Russ Rew wrote:
Hi Jeff,
In netcdf-4.0, I don't see how to create variables which are arrays of
strings with length > 1. I see how to create arrays of
single-characters, and arrays of variable-length strings, but not
strings of a specified length.
...
Maybe Russ or Ed can tell me whether this is possible in netcdf-4.0?
Being limited to single character arrays in netcdf-3 was one of things
I really wanted netcdf-4 to overcome. Variable length strings are
nice, but they don't map well to commonly used data structures
(i.e. arrays) in fortran or python.
The distinction between an N-dimensional array of chars and an
(N-1)-dimensional array of fixed-length strings is pretty subtle, and it
looks like we assumed you could use the former if you needed the latter.
Multidimensional arrays of chars are supported in netCDF-4 just as in
netCDF-3. Maybe a few convenience functions that treat an N-dimensional
array of chars as an (N-1)-dimensional array of strings that happen to
be fixed length are all that's needed. If not, we may need a more
detailed use case that would make clear why that's inadequate.
--Russ
Russ: I realize you can use a array of shape ndim,8 to store an array of
ndim 8 character strings. Thats the way I've done it with netcdf-3 - it
just feels clunky. A typical use case for me is station data, where you
want to store the name of the station. I end up the with an array of
characters shaped (nstations,ncars) - in fortran I read it into an
(nstations,nchars) character(len=1) array (after first finding out what
both nstations and nchars are), then reshape it into a (nstations)
character(len=nchars) array. I'd rather just read it into a
character(len=nchars) array straight off. Not a show stopper for sure,
but it would be more convenient. I realize that specifying the data
type would be tricky, instead of NC_CHAR, do you have a bunch of new
types NC_CHAR1, NC_CHAR2, ... NC_CHAR120? Or a new function datatype =
nc_set_chartype(nchars)? However, I bet it would get used a lot more
than the esoteric datatypes you have in netcdf-4 already (enums and
opaque for example).
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.
My 0.02$, thanks for listening.
-Jeff
--
Jeffrey S. Whitaker Phone : (303)497-6313
NOAA/OAR/CDC R/PSD1 FAX : (303)497-6449
325 Broadway Boulder, CO, USA 80305-3328
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