ncdigest V1 #1119

[Jeff Whitaker]

Ed Hartnett wrote:
> Jeff Whitaker <jswhit at fastmail.fm> writes:
>
>   
>> By the way, while I'm ranting ...  Another potential problem is see is
>> nested user-defined types.  Since this is allowed in netcdf-4, I can
>> see the potential for creating horribly complicated files with
>> compounds of vlens containing compounds containing enums etc.  Don't
>> think many people would be crazy enough to use it, but it makes it
>> very hard to create client code to read arbitrary netcdf-4 files,
>> since you have to check for all those possibilities in general.  I
>> wonder whether it might not be wise to sacrifice some generality for
>> simplicity and just say that user-defined types can only be composed
>> of primitive data types.
>>
>>     
>
> Howdy Jeff!
>
> Here at NetCDF World Headquarters, we also wonder how these new
> features will be used by the netCDF community!
>
> Perhaps some of the HDF5 readers of this mailing list can provide some
> real world examples of nested structures.
>
> To read them, the only general purpose answer I know of (as with
> groups) is a recursive function. This is how the netCDF-4 file reading
> code handles the problem, and, I believe, how ncdump handles the
> problem (Russ will correct me if I'm wrong about ncdump). 
>
> Both of these are in C (for example libsrc4/nc4hdf5.c, function
> rec_read_metadata().
>
> The problem, perhaps, is what to do with this information once you
> have it. 
>
> For example, it's easy to plot a data file full of NC_INT, or
> NC_FLOAT, but how do you plot a file full of data in compound types?
>
> It is a more complicated problem, but I'm sure the very clever
> programmers working on the IDV will come up with something. ;-)
>
> Although these new features present many new complexities, the
> CLASSIC_MODEL flag allows users to produce netCDF-4 files which don't
> use any of the user-defined types, and thus can be read (and, with a
> little modification on the nc_create call, written) by existing
> software.
>
> Thanks!
>
> Ed
>
>   
Ed:  I think it's relatively easy (and productive)  to deal with 
compound types, where the elements are primitive (fixed size) data 
types.  I think that allowing nested compound types (or compound types 
containing vlens) increases the complexity exponentially though,   
Unless there are some real use cases out there, I wonder if it is really 
worth it.

-Jeff

-- 
Jeffrey S. Whitaker         Phone  : (303)497-6313
Meteorologist               FAX    : (303)497-6449
NOAA/OAR/PSD  R/PSD1        Email  : Jeffrey.S.Whitaker at noaa.gov
325 Broadway                Office : Skaggs Research Cntr 1D-124
Boulder, CO, USA 80303-3328 Web    : http://tinyurl.com/5telg

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