Re: 19990621: netCDF support for variable removal?

>From: "Jessica Thomale" <address@hidden>
>Subject: Support for variable removal?
>Organization: OU
>Keywords: 199906211607.KAA15609 netCDF

Hi Jessica,

> I was wondering if there were any plans for variable removal?  
> 
> There are methods for removing attributes and other elements of the netcdf
> files, but no removal of variables.
> 
> The reason I bring up this topic is that when I need to remove a variable. 
> I have to either rename it which means it is still taking up space in the
> netcdf file or I have to remake the file without that variable.  Neither
> solution is very efficient. 
> 
> Is there a work around?

No, sorry, there are no plans to implement variable removal in netCDF.
To implement variable removal so that the variable would not still
take up space in the file would essentially require copying the file,
unless the variable just happened to be the last variable in a file
with no unlimited dimension.  Some efficiency could be gained for
deleting multiple variables by only copying the remaining variables
once, but users can do this also with the current interface.  There is
also the question of whether to delete dimensions only used by a
variable to be deleted.  Finally, there have not been many requests
for this capability, compared to other desired enhancements.

There are utilities that make deleting variables easier, by doing the
work of copying all but specified variables from an input netCDF file
to an output netCDF file.  For example, see ncks, part of the NCO
package:

  ncks will extract (and optionally create a new netCDF file comprised
  of) only selected variable from the input file, like ncextr(1) but
  with these differences: Only variables and coordinates may be
  specifically included or excluded--all global attributes and any
  attribute associated with an extracted variable will be copied to
  the screen and/or output netCDF file.  Options `-c', `-C', `-v', and
  `-x' control which variables are extracted.

The NCO package is freely-available from
  
  http://goldhill.cgd.ucar.edu/cms/nco/

> Also, when is version 4 for C/C++ due to come out? I am really looking
> forward to some of the new features that were announced.

The plans for netCDF 4 are on hold while we evaluate how we will make
use of HDF-5, whether to write a new C++ layer for both the current
XDR format and a new HDF-5 representation.  Our plans had been to use
what we learned in developing the Java interface to write the new C++
implementation, but the loss of the main developer, Glenn Davis, has
forced a change in plans.  We will have to reevaluate how we will
continue netCDF development after we have hired a replacement.

Sorry I can't be more definite, but things are still up in the air ...

--Russ

_____________________________________________________________________

Russ Rew                                         UCAR Unidata Program
address@hidden                     http://www.unidata.ucar.edu