ncdf4 (version 1.23)

ncatt_get: Get attribute from netCDF file

Description

Reads an attribute from a netCDF file.

Usage

ncatt_get( nc, varid, attname=NA, verbose=FALSE )

Value

If an attribute name is supplied (i.e., argument attname is given), this returns a list with two components, "hasatt" and "value". "hasatt" is TRUE if the named attribute was found, and FALSE otherwise. "value" is the (possibly vector) value of the attribute. If the on-disk type of the attribute is short or integer, then an integer value is returned. If the on-disk type is float or double, than a double value is returned. If the on-disk type is character, than a character string is returned.

If no attribute name is supplied, then this returns a list containing ALL the attributes for the specified variable along with their associated values. For example, if attlist is the list returned by this call, then names(attlist) shows all the attributes defined for the variable, and attlist[[N]] is the value of the N'th attribute.

Arguments

nc

An object of class ncdf4 (as returned from nc_open), indicating what file to read from.

varid

The variable whose attribute is to be read. Can be a character string with the variable's name or an object of class ncvar4. As a special case, if varid==0, then a global (file) attribute will be read rather than a particular variable's attribute. In netcdf version 4 files, attributes can be stored in a group without an associated variable (as if they were global attributes for the group instead of for the file). In this case, set varid to a string holding the fully qualified group name using forward slashes for subgroups. For example, "group1/metadata".

attname

Name of the attribute to read; if not specified, a list containg ALL attributes of the selected variable or file is returned.

verbose

If TRUE, then debugging information is printed.

Author

David W. Pierce

Details

This function gets an attribute from a netCDF variable (or a global attribute from a netCDF file, if the passed argument "varid" is zero). Multiple attributes are returned in a vector.

References

http://dwpierce.com/software

See Also

ncatt_put.

Examples

if (FALSE) {
# Make a simple netCDF file
filename <- "atttest_types.nc"
dim <- ncdim_def( "X", "inches", 1:12 )
var <- ncvar_def( "Data", "unitless", dim, -1 ) 
ncnew <- nc_create( filename, var )

# Define some attributes of various types
attvaldbl <- 3.1415926536
ncatt_put( ncnew, var, "testatt_dbl", attvaldbl, prec="double" )
attvalfloat <- c(1.0,4.0,9.0,16.0)
ncatt_put( ncnew, var, "testatt_float", attvalfloat )
# varid=0 means it is a global attribute
ncatt_put( ncnew, 0, "globalatt_int", 32000, prec="int" ) 
ncatt_put( ncnew, 0, "globalatt_short", 7, prec="short" )
ncatt_put( ncnew, 0, "description", 
	"this is a test file with attributes of various types")
nc_close(ncnew)

# Now illustrate the use of the ncatt_get function by reading them back in
doitfor <- function( nc, var, attname ) {
	av <- ncatt_get( nc, var, attname )
	if( av$hasatt ) {
		print(paste("File",nc$filename,", var",var,"DOES have attribute",
				attname))
		print(paste("Storage mode:",storage.mode(av$value)))
		print("Attribute value:")
		print(av$value)
	} else {
		print(paste("File",nc$filename,", var",var,"does NOT have",
			"attribute", attname))
		}
}
	
nc <- nc_open( filename )
var <- "Data"
doitfor( nc, var, "testatt_dbl" )
doitfor( nc, var, "testatt_float" )
doitfor( nc, var, "testatt_wacko" )
doitfor( nc, 0,   "globalatt_int" )
doitfor( nc, 0,   "globalatt_short" )
doitfor( nc, 0,   "description" )
nc_close( nc )

# Clean up after our test
file.remove( filename )

}