Question

I want to replace directive

#define MAX_LINE 15

with variable value.

For example of behavior, if something ( text ) contains 15 or more lines #define MAX_LINE to return 15
else if the text contains less than 15 lines #define MAX_LINE to return numbers of lines.

for( i = 0; i < MAX_LINE; i++ ) {
   /* print lines in expandable menu window */
}

is this possible?

Answer 1

#defines simply replace some text with some other text. You could, for example, use this:

#define MAX_LINES ((num_lines > 15) ? 15 : num_lines)

Then you can use MAX_LINES as before in some statements. However, it won’t work in all situations. You may have had code that looked like this:

const char *lines[MAX_LINES];

With the old #define, MAX_LINES was 15, so it expanded to this:

const char *lines[15];

That should be valid in any standards-compliant compiler. However, say you used our new #define. Expanded, it would look like this:

const char *lines[((num_lines > 15) ? 15 : num_lines)];

Supposing that was a global declaration, that would be invalid. Even if it was just a local declaration, it would only be valid if the compiler supports variable-length arrays.

If I were you, I’d simply try to get whatever you’re trying to do to work without #defineing MAX_LINES to a non-constant expression; it’s clearer that way and it will be more obvious where it would be invalid to use MAX_LINES.