Author: James Swafford
Date: 04:54:04 01/18/06
Go up one level in this thread
On January 18, 2006 at 03:29:48, Chan Rasjid wrote: >On January 17, 2006 at 17:41:03, James Swafford wrote: > >>On January 17, 2006 at 14:59:15, Robert Hyatt wrote: >> >>>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 >> >> >> >>Why does it have to? If you say that's what most compilers *do*, then >>I believe you (I don't claim to be a compiler expert), but, mathematically, >>they don't *have to*, since 0 & b == 0 for any integer b. >>Please tell me what I'm missing. :) >>James > >I can guess compiler has to have a natural degree of dumbness. >You specifically ask to test if ( (a & b)== 0 ). > >I doubt they do extra work to add if (a == 0), ..or if (a == 0) && if (b == 0 ). >Rather the expected method is fetch, a, fetch b AND them and test. I thought the answer would be somewhere along those lines... Bob said the compiler "has to", but he must've meant it is "going to." It seems to me that in cases where the second operand is a complex expression you could potentially save a lot of work by simply testing the first operand for 0 (or vv). Thanks, -- James > >Rasjid
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