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Re: 20040312: Zero value variable

>To: address@hidden
>From: Roger Ting <address@hidden>
>Subject: Zero value variable
>Organization: UCAR/Unidata
>Keywords: 200403120402.i2C42ZrV006218

Hi Roger,

> Does netcdf subroutine allow a variable with no value? Currently, when i
> attempt to read a variable with empty value, it gives a negative value
> which causes my program to crash. The return code is still zero even
> though the value that been read in is obviously wrong
> 
> 
> This is how the variable appears in the dump file.
> 
> 
> No_Prof = _ ;
> 
> 
> This is how i read in the value
> rcode=NF_GET_VAR1_INT(fileid,nprofsid,1,No_Prof(option))
> call checkdataerr('Error in getting number of profile',rcode)
> 
> 
> I did inquire the variable id before reading it. Shouldn't netcdf
> subroutine return an error or zero value in this case?

No, netCDF allows variables with no values, and specifies what happens
when you try to read a value that has not been written:

  
http://www.unidata.ucar.edu/packages/netcdf/guidef/guidef-12.html#HEADING12-1381

  7.16 Fill Values

  What happens when you try to read a value that was never written in
  an open netCDF dataset?  You might expect that this should always be
  an error, and that you should get an error message or an error
  status returned.  You do get an error if you try to read data from a
  netCDF dataset that is not open for reading, if the variable ID is
  invalid for the specified netCDF dataset, or if the specified
  indices are not properly within the range defined by the dimension
  lengths of the specified variable.  Otherwise, reading a value that
  was not written returns a special fill value used to fill in any
  undefined values when a netCDF variable is first written.

  You may ignore fill values and use the entire range of a netCDF
  external data type, but in this case you should make sure you write
  all data values before reading them.  If you know you will be
  writing all the data before reading it, you can specify that no
  prefilling of variables with fill values will occur by calling
  NF_SET_FILL before writing.  This may provide a significant
  performance gain for netCDF writes.

  The variable attribute _FillValue may be used to specify the fill
  value for a variable.  Their are default fill values for each type,
  defined in the include file netcdf.inc: NF_FILL_CHAR, NF_FILL_INT1
  (same as NF_FILL_BYTE), NF_FILL_INT2 (same as NF_FILL_SHORT),
  NF_FILL_INT, NF_FILL_REAL (same as NF_FILL_FLOAT), and
  NF_FILL_DOUBLE. ...

This is not an error, because it often turns out to be useful to only
write parts of a multidimensional variable and be able to access the
variable later to determine which values were written and which values
are "missing".  For example, when storing sea surface temperature on a
grid, missing values are often used for grid points that are over
land.

--Russ

_____________________________________________________________________

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