Inexplainable “Error C141: syntax error” In C on Arduino [closed]

Answer 1

You have to remember that the preprocessor (which handles macros) is not the compiler and doesn't need semicolons to terminate the "statements".

Macros are replaced quite verbatim, so in the latter example the line

LEDS = 0x01;

will, after the preprocessor replaces the LEDS macro, be

P4; = 0x01;

which is not a valid C statement.

Answer 2

Don't use semicolons in macros.

#define LEDS P4;
#define SWITCH P5;