C error: missing whitespace after the macro name

Question

I wrote the following macro:

#define m[a,b] m.values[m.rows*(a)+(b)]

However gcc gives me this error:

error: missing whitespace after the macro name

What is wrong and how do I fix it?

Answer 1

You cannot use [ and ] as delimiters for macro arguments; you must use ( and ). Try this:

#define m(a,b) m.values[m.rows*(a)+(b)]

But note that defining the name of a macro as the name of an existing variable may be confusing. You should avoid shadowing names like this.

Answer 2

I'm not familiar with any C preprocessor syntax that uses square brackets. Change

  #define m[a,b] m.values[m.rows*(a)+(b)]

to

  #define m(a,b) m.values[m.rows*(a)+(b)]

And it should work.