of storage space.

Syntax : int <variable name>;like
int num 1;
short int num 2;
long int num 3;
Example : 5,6,100,2500

Integer Data Type Memory Allocation

 Floating Point Types :
  The float data type is used to store fractional numbers (real number) with 6 digits of precision. Floating point numbers are denoted by the keyword float. When the accuracy of the floating point number is insufficient, we can use the double to define the number. The double is same as float but with longer precision and takes double space (8 bytes) than float. To extend the precision further we can use long double which occupies 10 bytes of memory space.
Syntax : float <variable name>; like
float num 1;
double num 2;
long double num 3;
Example : 9.125, 3.1254

Floating Point Data Type Memory Allocation

 Character Type :
  Character type variable can hold a single character. As there are singed and unsigned int (either short or long), in the same way there are signed unsigned chars; both occupy 1 bytes each, but having different ranges. Unsigned character have values between 0 and 255, signed characters have values from -128 to 127.

Syntax : char <variable name>; like

char ch = 'a';

Example : a,b,g,S,j.

Void Type :
  The void type has no values therefore we cannot declare it as variable as we did in case of integer and float.
   The void data type is usually used with function to specify its type.Like in our first C program we declared we declared "main( )" as void type because it does not return any value. The concept of returning values will be discussed in detail in the C function hub.


Secondary Data Types 

    Array in C programming
          An array in C language is a collection of similar data-type, means an array can hold value of a particular data type for which it has been declared. Arrays can be created from any of the C data-types int,............. .

   Pointers in C programming
         In this blog we are going to discuss what pointer is and how to use them our C program. Many C programming learner thinks that pointer is one of the difficult topic in C language but its not......... .

  Structure in C programming
         We used variable in our C program to store value but one variable can store only single piece information (an integer can hold only one integer value) and to store similar type of values we had to declare.....

User defined type declaration
       C language supports a feature where user can define an identifier that characterizes an existing data type. This user defined data type identifiers can later be used to declare variables. In short its purpose is to redefine the name of an existing data type.

Syntax : typedef <type> <identifier>; like

typedef int number;
  
Now we can use number in lieu of int to declare integer variable. For example : "int x1" or "number x1" both statement declaring an integer variable. We have just changed the default keyword "int" to declare integer variable to "number".

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Variables

A variable is a value that can change any time. It is a memory location used to store a data value. A variable name should be care fully chosen by the programmer so that it use is reflected in a useful way in entire program. Variable names are case sensitive.
Example of variable names are
  sun
  Number
  Salary
  Emp_name
  Average1
  
Any variable declared in a program should confirm to the following-
1. They must always begin with a letter, although some systems permit underscore as first character.
2. The length of a variable must not be more than 8 characters.
3. White space is not allowed and
4. A variable should not be a keyword
5. It should not contain any special characters.

Example of invalid names are
123
(area)
6th
%abc

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Constants

A constant value is the one which does not change during the execution of a program. C supports several types of constants.
 1. Integer constants
 2. Real constants
 3. Single character constant
 4. String constants

Integer Constants
 An integer constant is a sequence of digits. There are 3 types of integers namely decimal integer, octal integers and hexadecimal integers.
  1. Decimal Integers consists of a set of digits 0 to 9 preceded by an optional + or -sign. Spaces, commas and non digit characters are not permitted between digits. Example for valid decimal integer constants are
123
-31
0
562321
+78

Some examples for invalid integer constant are
15 750
20,000
Rs. 1000


2. Octal Integer constant consists of any combination of digits from 0 through 7 with a O at the beginning.Some examples of octal integers are
O26
O
O347
O676

3. Hexadecimal Integer constants is preceded by OX or Ox, they may contain alphabets from A to F or a to f. The alphabets A to F refers to 10 to 15 in decimal digits. Example of valid hexadecimal integers are-
OX2
OX8C
OXbcd
Ox

Real Constants
   Real constants consists of a fractional part in their representation.Integer constant are inadequate to represent quantities that vary continuously. These quantities are represented by numbers containing fractional parts like 26.082.
Example of real constants are
0.0026
-0.97
435.29
+487.0
Real numbers can also be represented by exponential notation.

Single Character Constants
  A single Character constant represent a single character which is enclosed in a pair of quotation symbols.
Example for character constant are
'5'
'x'

String Constants
   A string constants is a set of character enclosed in double quotation marks. The character in a string constant sequence may be a alphabet, number, special character and blank space. Example of string constants are-
"IRAWEN"
"1234"
"God Bless"
"!....?"

Backslash Character Constant [Escape Sequence]
 Backslash character constants are special character used in output functions. Although they contain two characters they represent only one character.
For example "\n" is the new line character constant. and
    "\t" is to give a tab space for formatting.

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Character Set, Keywords and Identifiers

Character Set
  The character set in C language can be grouped into the following categories.
   1. Letters
   2. Digits
   3. Special characters
   4. White spaces

White spaces are ignored by the compiler until they are a part of string constant.White space may be used to separate words, but are strictly prohibited while using between characters of keywords or identifiers.

C Character-set table

Special Characters

Keywords and Identifiers
  Every word in C language is a keyword or an identifier. Keywords in C language cannot be used as a variable name. They are specifically used by the compiler for own purpose and they serve as building blocks of a C program.
  
The following are the keyword set of C language.

Some compilers may have additional keywords.
  
Identifiers refers to the name of user-defined variables, array and functions. A variables should be essentially a sequence of letters and or digits and the variable name should begin with a character.
  
  Both uppercase and lowercase letters letters are permitted. The underscore character is also permitted in identifiers.

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Friend Function

The private member function can not be accessed from outside the class. The non Member function can not access private data.

Sometimes there can be situation where we would like two classes to share a particular function.

In such a case, c++ allows the common function to be made friendly with both classes and so allows the function to have access to private data of the class.

Syntax:
  class abc
   {
     ........
     .........
     public:
     friend void xyz( );   //declaration
    };

The function definition does not use either keyword 'friend' or the scope resolution operator
  Definition void xyz ( )
                       {
                          .................
                          ..................
                         }

#include<iostream.h>
#include<conio.h>
class sample
{
int a;
int b;
public:
void setvalue( )
{
a=25;
b=40;
}
friend float mean(sample s);
};
float mean(sample s)
{
return float(s.a+s.b)/2.0;
}
void main( )
{
sample s1;
clrscr( );
s1.setvalue( );
mean(s1);
getch( );
}

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Array of Objects

Any object whether built in or user defined, can be stored in an array. When you declare the array, you tell the compiler the type of object to store and the number of objects for which to allocate room.

The compiler know how much room is needed for each object based on the class declaration.

Consider the following class definition :
class vehicle
{
int vcode;
char vname[30];
public:
void getinfo( );
void disinfo( );
};

The identifier vehicle is a user defined data type and can be used to create objects that relate to a different categories of vehicles.


Example:

vehicle v1[3];     vehicle v2[4];

The array contains three objects namely , v1[0] , v1[1], v1[2] of type vehicle class. Similarly, v2 contains 4 objects namely v2[0], v2[1], v[2], v[3].

Accessing member data in an array of objects is a two step process. You identify the member of the array by using the index operator([]) and then you add the member operator(.) to access the particular member variable.

For Example:
v[i] disinfo( );

will display the data of the ith element of the array v1. That is, this statement request the objects v[i] to invoke the member function disonfo( ). An array of object is stored inside the memory in same way as a multi-dimensional array.

The array v1 represented in figure below

Here only the space for data items of object is created. Member function are stored separately and will be used by all the objects.

Write a program to declare a class book having data members as a title and author. Accept the data for five books and display the accepted data.

#include<iostream.h>
#include<conio.h>
class book
{
char auth[20];
char title[50];
public:
void accept( );
void display( );
};
void book::accept( )
{
cout<<"\n\n\tEnter Title of the book";
cin>>title;
cout<<"\n\n\tEnter Author Name";
cin>>auth;
}
void book::display( )
{
cout<<"\n\n\tBook Name--->"<<title;
cout<<"\n\n\tAuthor is--->"<<auth;
}
void main( )
{
book b[5];
int i;
clrscr( );
for(i=0;i<5;i++)
{
 b[i].accept( );
}
for(i=0;i<5;i++)
{
 b[i].display( );
}
getch( );
}


Output:-
   Enter Title of the book c_basic
   Enter Author Name ABC
   Enter Title of the book cpp
   Enter Author Name DEF
   Enter Title of the book java
   Enter Author Name PQR
   Enter Title of the book sql
   Enter Author Name XYZ
   Enter Title of the book php
   Enter Author Name MNO

Book Name--->c_basic
 Author is--->ABC
Book Name--->cpp
 Author is--->DEF
Book Name--->java
 Author is--->PQR
Book Name--->sql
 Author is--->XYZ
Book Name--->php
Author is--->MNO

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Functions returning Objects

A function can not only receive the objects as argument but also returns objects from function.

Ex:
#include<iostream.h>
#include<conio.h>
class complex
{
float x;
float y;
public:
void input(float real,float img)
{
x=real;
y=img;
}
friend complex sum(complex, complex);
void show(complex);
};
complex sum(complex c1, complex c2)
{
complex c3;
c3.x=c1.x+c2.x;
c3.y=c1.y+c2.y;
return c3;
}
void complex::show(complex c)
{
cout<<"\t"<<c.x<<"+j"<<c.y;
}
void main( )
{
complex A,B,C;
clrscr( );
A.input(3.1,5.65);
B.input(2.75,1.2);
C=sum(A,B);
cout<<"\n\n\tComplex No A is--->";
A.show(A);
cout<<"\n\n\tComplex No B is--->";
B.show( );
cout<<"\n\n\tComplex No C is--->";
C.show( );
getch( );
}

Output:-
    Complex No A is--->3.1+j5.65
    Complex No B is--->2.75+j1.2
    Complex No C is--->5.85+j6.85

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Object as Function Argument

An object can be used as a function argument.This can be done in two ways
 1. Pass by value
 2. Pass by reference

Pass by value :
  In pass by value, a copy of entire object is passed to the function so any change made to the object inside the function do not affect the object used to call the function.

Pass by reference :
  In pass by reference , only the object's address is transferred to the function. When an address of the object is passed, the called function works directly on the actual object used in the call.

So any change made to the object inside the function will affect in actual object.

Write a program to illustrate the use of objects as an function argument

#include<iostream.h>
#include<conio.h>
class time
{
int hours;
int minutes;
public:
void gettime(int h, int m)
{
hours=h;
minutes=m;
}
void puttime( )
{
cout<<hours<<"Hours And"<<minutes<<"Minutes"<<"\n";
}
void sum(time,time);
};
void time::sum(time t1, time t2)
{
minutes=t1.minutes+t2.minutes;
hours=minutes/60;
minutes=minutes%60;
hours=hours+t1.hours+t2.hours;
}
void main( )
{
time t1,t2,t3;
clrscr( );
t1.gettime(2,45);
t2.gettime(3,30);
t3.sum(t1,t2);
cout<<"\n\tT1::";
t1.puttime( );
cout<<"\n\tT2::";
t2.puttime( );
cout<<"\n\tT3::";
t3.puttime( );
getch( );
}


Output :-
  T1::2  Hours And 45 Minutes
  T2::3  Hours And 30 Minutes
  T3::6  Hours And 15 Minutes

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