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209 lines
6.6 KiB
C
209 lines
6.6 KiB
C
/* Copyright 2019 University Corporation for Atmospheric
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Research/Unidata. See COPYRIGHT file for conditions of use. */
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/**
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* @file
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* @ingroup tutorial
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* A more complex example of writing a netCDF file.
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*
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* This is an example program which writes some 4D pressure and
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* temperatures. It is intended to illustrate the use of the netCDF C
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* API. The companion program pres_temp_4D_rd.c shows how to read the
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* netCDF data file created by this program.
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*
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* @author Ed Hartnett
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*/
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#include <stdio.h>
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#include <string.h>
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#include <netcdf.h>
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/* This is the name of the data file we will create. */
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#define FILE_NAME "pres_temp_4D.nc"
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/* We are writing 4D data, a 2 x 6 x 12 lvl-lat-lon grid, with 2
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timesteps of data. */
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#define NDIMS 4
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#define NLAT 6
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#define NLON 12
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#define LAT_NAME "latitude"
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#define LON_NAME "longitude"
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#define NREC 2
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#define REC_NAME "time"
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#define LVL_NAME "level"
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#define NLVL 2
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/* Names of things. */
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#define PRES_NAME "pressure"
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#define TEMP_NAME "temperature"
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#define UNITS "units"
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#define DEGREES_EAST "degrees_east"
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#define DEGREES_NORTH "degrees_north"
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/* These are used to construct some example data. */
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#define SAMPLE_PRESSURE 900
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#define SAMPLE_TEMP 9.0
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#define START_LAT 25.0
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#define START_LON -125.0
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/* For the units attributes. */
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#define UNITS "units"
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#define PRES_UNITS "hPa"
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#define TEMP_UNITS "celsius"
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#define LAT_UNITS "degrees_north"
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#define LON_UNITS "degrees_east"
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#define MAX_ATT_LEN 80
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/* Handle errors by printing an error message and exiting with a
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* non-zero status. */
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#define ERR(e) {printf("Error: %s\n", nc_strerror(e)); return 2;}
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int
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main()
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{
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/* IDs for the netCDF file, dimensions, and variables. */
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int ncid, lon_dimid, lat_dimid, lvl_dimid, rec_dimid;
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int lat_varid, lon_varid, pres_varid, temp_varid;
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int dimids[NDIMS];
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/* The start and count arrays will tell the netCDF library where to
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write our data. */
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size_t start[NDIMS], count[NDIMS];
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/* Program variables to hold the data we will write out. We will only
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need enough space to hold one timestep of data; one record. */
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float pres_out[NLVL][NLAT][NLON];
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float temp_out[NLVL][NLAT][NLON];
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/* These program variables hold the latitudes and longitudes. */
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float lats[NLAT], lons[NLON];
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/* Loop indexes. */
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int lvl, lat, lon, rec, i = 0;
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/* Error handling. */
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int retval;
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/* Create some pretend data. If this wasn't an example program, we
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* would have some real data to write, for example, model
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* output. */
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for (lat = 0; lat < NLAT; lat++)
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lats[lat] = START_LAT + 5.*lat;
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for (lon = 0; lon < NLON; lon++)
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lons[lon] = START_LON + 5.*lon;
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for (lvl = 0; lvl < NLVL; lvl++)
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for (lat = 0; lat < NLAT; lat++)
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for (lon = 0; lon < NLON; lon++)
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{
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pres_out[lvl][lat][lon] = SAMPLE_PRESSURE + i;
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temp_out[lvl][lat][lon] = SAMPLE_TEMP + i++;
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}
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/* Create the file. */
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if ((retval = nc_create(FILE_NAME, NC_CLOBBER, &ncid)))
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ERR(retval);
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/* Define the dimensions. The record dimension is defined to have
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* unlimited length - it can grow as needed. In this example it is
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* the time dimension.*/
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if ((retval = nc_def_dim(ncid, LVL_NAME, NLVL, &lvl_dimid)))
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ERR(retval);
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if ((retval = nc_def_dim(ncid, LAT_NAME, NLAT, &lat_dimid)))
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ERR(retval);
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if ((retval = nc_def_dim(ncid, LON_NAME, NLON, &lon_dimid)))
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ERR(retval);
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if ((retval = nc_def_dim(ncid, REC_NAME, NC_UNLIMITED, &rec_dimid)))
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ERR(retval);
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/* Define the coordinate variables. We will only define coordinate
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variables for lat and lon. Ordinarily we would need to provide
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an array of dimension IDs for each variable's dimensions, but
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since coordinate variables only have one dimension, we can
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simply provide the address of that dimension ID (&lat_dimid) and
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similarly for (&lon_dimid). */
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if ((retval = nc_def_var(ncid, LAT_NAME, NC_FLOAT, 1, &lat_dimid,
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&lat_varid)))
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ERR(retval);
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if ((retval = nc_def_var(ncid, LON_NAME, NC_FLOAT, 1, &lon_dimid,
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&lon_varid)))
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ERR(retval);
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/* Assign units attributes to coordinate variables. */
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if ((retval = nc_put_att_text(ncid, lat_varid, UNITS,
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strlen(DEGREES_NORTH), DEGREES_NORTH)))
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ERR(retval);
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if ((retval = nc_put_att_text(ncid, lon_varid, UNITS,
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strlen(DEGREES_EAST), DEGREES_EAST)))
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ERR(retval);
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/* The dimids array is used to pass the dimids of the dimensions of
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the netCDF variables. Both of the netCDF variables we are
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creating share the same four dimensions. In C, the
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unlimited dimension must come first on the list of dimids. */
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dimids[0] = rec_dimid;
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dimids[1] = lvl_dimid;
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dimids[2] = lat_dimid;
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dimids[3] = lon_dimid;
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/* Define the netCDF variables for the pressure and temperature
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* data. */
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if ((retval = nc_def_var(ncid, PRES_NAME, NC_FLOAT, NDIMS,
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dimids, &pres_varid)))
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ERR(retval);
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if ((retval = nc_def_var(ncid, TEMP_NAME, NC_FLOAT, NDIMS,
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dimids, &temp_varid)))
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ERR(retval);
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/* Assign units attributes to the netCDF variables. */
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if ((retval = nc_put_att_text(ncid, pres_varid, UNITS,
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strlen(PRES_UNITS), PRES_UNITS)))
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ERR(retval);
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if ((retval = nc_put_att_text(ncid, temp_varid, UNITS,
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strlen(TEMP_UNITS), TEMP_UNITS)))
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ERR(retval);
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/* End define mode. */
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if ((retval = nc_enddef(ncid)))
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ERR(retval);
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/* Write the coordinate variable data. This will put the latitudes
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and longitudes of our data grid into the netCDF file. */
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if ((retval = nc_put_var_float(ncid, lat_varid, &lats[0])))
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ERR(retval);
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if ((retval = nc_put_var_float(ncid, lon_varid, &lons[0])))
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ERR(retval);
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/* These settings tell netcdf to write one timestep of data. (The
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setting of start[0] inside the loop below tells netCDF which
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timestep to write.) */
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count[0] = 1;
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count[1] = NLVL;
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count[2] = NLAT;
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count[3] = NLON;
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start[1] = 0;
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start[2] = 0;
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start[3] = 0;
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/* Write the pretend data. This will write our surface pressure and
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surface temperature data. The arrays only hold one timestep worth
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of data. We will just rewrite the same data for each timestep. In
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a real application, the data would change between timesteps. */
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for (rec = 0; rec < NREC; rec++)
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{
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start[0] = rec;
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if ((retval = nc_put_vara_float(ncid, pres_varid, start, count,
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&pres_out[0][0][0])))
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ERR(retval);
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if ((retval = nc_put_vara_float(ncid, temp_varid, start, count,
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&temp_out[0][0][0])))
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ERR(retval);
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}
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/* Close the file. */
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if ((retval = nc_close(ncid)))
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ERR(retval);
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printf("*** SUCCESS writing example file %s!\n", FILE_NAME);
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return 0;
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}
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