Re: 19990924: nc_inq_var of netcdf

>To: address@hidden
>From: address@hidden
>Subject: nc_inq_var of netcdf
>Organization: .
>Keywords: 199909241257.GAA06898

Hi Thomas,

> I am working with the 32-bit version of your netcdf.dll on Windows NT.
> nc_inq_libvers brings "VERSION of Mar 17 1998 16:25:22 $".
> Writing C++ code under Borland C++ Builder I have a problem with function 
> nc_inq_var: the varId that is passed as value is overwritten, see below.
> Could you provide a solution or explanation to this problem?

I am unable to duplicate the problem, and suspect something else is
happening to make you think that varId is overwritten, but varId
cannot be overwritten by a function to which you only pass its value
rather than its address.

> 
> static int GetNcValue( const String andiAttr,
>                        int          cdfId,
>                        double      *pAndiDbl )
> {
>     int     err;
>     int     varId;
>     nc_type type;
>     int     dims;
> 
>     err = nc_inq_varid( cdfId, andiAttr.c_str(), &varId );
> >>>>>>>>>>>>>>>> varId == 3
>     if (NC_NOERR != err)
>     {
>         return( err );
>     }
>     err = nc_inq_var( cdfId,
>                       varId,
>                       NULL,     // name
>                       &type,
>                       &dims,
>                       NULL,     // dimids
>                       NULL );   // natts
> >>>>>>>>>>>>>>>> varId == 0

Passing the argument "varId" to any function cannot change its
value, since only a copy of the value of varId is passed to the
function.  You would have to pass "&varId" as a parameter to the
function for it to overwrite the value of varId.  Even then,
nc_inq_var does not overwrite its second parameter.

I have appended a program that creates a netCDF file and runs code
similar to what you have provided, printing the value of "varId"
before and after the call to nc_inq_var(), and the value is unchanged.
Could you compile this, link it against netcdf.dll you have, and
verify that it behaves as expected, producing the following output:

 varId = 5
 after nc_inq_var(), varId = 5

So I'm afraid I cannot explain how you could be seeing the value of
varId changed by a call to c_inq_var().

Incidentally, if you are using C++, you might find the C++ interface
to netCDF easier to use.  It detects more errors at compile time than
the C interface.

--Russ

_____________________________________________________________________

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

#include "netcdf.h"
#include <stdio.h>

static int GetNcValue( const char *varName,
                       int          cdfId,
                       double      *pAndiDbl )
{
    int     err;
    int     varId;
    nc_type type;
    int     dims;

    err = nc_inq_varid( cdfId, varName, &varId );
    printf( "varId = %d\n", varId);
    if (NC_NOERR != err)
    {
        return( err );
    }
    err = nc_inq_var( cdfId,
                      varId,
                      NULL,     // name
                      &type,
                      &dims,
                      NULL,     // dimids
                      NULL );   // natts
    printf ("after nc_inq_var(), varId = %d\n", varId);
    if (NC_NOERR != err)
    {
        return( err );
    }
    if (NC_CHAR == type)    // this item is no numeric value
    {
      printf("expected numeric value\n");
        return( -1 );
    }
    if (0 != dims)          // this is an array instead of a single value
    {
      printf("expected scalar\n");
        return( -2 );
    }

    // get the the value as double
    err = nc_get_var_double( cdfId, varId, pAndiDbl );
    if (NC_NOERR != err)
    {
        return( err );
    }

    return( 0 );

}



int
main()
{
   int  status;
   int  ncid;

   /* dimension ids */
   int  lat_dim, lon_dim, frtime_dim, timelen_dim;

   /* variable ids */
   int  P_id, lat_id, lon_id, frtime_id, reftime_id, scalarv_id;

   /* variable shapes */
   int dims[3];

   /* containers for scalar attributes */
   float  float_val;
   double  double_val;

   /* attribute vectors */
   float  P_valid_range[2];

   status = nc_create("example.nc", NC_CLOBBER,&ncid);

   /* define dimensions */
   status       = nc_def_dim(ncid, "lat",               4L ,                    
&lat_dim);
   status       = nc_def_dim(ncid, "lon",               3L,                     
        &lon_dim);
   status       = nc_def_dim(ncid, "frtime",    NC_UNLIMITED,   &frtime_dim);
   status       = nc_def_dim(ncid, "timelen",   20L,                    
&timelen_dim);

   /* define variables and assign attributes */

   dims[0] = frtime_dim;
   dims[1] = lat_dim;
   dims[2] = lon_dim;

   status  = nc_def_var(ncid, "P", NC_FLOAT, 3, dims, &P_id);

   nc_put_att_text(ncid, P_id, "long_name", 24, "pressure at maximum wind");
   nc_put_att_text(ncid, P_id, "units",         12, "hectopascals");

   P_valid_range[0] = 0;
   P_valid_range[1] = 1500;
   nc_put_att_float(ncid, P_id, "valid_range", NC_FLOAT , 2 , P_valid_range);

   float_val = -9999;
   nc_put_att_float(ncid, P_id, "_FillValue",  NC_FLOAT,  1 , &float_val);

   dims[0] = lat_dim;
   status  = nc_def_var(ncid, "lat", NC_FLOAT, 1, dims, &lat_id);
   nc_put_att_text(ncid, lat_id, "long_name",   8,  "latitude");
   nc_put_att_text(ncid, lat_id, "units",               13, "degrees_north");

   dims[0] = lon_dim;
   status = nc_def_var(ncid, "lon", NC_FLOAT, 1, dims , &lon_id);
   nc_put_att_text(ncid, lon_id, "long_name",    9, "longitude");
   nc_put_att_text(ncid, lon_id, "units",               12, "degrees_east");

   dims[0] = frtime_dim;
   status = nc_def_var(ncid, "frtime", NC_LONG, 1, dims , &frtime_id);
   nc_put_att_text(ncid, frtime_id, "long_name",        13, "forecast time");
   nc_put_att_text(ncid, frtime_id, "units",             5, "hours");

   dims[0] = timelen_dim;
   status = nc_def_var(ncid, "reftime", NC_CHAR, 1, dims , &reftime_id);
   nc_put_att_text(ncid, reftime_id, "long_name", 14,"reference time");
   nc_put_att_text(ncid, reftime_id, "units",      9,"text_time");

   status = nc_def_var(ncid, "scalarv", NC_DOUBLE, 0, 0 , &scalarv_id);
   double_val = 1;
   nc_put_att_double(ncid, scalarv_id, "scalar_att", NC_DOUBLE, 1, &double_val);

   /* Global attributes */
   nc_put_att_text(ncid, NC_GLOBAL, "history", 41,"created by Unidata LDM from 
NPS broadcast");
   nc_put_att_text(ncid, NC_GLOBAL, "title",   48,"NMC Global Product Set: 
Pressure at Maximum Wind");

   /* leave define mode */
   nc_enddef(ncid);
  
   {                    /* store lat */
    static unsigned int lat_start[] = {0};
    static unsigned int lat_edges[] = {4};
    static float lat[] = {-90, -87.5, -85, -82.5};
    nc_put_vara_float(ncid, lat_id, lat_start, lat_edges, lat);
   }

   {                    /* store lon */
    static unsigned int lon_start[] = {0};
    static unsigned int lon_edges[] = {3};
    static float lon[] = {-180, -175, -170};
    nc_put_vara_float(ncid, lon_id, lon_start, lon_edges, lon);
   }

   {                    /* store frtime */
    static unsigned int frtime_start[] = {0};
    static unsigned int frtime_edges[] = {1};
    static long frtime[] = {12};
    nc_put_vara_long(ncid, frtime_id, frtime_start, frtime_edges,frtime);
   }

   {                    /* store frtime */
    static unsigned int frtime_start[] = {1};
    static unsigned int frtime_edges[] = {1};
    static long frtime[] = {18};
    nc_put_vara_long(ncid, frtime_id, frtime_start, frtime_edges,frtime);
   }

   {                    /* store reftime */
    static unsigned int reftime_start[] = {0};
    static unsigned int reftime_edges[] = {20};
    static char reftime[] = {"1992 03 04 12:00"};
    nc_put_vara_text(ncid, reftime_id, reftime_start, reftime_edges,reftime);
   }

   {                    /* store P */
    static unsigned int P_start[] = {0, 0, 0};
    static unsigned int P_edges[] = {2, 4, 3};
    static float P[] = {
        950, 951, 952, 953, 954, 955, 956, 957, 958, 959, 960, 961,
        962, 963, 964, 965, 966, 967, 968, 969, 970, 971, 972, 973
      };
    nc_put_vara_float(ncid, P_id, P_start, P_edges, P);
   }

   {                    /* store scalarv */
    static double scalarv = {3.14159265358979};
    nc_put_var1_double(ncid, scalarv_id, (unsigned int *)0, &scalarv);
    }

   double dval;
   status = GetNcValue( "scalarv", ncid, &dval );

   nc_close (ncid);
   return 0;
}