C macro: #if check for equality

Question

Is there a way to do check for numerical equality in macros?

I want to do something like

#define choice 3

#if choice == 3
  ....
#endif

#if choice == 4
 ...
#endif

Does C macros have support for things like this?

Rania

What did your compiler tell you? – Grose

Better yet, what does the textbook / reference book say? – Slumlord

@Grose My compiler is currently saying, "No, I don't support this." I'm trying to figure out why it's not working. So, your question is not helpful in my situation. – Endocarp

Answer 1

Indeed that should work. See http://gcc.gnu.org/onlinedocs/cpp/If.html#If

That reference is accurate, but written in "standards format":  abstractly without examples.

Maestoso

Answer 2

Another way to write your code uses chained #elif directives:

#if choice == 3
  ...
#elif choice == 4
  ...
#else
  #error Unsupported choice setting
#endif

Note that if choice is not #defined, the compiler (preprocessor) treats it as having the value 0.

Ferreira

How can we compare strings? Namely if I have a macro defined by #define MY_MACRO Hello. How can I make if to check its string value? – Orthodoxy

The ISO C preprocessor does not have a way to compare strings. – Ferreira

Answer 3

As far as i know that should work. What compiler are you using ?

PS : Just for information, the defines names are usually written in caps !

#define CHOICE 3

Bituminize