[CF-metadata] bounds

From: John Caron <caron>
Date: Thu, 22 May 2003 13:56:36 -0600

Russ Rew wrote:

>John Caron wrote:
>
>
>
>>I would prefer "equal within tolerence". if you need a definition of
>>tolerence, i would use something like
>> abs( (a-b)/b) < 1.0E-5
>>
>>The reason is that post processors shouldnt have to preserve floating
>>point to the last bitter bit. ;^}
>>
>>
>
>I disagree for a couple of reasons:
>
> - Post processors don't have to preserve floating point to the last
> bitter bit, each one just has to be consistent in changing the same
> floating point value to a different floating point value. I can't
> imagine any post processor (or other program) that would sometimes
> change 0.66667 to 0.6666 and other times to 0.6667 (I'm just using
> base 10 for illustration). As long as the same processor munges
> the same floating point value in the same way, equality tests will
> work OK. Some programs depend on that fact for processing missing
> values for floating point.
>
> - Approximate comparison is more complex than
>
> abs( (a-b)/b) < 1.0E-5
>
> in case b is identically zero, and because there is no "epsilon"
> that is always appropriate for such tests. You can use an epsilon
> appropriate for IEEE single precision floating point, but then you
> can contrive cases where that would be wrong and exact equality
> would be the only appropriate test.
>
>I think Jonathan is right to recommend exact equality tests in this
>case.
>
>--Russ
>
>
ok, let me give a use case:

Someone has tried to create a CF netcdf file and bungled it, by not
getting the coordinate bounds correct. So I am going to use NcML to fix
the problem, by adding some of the values in "by hand". Do I need to
worry about if "1.0" is really 0.999999999999999999999999 in double ? So
the problem is needing to match a floating point representation in
ascii. how many significant places are needed? From my POV, getting the
last bits right is not needed, and could be onerous.

Obviously I didnt completely specify the tolerence above for the b = 0
case. One could also specify a stricter tolerence for doubles.

What would be a "contrived" example where 1E-5 would not work ? We are
not trying to preserve numerical results in the face of complex
algorthims, just trying to answer the question if the bounds are continuous.
Received on Thu May 22 2003 - 13:56:36 BST