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>To: address@hidden >From: Dennis Shea <address@hidden> >Subject: Re: 20030224: netCDF and NaN >Organization: >Keywords: Hi Dennis, > FYI ... I had a user ask me about the NaN issue. > Below is my response. > > [snip] > > Yes, I agree using NaN for missing values is non-portable. > > >In the the "Missing Data Values" section . .. > > > > * One standard way of indicating missing values for real type data, > > is to store an IEEE NaN floating point value. The advantage of this > > is that any computation using a NaN results in a NaN. > > Client software must know to look for NaNs, however. > > ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ > > This is a poor qualifier. It should read: > Using NaNs could compromise file portability, the main > reason for using netCDF, since it is not guaranteed > that a local system will support detection of IEEE NaNs. Even if a platform does not support propagation of NaNs in arithmetic operations, I think a NaN value can be detected and compared for equality with a variable value on every system I know of that is used for scientific computing. For example, a Cray may not support IEEE arithmetic, but it still can detect whether a floating-point value is equal to a specific NaN value stored as a missing data value attribute, can't it? So I think the above recommended rewrite is too strong. What compromises portability is not the use of NaNs as missing values, but an assumption that they will be propagated in arithmetic operations. So I would recommend changing > > Client software must know to look for NaNs, however. to Client software must know to look for NaNs, however, and there are some platforms that don't propagate NaN values in arithmetic operations. > P.S. Any word on the status of your proposal > regarding netCDF development? No, not yet. The last time I asked, I was informed that the selection decision was being delayed "until mid February", but there has still been no announcement. --Russ