Though the pre processor assigns i = 3.14159265. When i compile it, gives an error saying expected an expression before \'=\' token what and why is that ?
#incl
#define PI = 3.14159265
defines PI to be = 3.14159265 literally including the equals sign. Thus i = PI; is the same as:
= 3.14159265
i = PI;
i = = 3.14159265;
which clearly won't compile. To fix it, remove the = from your definition of PI:
=
#define PI 3.14159265