Help with Understanding "#define"

[kinghajj]

In my book, it says that the syntax for #define is

#define macro_name macro_body

However, I've seen lots of code like this:

#define macro_name

What does exp. 2 mean?

[vinkesh]

the working of both r similar.

1. #define MACR0 10

what this does is it replaces the word MACRO with 10 everywhere in the programme.
This is used from the clarity of the programme.

2. # define MACRO

here u r just defining a macro
it acts like an if.

ex.

#ifdef MACRO
printf(....)
#else
printf(....)

[kinghajj]

That makes sense. I can see lots of ways to use that. Thanks.