In computing, 'real number' often refers to non-complex floating-point numbers. It include both rational numbers, such as 42 and 3/4, and irrational numbers such as pi = 3.14159265...

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.

C data types classification are

1. Primary data types
   1. int
   2. char
   3. float
   4. double
   5. void

Declaring is the way a programmer tells the compiler to expect a particular type, be it a variable, class/struct/union type, a function type (prototype) or a particular object instance. (ie. extern int i)

Declaration never reserves any space for the variable or instance in the program's memory; it simply a "hint" to the compiler that a use of the variable or instance is expected in the program. This hinting is technically called "forward reference".

extern int x; - is an external variable declaration.
double pow(double, double); - is a function prototype declaration.
Therefore, 1 and 3 are declarations. 2 is definition.

- During declaration we tell the datatype of the Variable.

- During definition the value is initialized.