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sfc_pres_temp_rd.c
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1 
14 #include <stdio.h>
15 #include <string.h>
16 #include <netcdf.h>
17 
18 /* This is the name of the data file we will read. */
19 #define FILE_NAME "sfc_pres_temp.nc"
20 
21 /* We are reading 2D data, a 6 x 12 lat-lon grid. */
22 #define NDIMS 2
23 #define NLAT 6
24 #define NLON 12
25 
26 #define LAT_NAME "latitude"
27 #define LON_NAME "longitude"
28 #define PRES_NAME "pressure"
29 #define TEMP_NAME "temperature"
30 
31 /* These are used to calculate the values we expect to find. */
32 #define SAMPLE_PRESSURE 900
33 #define SAMPLE_TEMP 9.0
34 #define START_LAT 25.0
35 #define START_LON -125.0
36 
37 /* For the units attributes. */
38 #define UNITS "units"
39 #define PRES_UNITS "hPa"
40 #define TEMP_UNITS "celsius"
41 #define LAT_UNITS "degrees_north"
42 #define LON_UNITS "degrees_east"
43 #define MAX_ATT_LEN 80
44 
45 /* Handle errors by printing an error message and exiting with a
46  * non-zero status. */
47 #define ERR(e) {printf("Error: %s\n", nc_strerror(e)); return 2;}
48 
49 int
50 main()
51 {
52  int ncid, pres_varid, temp_varid;
53  int lat_varid, lon_varid;
54 
55  /* We will read surface temperature and pressure fields. */
56  float pres_in[NLAT][NLON];
57  float temp_in[NLAT][NLON];
58 
59  /* For the lat lon coordinate variables. */
60  float lats_in[NLAT], lons_in[NLON];
61 
62  /* To check the units attributes. */
63  char pres_units_in[MAX_ATT_LEN], temp_units_in[MAX_ATT_LEN];
64  char lat_units_in[MAX_ATT_LEN], lon_units_in[MAX_ATT_LEN];
65 
66  /* We will learn about the data file and store results in these
67  program variables. */
68  int ndims_in, nvars_in, ngatts_in, unlimdimid_in;
69 
70  /* Loop indexes. */
71  int lat, lon;
72 
73  /* Error handling. */
74  int retval;
75 
76  /* Open the file. */
77  if ((retval = nc_open(FILE_NAME, NC_NOWRITE, &ncid)))
78  ERR(retval);
79 
80  /* There are a number of inquiry functions in netCDF which can be
81  used to learn about an unknown netCDF file. NC_INQ tells how
82  many netCDF variables, dimensions, and global attributes are in
83  the file; also the dimension id of the unlimited dimension, if
84  there is one. */
85  if ((retval = nc_inq(ncid, &ndims_in, &nvars_in, &ngatts_in,
86  &unlimdimid_in)))
87  ERR(retval);
88 
89  /* In this case we know that there are 2 netCDF dimensions, 4
90  netCDF variables, no global attributes, and no unlimited
91  dimension. */
92  if (ndims_in != 2 || nvars_in != 4 || ngatts_in != 0 ||
93  unlimdimid_in != -1) return 2;
94 
95  /* Get the varids of the latitude and longitude coordinate
96  * variables. */
97  if ((retval = nc_inq_varid(ncid, LAT_NAME, &lat_varid)))
98  ERR(retval);
99  if ((retval = nc_inq_varid(ncid, LON_NAME, &lon_varid)))
100  ERR(retval);
101 
102  /* Read the coordinate variable data. */
103  if ((retval = nc_get_var_float(ncid, lat_varid, &lats_in[0])))
104  ERR(retval);
105  if ((retval = nc_get_var_float(ncid, lon_varid, &lons_in[0])))
106  ERR(retval);
107 
108  /* Check the coordinate variable data. */
109  for (lat = 0; lat < NLAT; lat++)
110  if (lats_in[lat] != START_LAT + 5.*lat)
111  return 2;
112  for (lon = 0; lon < NLON; lon++)
113  if (lons_in[lon] != START_LON + 5.*lon)
114  return 2;
115 
116  /* Get the varids of the pressure and temperature netCDF
117  * variables. */
118  if ((retval = nc_inq_varid(ncid, PRES_NAME, &pres_varid)))
119  ERR(retval);
120  if ((retval = nc_inq_varid(ncid, TEMP_NAME, &temp_varid)))
121  ERR(retval);
122 
123  /* Read the data. Since we know the contents of the file we know
124  * that the data arrays in this program are the correct size to
125  * hold all the data. */
126  if ((retval = nc_get_var_float(ncid, pres_varid, &pres_in[0][0])))
127  ERR(retval);
128  if ((retval = nc_get_var_float(ncid, temp_varid, &temp_in[0][0])))
129  ERR(retval);
130 
131  /* Check the data. */
132  for (lat = 0; lat < NLAT; lat++)
133  for (lon = 0; lon < NLON; lon++)
134  if (pres_in[lat][lon] != SAMPLE_PRESSURE + (lon * NLAT + lat) ||
135  temp_in[lat][lon] != SAMPLE_TEMP + .25 * (lon * NLAT + lat))
136  return 2;
137 
138  /* Each of the netCDF variables has a "units" attribute. Let's read
139  them and check them. */
140  if ((retval = nc_get_att_text(ncid, lat_varid, UNITS, lat_units_in)))
141  ERR(retval);
142  if (strncmp(lat_units_in, LAT_UNITS, strlen(LAT_UNITS)))
143  return 2;
144 
145  if ((retval = nc_get_att_text(ncid, lon_varid, UNITS, lon_units_in)))
146  ERR(retval);
147  if (strncmp(lon_units_in, LON_UNITS, strlen(LON_UNITS)))
148  return 2;
149 
150  if ((retval = nc_get_att_text(ncid, pres_varid, UNITS, pres_units_in)))
151  ERR(retval);
152  if (strncmp(pres_units_in, PRES_UNITS, strlen(PRES_UNITS)))
153  return 2;
154 
155  if ((retval = nc_get_att_text(ncid, temp_varid, UNITS, temp_units_in)))
156  ERR(retval);
157  if (strncmp(temp_units_in, TEMP_UNITS, strlen(TEMP_UNITS))) return 2;
158 
159  /* Close the file. */
160  if ((retval = nc_close(ncid)))
161  ERR(retval);
162 
163  printf("*** SUCCESS reading example file sfc_pres_temp.nc!\n");
164  return 0;
165 }
Main header file for the C API.
EXTERNL int nc_inq(int ncid, int *ndimsp, int *nvarsp, int *nattsp, int *unlimdimidp)
Inquire about a file or group.
Definition: dfile.c:1817
#define NC_NOWRITE
Set read-only access for nc_open().
Definition: netcdf.h:123
EXTERNL int nc_close(int ncid)
Close an open netCDF dataset.
Definition: dfile.c:1470
EXTERNL int nc_get_att_text(int ncid, int varid, const char *name, char *ip)
Get an attribute.
Definition: dattget.c:187
EXTERNL int nc_open(const char *path, int mode, int *ncidp)
Open an existing netCDF file.
Definition: dfile.c:828
EXTERNL int nc_inq_varid(int ncid, const char *name, int *varidp)
Find the ID of a variable, from the name.
Definition: dvarinq.c:62
int nc_get_var_float(int ncid, int varid, float *ip)
Read an entire variable in one call.
Definition: dvarget.c:1087

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