The usefulness of the "Hello World" programs shown in the previous section is quite questionable. We had to write  several lines of code, compile them, and then execute the resulting program just to obtain a simple sentence  written on the screen as result.

Let us think that I ask you to retain the number 5 in your mental memory, and then I ask you to memorize also  the number 2 at the same time. You have just stored two different values in your memory.   

The whole process that you have just done with your mental memory is a simile of what a computer can do with  two variables. The same process can be expressed in C++ with the following instruction set: 

    a = 5;  
    b = 2;  
    a = a + 1;  
    result = a - b;  

Obviously, this is a very simple example since we have only used two small integer values, but consider that your  computer can store millions of numbers like these at the same time and conduct sophisticated mathematical  operations with them.

Declaration of variables:
In order to use a variable in C++, we must first declare it specifying which data type we want it to   e. The syntax  to declare a new variable is to write the specifier of the desired data type (like int, bool, float...) followed by a valid  variable identifier. For example:  

int  a =0 ; 
double   val = 0.0; 

These are two valid declaration and initialization  of variables. The first one declares a variable of type int with the identifier a. The  second one declares a variable of type float with the identifier val. Once declared, the variables a and  mynumber can be used within the rest of their scope in the program.  

If you are going to declare more than one variable of the same type, you can declare all of them in a single 
statement by separating their identifiers with commas. For example:  

int a, b, c, asl, pet; 

This declares three variables (a, b and c  ----), all of them of type int, and has exactly the same meaning as:  

    int a; 
    int b; 
    int c; 
    int asl;
    int pet;

Local and Global Variable:
Variables declared outside of a block (outside the main function) are called global variables. Global variables have program scope, which means they can be accessed everywhere in the program.

A variable declared inside the a function is called local variable.
A local variable is local in the sense that,when the operating point or control moves out of teh function at the end of it, the variable become undeclared.

Here is an example of declaring a global variable and local variable.

  including namepace std;
   int  global =5;          // declaration of global variable

  double myfunc(double x, double y)
  double sum=0.0;      // here sum is the local variable
   int i=0;                      //declaration of local variable

  int main() 
    int local=6;        //declaration of local variable


Defined constants ( # define)

You can define your own names for constants that you use very often without having to resort to memory-consuming variables, simply by using the   #define   preprocessor directive. Its format is:

 #define PI  3.14159

This defines two new constants: PI  .Once this defined, you can use them in the rest of the code as if they were any other regular constant, for example:

// defined constants: calculate circumference                      
#include <iostream> 
using namespace std; 
#define PI 3.14159 

int main () 

  double r=5.0;               // radius 
  double circle; 
  circle = 2 * PI * r; 
  cout << circle <<endl; 

  return 0; 

The output is :  31.4159 

Declared constants (const)

With the const prefix you can declare constants with a specific type in the same way as you would do with a variable:

const float PI = 3.14159;

Here, PI is a constants. They are treated just like regular variables except that 
their values cannot be modified after their definition.