[Rock-dev] [Discussion] usage of infinity for "no-data" tagging

[Leif Christensen]

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Has anyone already mentioned IEEE 754? There are quite some papers
around IEEE 754 and the conceptual use of NaN and infinities. There are
also different kinds of NaN (Wikiquote NaN):

"IEEE 754 NaNs are represented with the exponential field filled with
ones and some non-zero number in the significand. A bit-wise example of
a IEEE floating-point standard single precision (32-bit) NaN: s111 1111
1axx xxxx xxxx xxxx xxxx xxxx where s is the sign, x is the payload, and
a determines the type of NaN. If a = 1, it is a quiet NaN; if a is zero
and the payload is nonzero, then it is a signaling NaN."

Greetings,
Leif

On 29.07.2011 16:31, Alexander Duda wrote:
> On Fri, 2011-07-29 at 09:58 +0200, Jakob Schwendner wrote:
>> On 07/28/2011 02:10 PM, Sylvain Joyeux wrote:
>>> I've created a proposal for a guideline page for that
>>>
>>>     http://rock.opendfki.de/wiki/WikiStart/Standards/RG3
>>>
>>> Please discuss. I'll add the relevant bits to base when we agree on the
>>> proposal.
>>>
>> ok for me regarding unset values and the usage of NaN.
>> I don't agree with the usage of infinity though. Infinity has certain 
>> properties that do not hold true for an "unknown value".
>> I've done some tests on inf, because I wasn't 100% sure if my 
>> assumptions hold, but they do:
>> a : 1
>> inf : inf
>> minf : -inf
>>
>> a < inf : 1
>> a > inf : 0
>> a < minf : 0
>> a > minf : 1
>> a / inf : 0
>> a / minf : -0
>> a * inf : inf
>> a * minf : -inf
>> inf * -5.0 : -inf
>> inf / -5.0 : -inf
>>
>> I agree with the usage of inf in covariance matrices (since it's not an 
>> unknown value there), but not the example with the vector. If you really 
>> don't want to represent unknown values in a separate variable I would 
>> even prefer nan in this case.
>> The thing is you can do normal calculations with inf and even get a real 
>> number (0) out of it in case you divide another number by inf. Not what 
>> I would expect from an unknown number.
>>
>> Jakob
>>
> 
> For me inf is a sentinel value and cannot be measured by any physical
> sensor. Therefore in this case inf is just a concept to flag something
> like a unknown value.
> 
> Alex
> 

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