# Get attribute from netCDF file Reads an attribute from a netCDF file. ```r ncatt_get( nc, varid, attname=NA, verbose=FALSE ) ``` ## 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. ## Returns 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. ## References http://dwpierce.com/software ## 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. ## Author(s) David W. Pierce dpierce@ucsd.edu ## See Also `ncatt_put`. ## Examples ```r ## Not run: # 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 ) ## End(Not run) ```