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Re: 20010906: Help! - NetCDF

>To: <address@hidden>
>From: <address@hidden>
>Subject: Help! - NetCDF
>Organization: UCAR/Unidata
>Keywords: 200109061203.f86C3J125207

Hi,

> I have an array of integers with values from 0 to 255.
> I put them in NC_BYTE variables with the C function
> nc_put_vara_uchar(). But I when open the NetCDF file with
> ncgen, I find that all integers with values greater than 128
> are changed to -127. Could you kindly tell me is there
> any method to solve this problem? I will be very grateful
> to you. Thanks!

I assume you meant that you opened the file with ncdump rather than
ncgen?  (The ncdump program shows the values in a netCDF file, but
ncgen merely converts a CDL file into a binary netCDF file or
generates a C or Fortran program that creates an equivalent netCDF
file.)

When 8-bit data is stored in a netCDF file, it is just stored as an
8-bit integer.  In particular there is no netCDF type for "signed
8-bit integer" versus "unsigned 8-bit integer" as there is in C,
partly because some of the languages that read and write netCDF data,
for example Fortran, have no type corresponding to unsigned integer
data.  As it says in the netCDF User's Guide (section 3.1 netCDF
external data types):

  It is possible to interpret byte data as either signed (-128 to 127)
  or unsigned (0 to 255). However, when reading byte data to be
  converted into other numeric types, it is interpreted as signed.

The ncdump program similarly displays integer data as signed, since
there is no signed or unsigned property associated with the data in
the file.  However, you can still access the data as unsigned by using
the function nc_get_vara_uchar(), and you will get back exactly the
same unsigned values that you stored with nc_put_vara_uchar().

--Russ

_____________________________________________________________________

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