Re: ncdigest V1 #1119

Jeff Whitaker wrote:
Ed Hartnett wrote:
Jeff Whitaker <jswhit@xxxxxxxxxxx> 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.

This is an anecdotal datapoint, but I use nested structures now (only one level deep though) in my f95 code. I tend to write "atomic" I/O functions (wrappers around the required f90 API calls) for structures which translates to reading from separate files. It would be nice to be able to "layer" that sort of functionality. I guess it's a file-within-a-file type of thing.

Having said all that, I also value simplicity. netCDF is an *extremely* useful format from that perspective for what I do. Is there a hue and cry from the user community for multi-level nested compound type I/O in future netCDF versions? Otherwise, I think JeffW's initial idea,
  "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."
(which I interpret as similar to my "one level deep" nesting) is a very good 
one.


cheers,

paulv

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