Re: [Genesis-sim-users] Weird arithmetic error in Genesis

[james b. <bo...@ut...>]

thanks everyone

hope everyone who went enjoyed Prague  would have loved to be there - but way too busy here.

Jim


> On Jul 22, 2015, at 3:56 PM, Dave Beeman <db...@Co...> wrote:
> 
> Thanks to Hugo and Dieter for the explanations. Thet other thing to
> keep in mind is that at the scripting level, all floating point variables
> are 32 bit floats, not doubles.  This can be a source of roundoff errors
> and is the reason that some XODUS dialogs will report numbers like
> "0.0000000237" or similar. At the C code level, most important
> variables that are used in numerical solution (particularly those
> involving time) have been converted to doubles.
> On Wed, 22 Jul 2015, Cengiz Günay wrote:
> 
>> Thanks, Hugo! That's quite clear. It's good to hear that you're still on the
>> list. Thankfully my code did not actually contain this overflow error, it
>> was something I was trying to verify the calculations. Is it safe to say
>> that all floating point variables in Genesis are double precision? Do you
>> know if this is also true for "extended fields" in generic objects created
>> using addfield?
>> -Cengiz
>> On Wed, Jul 22, 2015 at 6:14 PM, Hugo Cornelis <hug...@gm...>
>> wrote:
>> Hi Cengiz,
>> I quickly checked the behaviour of both commands with GDB, and here is
>> what happens:
>> The first result 0.5500000385 is correct, GDB reports
>> 0.55000003851059043 as the double precisions result.
>> For the behaviour of the second command:
>> echo { 2012303744.1 / 3658733824 }
>> What happens goes as follows:
>> The second argument in the input, the integer 3658733824, is larger
>> than the highest signed 32bit integer (which is 2147483647).
>> GENESIS uses the sscanf() function to read the integer from the
>> command line.  This function does not generate an error when an
>> overflow occurs and simply returns 2147483647, so GENESIS continues
>> its operation with this result assuming everything is fine.
>> So internally the command is treated as this command:
>> echo { 2012303744.1 / 2147483647 }
>> for which GDB returns:
>> 0.93705195236813832
>> Obviously, this behaviour is totally undesired and GENESIS should emit
>> an error message for the integer overflow, but does not.
>> The work around is as you say, to suffix a .0 after the integer to
>> force GENESIS to use floating point calculations.
>> Hugo
>> On Wed, Jul 22, 2015 at 10:48 PM, Cengiz Günay <cg...@em...>
>> wrote:
>>      Hi,
>> I have been pulling my hair trying to debug arithmetic
>> inconsistencies in Genesis that are caused by large truncation
>> errors from simplest calculations and inconsistent PI
>> definitions, which I realize have been known and we need to be
>> cautious about. However, I wanted to report something really
>> disturbing:
>> > echo { 2012303744.1 / 3658733824.0 }
>> 0.5500000385
>> > echo { 2012303744.1 / 3658733824 }
>> 0.9370519524
>> Does anyone have an explanation for this behavior? This can
>> cause seriously wrong calculations and hidden bugs.
>> I think the take home message is to always put a .0 behind
>> integer numbers that are going to be in floating point
>> calculations. :)
>> Best,
>> -Cengiz
>> --
>> Cengiz Gunay
>> Postdoctoral Fellow, Dept. of Biology
>> Visiting Faculty, Dept. of Math & CS
>> Emory University
>> http://www.biology.emory.edu/research/Prinz/Cengiz/
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>> --
>> Hugo
>> --
>>                   Hugo Cornelis, Ph.D.
>>                     Agora Classica -- CTO
>>               http://www.agoraclassica.com/
>>             GENESIS-3 -- lead architect
>>               http://www.genesis-sim.org/
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