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[netCDF #XQR-671194]: How to detect incorrect number of data

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  • Subject: [netCDF #XQR-671194]: How to detect incorrect number of data
  • Date: Thu, 17 Sep 2009 13:20:14 -0600
Hi Ramesh,

> In my program I read existing netcdf files. One of the dimension in the
> file is an 'unlimited dimension' and I have several variables that use
> this dimension.
> 
> e.g.
> dimensions:
> index = UNLIMITED ;
> short var1(index, oversampling) ;
> short var2(index, oversampling) ;
> 
> data:
> var1 = ....
> ....;
> 
> var2 = ....
> ....;
> 
> When I query for the dimension of 'index' it reports the largest number
> for 'index' across all the variables that use this dimension.
> 
> My question is how can I detect that the variables (var1 and var2 in
> the example above) have different number of data samples for 'index'
> dimension?

Just like other dimensions used with netCDF-3, the unlimited dimension
is shared, so it always has only one length, which corresponds to the
largest value it has grown to as the result of writing data to
variables that share that dimension.

One way to figure out how many data samples actually exist for
different variables that use the unlimited dimension is to take
advantage of "fill values", the value given to data that has never
been written.  The default fill value for short variables (as used in
your example) is NC_FILL_SHORT in C (or a corresponding constant for
other language APIs), which is defined as -32767.  So you could read
in the data for var2 and search backwards for the first value not
equal to -32767 to determine how many values were actually written for
that variable.

If that's too inefficient, you could store the number of values
written for each variable in a variable-specific attribute or in a
separate variable, and increment it when you append new data.

Alternatively, if you can use the enhanced data model of netCDF-4, it
supports any number of unlimited dimensions.  By using a different
unlimited dimension for each variable, you can get the desired
functionality, but your data will only be accessible to applications
that can interpret netCDF-4 data.

--Russ

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



Ticket Details
===================
Ticket ID: XQR-671194
Department: Support netCDF
Priority: Normal
Status: Closed