"expected primary-expression before '.' token" in debug code

Question

I'm making some code for school, and is adding debug code to it. When compiling, I get the error as written in the title. This is the code in question:

#define DEBUG 1

#ifdef DEBUG
#define Serial.println(x) DEBUG_PRINT(x)
#else
#define DEBUG_PRINT(x)
#endif

And the code in my void setup, which has Serial.begin:

#ifdef DEBUG
Serial.begin(9600);
DEBUG_PRINT("Asteroid");
#endif

What am I getting wrong? I get the feeling it's my define text, but I'm unsure.

Thank you for your time Gaarden

Answer 1

In short: you have your macro backwards.

Macros are not formed as "Take this set of commands and call it X" but "Make this macro X and have it equate to this set of commands".

Where you have:

#define Serial.println(x) DEBUG_PRINT(x)

it should instead read:

#define DEBUG_PRINT(x) Serial.println(x)

When you call it, because the macro doesn't contain a semi-colon in it, you need to specify the semi-colon as if you were calling a function:

DEBUG_PRINT("Aardvark");

You can omit the semi-colon only if you have a semi-colon in your macro:

#define DEBUG_PRINT(x) Serial.println(x);

and then:

DEBUG_PRINT("Aardvark")

By doing it that way your "empty" macro now does become truly empty when used. Without the semi-colon in the macro you would have to use:

DEBUG_PRINT("Aardvark");

Which for the empty variant would end up as:

;

Not always desirable, and can cause some problems.

So by adding the semi-colon into the macro you can omit it from the usage of the macro, and your empty variant now becomes:

Answer 2

You defined the pre-processor macro the wrong way around.
Think it is like an assign statement. You come up with a name and assign something to it.

#define SHORT_NAME(ARG) some_real_function_that_has_an_awkward_long_name(ARG)

See the manual of your favourite compiler GCC for reference.
It is always a good idea to do that when using features you haven't used before. Sometimes it can be enlighting.