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;

                }