Values are converted to the appropriate type before being written to a netCDF file. For example, in the commands below, IDL converts the string "12" to a floating-point 12.0 before writing it:
varid=NCDF_VARDEF(fileid, 'VarName', [d0,d1,d2+d3], /FLOAT)
NCDF_VARPUT, fileid, 'VarName', '12'
