Thread: String Reversal (kinda different from the rest)

I got a comment about my code that it was C99. Just for my own understanding, can someone clear up for me what's C99 about it? Is it the way I declared strarray? I really should get my hands on some standards books

Thanks

Here's another one. It uses the XOR trick, but you can use another temporary variable to swap characters.

Code:

char *revstr(char *const string)
{
		register char   *begin;
		register char   *end;
		const size_t	 length = strlen(string);


		for (begin = string, end = string + length - 1; begin < end; begin++, end--)
				*begin ^= *end ^= *begin ^= *end;

		return string;
}

PS: itsme86: Yes, it's the way you declared strarray (array size computed in run-time).

You know there's really no point in ever using the 'register' keyword, right?

Quzah.

> *begin ^= *end ^= *begin ^= *end;
Oh please, not this nonsense again!!!!

http://www.eskimo.com/~scs/C-faq/q10.3.html
Pay REALLY close attention now - read the bit which says

"and the ``obvious'' supercompressed implementation for integral types a^=b^=a^=b is illegal due to multiple side-effects; see question 3.2)."

I seem to remember a post of mine some time ago which showed that the compiler was way smarter than the "clever" programmer in that the blindingly obvious "use a temporary variable" actually produced the smallest code, and that all these arithmetic and logical tricks produced longer instruction sequences.

Originally Posted by itsme86

I got a comment about my code that it was C99. Just for my own understanding, can someone clear up for me what's C99 about it? Is it the way I declared strarray? I really should get my hands on some standards books

Thanks

Variable-size arrays in C have always been (pre-C99) a no-no:

Code:

void revstr(char *str)
{

  struct str_s
  {
    char ch;
    char *ptr;
  } strarray[strlen(str)], temp;

(You do realize, don't you, that there's a little elf ---near the East Pole--- who collects a royalty of .01 euros for every use of malloc() in a commercial program? If everybody started using variable-length arrays, he would have to declare bankruptcy. The next thing you know, he would given his own reality-TV show. Too horrible to imagine!)

Regards,

Dave