Why is this C or C++ macro not expanded by the preprocessor?

Question

Can someone points me the problem in the code when compiled with gcc 4.1.0.

#define X 10 int main() {   double a = 1e-X;   return 0; } 

I am getting error:Exponent has no digits.

When I replace X with 10, it works fine. Also I checked with g++ -E command to see the file with preprocessors applied, it has not replaced X with 10. I was under the impression that preprocessor replaces every macro defined in the file with the replacement text with applying any intelligence. Am I wrong?

I know this is a really silly question but I am confused and I would rather be silly than confused :).

Any comments/suggestions?

Answer 1

The preprocessor is not a text processor, it works on the level of tokens. In your code, after the define, every occurence of the token X would be replaced by the token 10. However, there is not token X in the rest of your code.

1e-X is syntactically invalid and cannot be turned into a token, which is basically what the error is telling you (it says that to make it a valid token -- in this case a floating point literal -- you have to provide a valid exponent).

Answer 2

When you write 1e-X all together like that, the X isn't a separate symbol for the preprocessor to replace - there needs to be whitespace (or certain other symbols) on either side. Think about it a little and you'll realize why.. :)

Edit: "12-X" is valid because it gets parsed as "12", "-", "X" which are three separate tokens. "1e-X" can't be split like that because "1e-" doesn't form a valid token by itself, as Jonathan mentioned in his answer.

As for the solution to your problem, you can use token-concatenation:

#define E(X) 1e-##X int main() {   double a = E(10); // expands to 1e-10   return 0; }