netcdf-c/examples/C/pres_temp_4D_rd.c

/*! \file
Example program for reading a 4D netCDF file

\ingroup tutorial

This is an example which reads some 4D pressure and temperatures. The
data file read by this program is produced by the companion program
pres_temp_4D_wr.c. It is intended to illustrate the use of the netCDF
C API.

This is part of the netCDF package. Copyright 2006-2011 University
Corporation for Atmospheric Research/Unidata. See COPYRIGHT file for
conditions of use. Full documentation of the netCDF can be found at
http://www.unidata.ucar.edu/software/netcdf/docs.

*/

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

/* This is the name of the data file we will read. */
#define FILE_NAME "pres_temp_4D.nc"

/* We are reading 4D data, a 2 x 6 x 12 lvl-lat-lon grid, with 2
   timesteps of data. */
#define NDIMS 4
#define NLAT 6
#define NLON 12
#define LAT_NAME "latitude"
#define LON_NAME "longitude"
#define NREC 2
#define REC_NAME "time"
#define LVL_NAME "level"
#define NLVL 2

/* Names of things. */
#define PRES_NAME "pressure"
#define TEMP_NAME "temperature"
#define UNITS "units"
#define DEGREES_EAST "degrees_east"
#define DEGREES_NORTH "degrees_north"

/* 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;

   /* The start and count arrays will tell the netCDF library where to
      read our data. */
   size_t start[NDIMS], count[NDIMS];

   /* Program variables to hold the data we will read. We will only
      need enough space to hold one timestep of data; one record. */
   float pres_in[NLVL][NLAT][NLON];
   float temp_in[NLVL][NLAT][NLON];

   /* These program variables hold the latitudes and longitudes. */
   float lats[NLAT], lons[NLON];

   /* Loop indexes. */
   int lvl, lat, lon, rec, i = 0;
   
   /* Error handling. */
   int retval;

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

   /* 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[0])))
      ERR(retval);
   if ((retval = nc_get_var_float(ncid, lon_varid, &lons[0])))
      ERR(retval);

   /* Check the coordinate variable data. */
   for (lat = 0; lat < NLAT; lat++)
      if (lats[lat] != START_LAT + 5.*lat)
	 return 2;
   for (lon = 0; lon < NLON; lon++)
      if (lons[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 one timestep. */
   count[0] = 1;
   count[1] = NLVL;
   count[2] = NLAT;
   count[3] = NLON;
   start[1] = 0;
   start[2] = 0;
   start[3] = 0;

   /* Read and check one record at a time. */
   for (rec = 0; rec < NREC; rec++)
   {
      start[0] = rec;
      if ((retval = nc_get_vara_float(ncid, pres_varid, start, 
				      count, &pres_in[0][0][0])))
	 ERR(retval);
      if ((retval = nc_get_vara_float(ncid, temp_varid, start,
				      count, &temp_in[0][0][0])))
	 ERR(retval);

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

   } /* next record */

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

   printf("*** SUCCESS reading example file pres_temp_4D.nc!\n");
   return 0;
}