[netcdfgroup] Dealing with null datasets in NetCDF

• To: netcdfgroup@xxxxxxxxxxxxxxxx
• Subject: [netcdfgroup] Dealing with null datasets in NetCDF
• From: shaalin23 <shaalinnathwani@xxxxxxxxxxxxx>
• Date: Tue, 23 Jan 2018 07:48:21 -0700 (MST)

Hi all,
I’m having an issue with using netcdf with fortran – I have several netcdf
files that need to be read into a fortran program and some of the variables
in the netcdf files have null datasets. 
E.g. doing an ncdump operation as follows: 
$> ncdump –v hello hello_world.nc 
Will output
hello = _ ;
To the terminal because the dataset associated with it is null. 
Even though it is null, I need it to still be read in in the FORTRAN code
because the code will be performing operations on several netcdf files – and
in some of them, the variable “hello” has actual values. 
Currently in the FORTRAN code, I’m currently doing the following:
-       Allocating an array big enough to hold the “hello” dataset. 
-       Performing an NF90_get_var operation as follows:
o       Err = NF90_GET_VAR(ncid, 10, helloArray)
o       Where ncid is the id for the file, 10 is the variable ID, and helloArray
is the location where the data needs to be stored.
-         Writing the array to the screen

The last step, writing the array to the screen, currently produces garbage,
I assume that it is the memory address – a random negative number completely
unrelated to the value (null) in the ncdump output. 

Is there a way to check for nulls using the netcdf API? At the moment I
can’t think of a way to do this and therefore my data gathering from the
netcdf file is not accurate. 

A work flow as follows would be appropriate:
-       Allocate an array big enough
-       Perform NF90_get_var operation
-       Check if the result obtained with null
-       Deal with it appropriately. 

Look forward to hearing people’s thoughts on this!

Cheers




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