error with define macro definition

Question

when i am using #define function,I observe something bizarre. In the below code if I gave i value as \'10\' from input i

Answer 1

Macro's have subtleties. What your macro does is: Double(++i) -> ++i*++i

in your case 11*12 or 12*11

Answer 2

What you have is undefined behaviour.

You Double(++i) is changed to ++i * ++i, when you compile you code.

Answer 3

Double(++i) will expand to ++i * ++i. In this expression, i is modified twice without an intervening sequence point, which is undefined behavior.

Read: So, what's wrong with using macros?