Re: Loop optimization

haroon wrote: 
Hi,
I have two versions of a loop. I want to know which one is more
efficient. and is there any way to make it even more efficient?

1- for (i = 0; str[i++] != '\0';) ;

2- for (i = 0; str[] != '\0'; ++i) ; 

    Version 2 is more efficient, because it won't compile
and will therefore consume no time at all in execution.

    More seriously, your question is ill-posed.  If we
correct the second version by changing `str[]' to `str[i]'
(I'm just guessing, but it's your error that forces me to
make the guess), observe that the two loops do not perform
the same task.  Since they perform different tasks, it is
senseless to ask about their relative efficiency.  Which
is more efficient: A bicycle or a banana?

    Personally, I don't think I'd write either loop.  On
the supposition that `str' is a `char' array containing a
string (or is a pointer to the start of a string), I'd more
likely write

	i = strlen(str) + 1;  /* version 1 */
	i = strlen(str);      /* version 2, corrected */

.... depending on the desired outcome.  I'd do so even though
there's no guarantee that strlen() is more efficient than
your loops, because it is surpassingly unlikely that the
calculation of a string length is the bottleneck in the
program.  If it does turn out to be the bottleneck, you'll
discover that only by measurement -- and the measurement (and
whatever improvements might be possible) will be specific to
the particular C implementation you're using at the moment.

--
Eric Sosman
esosman@xxxxxxxxxxxxxxxxxxx
.

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