Writing c function in preprocessror MACRO

Asked Oct 03 '22 at 13:49

Active Oct 03 '22 at 14:18

Viewed 86 times

I am working with converting C functions into pre-processor MACRO in c language. I know that for example:

int plus(int x, int y) {
  return x + y;
}

will be as:

#define PLUS(x, y) ((x) + (y))

in pre-processor MACRO in C language. My question is what would:

int factorial(int n) {
  if (n == 0)
    return 1;
  else
    return n * factorial(n - 1);
}

look like as pre-processor MACRO in C language. I have tried:

#define FACTORIAL(n) (n == 0 ? : n * FACTORIAL(n - 1))

But it is not working. Any tips?

edited Oct 03 '22 at 14:18

ryyker

asked Oct 03 '22 at 13:49

Glory

You can't do this in a macro because it can't test the variable value to know when to stop expanding. – Barmar Oct 03 '22 at 13:51
https://stackoverflow.com/questions/12447557/can-we-have-recursive-macros – Iłya Bursov Oct 03 '22 at 13:52
https://stackoverflow.com/questions/13614992/c-preprocessor-recursive-macro – Iłya Bursov Oct 03 '22 at 13:52
1

Basically, recursion can't be done in a macro because the base case is determined when the code is running, but the macro expands at compile time. – Barmar Oct 03 '22 at 13:52
@IłyaBursov You have enough rep to vote to close as a dupliate. – Barmar Oct 03 '22 at 13:53
1

@Barmar they're are not direct answers to factorial question, so I want them to be as linked questions only – Iłya Bursov Oct 03 '22 at 13:54
If you ditch the recursion you could likely do a preprocessor calculation based on `n`, up to `n == 12` or whatever. Larger than that doesn't make sense, because then you get integer overflow. – Lundin Oct 03 '22 at 13:55
I found a better dupe, added to the duplicate list. – Lundin Oct 03 '22 at 13:59
And well, this took me 5 minutes to write https://godbolt.org/z/6rc17PT7a. – Lundin Oct 03 '22 at 14:08
You probably know this, but in ordinary code, replacing functions by macros is *not* generally considered a Good Thing. – Steve Summit Oct 03 '22 at 14:16

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