Open a netCDF File

Description

Opens an existing netCDF file for reading (or, optionally, writing).

Usage

1
 nc_open( filename, write=FALSE, readunlim=TRUE, verbose=FALSE )

Arguments

filename

Name of the existing netCDF file to be opened.

write

If FALSE (default), then the file is opened read-only. If TRUE, then writing to the file is allowed.

readunlim

When invoked, this function reads in the values of all dimensions from the associated variables. This can be slow for a large file with a long unlimited dimension. If set to FALSE, the values for the unlimited dimension are not automatically read in (they can be read in later, manually, using ncvar_get()).

verbose

If TRUE, then messages are printed out during execution of this function.

Details

This routine opens an existing netCDF file for reading (or, if write=TRUE, for writing). To create a new netCDF file, use nc_create instead.

In addition to simply opening the file, information about the file and its contents is read in and stored in the returned object, which is of class ncdf4. This class has the following user-accessible fields, all of which are read-only: 1) filename, which is a character string holding the name of the file; 2) ndims, which is an integer holding the number of dimensions in the file; 3) nvars, which is an integer holding the number of the variables in the file that are NOT coordinate variables (aka dimensional variables); 4) natts, which is an integer holding the number of global attributes; 5) unlimdimid, which is an integer holding the dimension id of the unlimited dimension, or -1 if there is none; 6) dim, which is a list of objects of class ncdim4; 7) var, which is a list of objects of class ncvar4; 8) writable, which is TRUE or FALSE, depending on whether the file was opened with write=TRUE or write=FALSE.

The concept behind the R interface to a netCDF file is that the ncdf object returned by this function, as well as the list of ncdim objects contained in the ncdf object's "dim" list and the ncvar objects contained in the ncdf object's "var" list, completely describe the netCDF file. I.e., they hold the entire contents of the file's metadata. Therefore, there are no R interfaces to the explicit netCDF query functions, such as "nc_inq_nvars" or "nc_inq_natts". The upshot is, look in the ncdf object or its children to get information about the netCDF file. (Note: the ncdim object is described in the help file for ncdim_def; the ncvar object is described in the help file for ncvar_def).

Value

An object of class ncdf4 that has the fields described above.

Author(s)

David W. Pierce dpierce@ucsd.edu

References

http://dwpierce.com/software

See Also

ncdim_def, ncvar_def.

Examples

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## Not run: 
# Define an integer dimension 
dimState <- ncdim_def( "StateNo", "count", 1:50 )

# Make an integer variable.  Note that an integer variable can have
# a double precision dimension, or vice versa; there is no fixed
# relationship between the precision of the dimension and that of the
# associated variable.  We just make an integer variable here for
# illustration purposes.
varPop <- ncvar_def("Pop", "count", dimState, -1, 
	longname="Population", prec="integer")

# Create a netCDF file with this variable
ncnew <- nc_create( "states_population.nc", varPop )

# Write some values to this variable on disk.
popAlabama <- 4447100
ncvar_put( ncnew, varPop, popAlabama, start=1, count=1 )

# Add source info metadata to file
ncatt_put( ncnew, 0, "source", "Census 2000 from census bureau web site")

nc_close(ncnew)

# Now open the file and read its data
ncold <- nc_open("states_population.nc")
data <- ncvar_get(ncold)
print("here is the data in the file:")
print(data)
nc_close( ncold )

## End(Not run)