netcdf-c/examples/C/sfc_pres_temp_rd.c

/* This is part of the netCDF package.
   Copyright 2006 University Corporation for Atmospheric Research/Unidata.
   See COPYRIGHT file for conditions of use.

   This is an example which reads some surface pressure and
   temperatures. The data file read by this program is produced by the
   companion program sfc_pres_temp_wr.c. It is intended to illustrate
   the use of the netCDF C API.

   This program is part of the netCDF tutorial:
   http://www.unidata.ucar.edu/software/netcdf/docs/netcdf-tutorial

   Full documentation of the netCDF C API can be found at:
   http://www.unidata.ucar.edu/software/netcdf/docs/netcdf-c

   $Id: sfc_pres_temp_rd.c,v 1.5 2007/02/14 20:59:21 ed Exp $
*/

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

/* This is the name of the data file we will read. */
#define FILE_NAME "sfc_pres_temp.nc"

/* We are reading 2D data, a 6 x 12 lat-lon grid. */
#define NDIMS 2
#define NLAT 6
#define NLON 12

#define LAT_NAME "latitude"
#define LON_NAME "longitude"
#define PRES_NAME "pressure"
#define TEMP_NAME "temperature"

/* These are used to calculate the values we expect to find. */
#define SAMPLE_PRESSURE 900
#define SAMPLE_TEMP 9.0
#define START_LAT 25.0
#define START_LON -125.0

/* For the units attributes. */
#define UNITS "units"
#define PRES_UNITS "hPa"
#define TEMP_UNITS "celsius"
#define LAT_UNITS "degrees_north"
#define LON_UNITS "degrees_east"
#define MAX_ATT_LEN 80

/* Handle errors by printing an error message and exiting with a
 * non-zero status. */
#define ERR(e) {printf("Error: %s\n", nc_strerror(e)); return 2;}

int
main()
{
   int ncid, pres_varid, temp_varid;
   int lat_varid, lon_varid;

   /* We will read surface temperature and pressure fields. */
   float pres_in[NLAT][NLON];
   float temp_in[NLAT][NLON];

   /* For the lat lon coordinate variables. */
   float lats_in[NLAT], lons_in[NLON];

   /* To check the units attributes. */
   char pres_units_in[MAX_ATT_LEN], temp_units_in[MAX_ATT_LEN];
   char lat_units_in[MAX_ATT_LEN], lon_units_in[MAX_ATT_LEN];

   /* We will learn about the data file and store results in these
      program variables. */
   int ndims_in, nvars_in, ngatts_in, unlimdimid_in;

   /* Loop indexes. */
   int lat, lon;

   /* Error handling. */
   int retval;

   /* Open the file. */
   if ((retval = nc_open(FILE_NAME, NC_NOWRITE, &ncid)))
      ERR(retval);

   /* There are a number of inquiry functions in netCDF which can be
      used to learn about an unknown netCDF file. NC_INQ tells how
      many netCDF variables, dimensions, and global attributes are in
      the file; also the dimension id of the unlimited dimension, if
      there is one. */
   if ((retval = nc_inq(ncid, &ndims_in, &nvars_in, &ngatts_in, 
			&unlimdimid_in)))
      ERR(retval);

   /* In this case we know that there are 2 netCDF dimensions, 4
      netCDF variables, no global attributes, and no unlimited
      dimension. */
   if (ndims_in != 2 || nvars_in != 4 || ngatts_in != 0 || 
       unlimdimid_in != -1) return 2;

   /* Get the varids of the latitude and longitude coordinate
    * variables. */
   if ((retval = nc_inq_varid(ncid, LAT_NAME, &lat_varid)))
      ERR(retval);
   if ((retval = nc_inq_varid(ncid, LON_NAME, &lon_varid)))
      ERR(retval);

   /* Read the coordinate variable data. */
   if ((retval = nc_get_var_float(ncid, lat_varid, &lats_in[0])))
      ERR(retval);
   if ((retval = nc_get_var_float(ncid, lon_varid, &lons_in[0])))
      ERR(retval);

   /* Check the coordinate variable data. */
   for (lat = 0; lat < NLAT; lat++)
      if (lats_in[lat] != START_LAT + 5.*lat)
	 return 2;
   for (lon = 0; lon < NLON; lon++)
      if (lons_in[lon] != START_LON + 5.*lon)
	 return 2;

   /* Get the varids of the pressure and temperature netCDF
    * variables. */
   if ((retval = nc_inq_varid(ncid, PRES_NAME, &pres_varid)))
      ERR(retval);
   if ((retval = nc_inq_varid(ncid, TEMP_NAME, &temp_varid)))
      ERR(retval);

   /* Read the data. Since we know the contents of the file we know
    * that the data arrays in this program are the correct size to
    * hold all the data. */
   if ((retval = nc_get_var_float(ncid, pres_varid, &pres_in[0][0])))
      ERR(retval);
   if ((retval = nc_get_var_float(ncid, temp_varid, &temp_in[0][0])))
      ERR(retval);

   /* Check the data. */
   for (lat = 0; lat < NLAT; lat++)
      for (lon = 0; lon < NLON; lon++)
	 if (pres_in[lat][lon] != SAMPLE_PRESSURE + (lon * NLAT + lat) ||
	     temp_in[lat][lon] != SAMPLE_TEMP + .25 * (lon * NLAT + lat))
	    return 2;

   /* Each of the netCDF variables has a "units" attribute. Let's read
      them and check them. */
   if ((retval = nc_get_att_text(ncid, lat_varid, UNITS, lat_units_in)))
      ERR(retval);
   if (strncmp(lat_units_in, LAT_UNITS, strlen(LAT_UNITS))) 
      return 2;

   if ((retval = nc_get_att_text(ncid, lon_varid, UNITS, lon_units_in)))
      ERR(retval);
   if (strncmp(lon_units_in, LON_UNITS, strlen(LON_UNITS))) 
      return 2;

   if ((retval = nc_get_att_text(ncid, pres_varid, UNITS, pres_units_in)))
      ERR(retval);
   if (strncmp(pres_units_in, PRES_UNITS, strlen(PRES_UNITS)))
      return 2;

   if ((retval = nc_get_att_text(ncid, temp_varid, UNITS, temp_units_in)))
      ERR(retval);
   if (strncmp(temp_units_in, TEMP_UNITS, strlen(TEMP_UNITS))) return 2;

   /* Close the file. */
   if ((retval = nc_close(ncid)))
      ERR(retval);

   printf("*** SUCCESS reading example file sfc_pres_temp.nc!\n");
   return 0;
}