Define the Variable:

A variable is a meaningful name of data storage location in computer memory. When using a variable you refer to memory address of compute

  

Naming Variables

The name of variable can be called identifier or variable name in a friendly way. It has to follow these rules:

• The name can contain letters, digits and the underscore but the first letter has to be a letter or the underscore. Be avoided underscore as the first letter because it can be clashed with standard system variables.
• The length of name can be up to 247 characters long in Visual C++ but 31 characters are usually adequate. Keywords cannot be used as a variable name. 

Of course, the variable name should be meaningful to the programming context.

Declaring Variables

To declare a variable you specify its name and kind of data type it can store. The variable declaration always ends with a semicolon, for example:

 int counter;

   char ch; 

You can declare variables at any point of your program before using it. The best practice suggests that you should declare your variables closest to their first point of use so the source code is easier to maintain. In C programming language, declaring a variable is also defining a variable.

Initializing Variables 

 You can also initialize a variable when you declare it, for example:

int x = 10

char ch = 'a'

Storage of Variables

Each variable has its own lifetime (the length of time the variable can be accessible) or storage duration. When you declare your variable you implicitly assign it a lifetime. Let take a look at this source code example:

                 

                      #include <stdio.h>

                      int global_variable=10;//global variable

                      void func();

                      void main()

                       {

                      int i; //test static variable

                      for(i=0; i<5; i++)

                       {

                      func();

                      printf("after %d call\n",i);

                        }

                        }

                      void func()

                        {

                       static int counter = 0;// static variable

                       counter++;

                         printf("func is called %d time(s)\n",counter);

                       int local_variable=10;

                               }