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;

                               }

Declare Variables

A Variable is a named memory location that can hold various values.Only the most trivial C programs do not include variables. In C Unlike some computer languages, all variables must be declared before they can be used. A variable's declaration serves one important purpose: it tells the compiler what type of variable is being used..C supports five different basic data types. as shown
Type                                                                  Keyword
character data                                                       char
signed whole numbers                                              int
floating-point numbers                                             float
double- precision floating-point numbers                     double
valueless                                                               void                                                                                
A variable of type char is 8 bits long and is most commonly used to hold a single character. Because C is very flexible, a variable of type char can also be used as a " little integer" if desired.
Integer variables (int) may hold signed whole numbers (numbers with no fractional part). For 16-bits long and is most commonly used to hold a single character.
Variables of types float and double hold signed floating-point values, which may have fractional components. One difference between float and double is that double providers about twice the precision as does float .
Shown a program use the variables:

               #include <stdio.h>

               int main (void)

                {

              char ch;

              float f;

              double d;

              ch= 'X';

              f=100.123;

              d=123.009;

              printf ("ch is %c,",ch);

              printf("f is %f,",f);

              printf("d is %f",d);

              return o;

                }

C Fundamentals

The C program is a set of functions. A C program always starts executing in a special function called main() function. Your program ends when main()'s closing curly brace is reached. One of the most common library functions is called printf(). This is C's general-purpose output function. The printf() function is quite versatile, allowing many variations. Its simplest from is shown here:

        #include<stdio.h>

        main()
          {

       printf("Hello programmer\n");

       return 0;
          }

The first line is the #include directive. C program uses this directive to load  external function library - stdio is C library which provides standard input/output printf is a function which is declared in the file called stdio.h .

The next is the main function the first entry point of all C programs.C program logic start from the beginning of main function to the its ending.

And finally is the printf function which accepts a string parameter. The printf function is used to print out the message to the screen.

Next You have learn how to write the first program  C