Author: Robert Hyatt
Date: 11:59:15 01/17/06
Go up one level in this thread
On January 17, 2006 at 12:52:40, James Swafford wrote: >On January 17, 2006 at 12:29:21, JW de Kort wrote: > >>Thank you for your reply. I check the boundries of the array's but that seems to >>be correct. >> >>Could you please clarify your response because i'am not 100% sure what you mean. >> > >I'll try. Let me preface this with this caveat: I doubt this is >your problem. :) Dann's advice is pretty sound. > >Let's say you are evaluating a bitwise expression "a & b". >If a == 0, then ( a & b ) == 0 for any b. So, it's not >necessary for the compiler to go to the trouble of figuring >out what b is. > >In your case, a & b represent items in an array. What is >really evaluated is probably implementation specific >(depends on the compiler), but it wouldn't even need to fetch >the item from the second array to do the bitwise comparison >if your first item (a) is 0. So, possibly you are skirting a >boundary bug when i==1 since your "a" is always 0. > >Again, I doubt that's the case in your program, but it's >a possiblity. > >-- >James > I don't see how that would cause a problem, since it has to fetch both values, and then AND them to produce the final zero/non-zero result for testing. This isn't the same as if (a && b) where if a is 0, b would not even be fetched usually... > > >> >>On January 17, 2006 at 11:54:29, James Swafford wrote: >> >>>On January 17, 2006 at 11:27:35, James Swafford wrote: >>> >>>>On January 16, 2006 at 16:22:10, Dann Corbit wrote: >>>> >>>>>On January 16, 2006 at 16:16:13, JW de Kort wrote: >>>>> >>>>>>Hi all, >>>>>> >>>>>>In my engine i want to use the following line: >>>>> >>>>>#include <assert.h> >>>>>... >>>>> >>>>> assert(kol+1 < sizeof bfZwart / sizeof bfZwart[0]); >>>>>> !(bfZwart[kol+1]&bfBoven[rij]) >>>> >>>>That's probably it, but another (less likely) possibility is that >>>>the short-circuit evaluation is not attempting fetch the value from >>>>the bfBoven[] array unless kol+1 > 1 (making bfZwart[kol+1] true). >>>> >>>>In other words, is it the case that bfZwart[kol+1] always evaluates >>>>to false? If so, I doubt the second part of the expression is even >>>>evaluated. Like I said, not likely, but weird things happen. >>>> >>> >>>By false/true I should say 'non-zero/zero', since we are dealing >>>with a bitwise operator. >>> >>> >>> >>>>-- >>>>James >>>> >>>> >>>>>> >>>>>>In only a few cases this lines crashes my engine. If i change the +1 to any >>>>>>other value there is no problem! >>>>>> >>>>>>Does anybody know were i have to look for to solve this problem? I do not >>>>>>understand how a comparison like the above can cause a program to crash. >>>>>> >>>>>>regards >>>>>>Jan Willem
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