Hi John, I've forwarded this question to esupport, because the answer is of more general interest and the question will probably come up again. For that reason, the response may also be a little more elaborate than you're looking for. > if i create a file with NC_FORMAT_CLASSIC: > > and i have a char (type=2) data variable, and i try to write to it with > > nc_put_vars_uchar (grpid, varid, origin, shape, stride, valb) > > or > > nc_put_vars_schar (grpid, varid, origin, shape, stride, valb) > > > i get error: > > NetCDF: Attempt to convert between text & numbers Right, the error message is correct in this case, because you are using a function intended for writing numeric data to write text data, and the API doesn't permit that. This needs a little explanation (and better documentation). The "schar" or "uchar" used as a suffix in the function name refers to the in-memory C type of the data you want to write, not the netCDF type of the variable to which you are writing. The only C declaration for 8-bit integers is "char". There is no "byte" declaration, although "char" can be qualified as signed or unsigned. So if you have some 8-bit numeric data you want to write to a numeric netCDF variable, you must use one of the nc_put_varX_schar() or nc_put_varX_uchar() functions, where X can be "1", "", "a", "s", or "m", depending on what kind of write you intend. Also the netCDF variable you are writing to can be any netCDF numeric type: NC_BYTE, NC_SHORT, NC_INT, NC_FLOAT, or, NC_DOUBLE, or an additional 5 numeric types for netCDF-4 files. The numeric data in the memory array of 8-bit values is converted to the numeric type on disk before it's written. But you can't mix numeric data and character strings, or ask the netCDF library to convert numeric data to characters in a string. If you want to write data to a netCDF variable declared to be of type NC_CHAR, rather than NC_BYTE, you have declared your intention that that variable is only for text data. The functions to use in that case are the nc_put_varX_text() family, and they all require a last argument of type const char*. So the API prevents you from inadvertently writing floats or ints into what is supposed to be an array of chars (unless you do it intentionally with casts). So you need to replace nc_put_vars_uchar (grpid, varid, origin, shape, stride, valb) with nc_put_vars_text (grpid, varid, origin, shape, stride, valb) (which will generally reek havoc on an array of characters being used for a UTF-8 encoded string, if there are any non-ASCII characters in the array and stride is greater than 1). > if i use > > nc4.nc_put_vars_text(grpid, varid, origin, shape, stride, valb); > > i get error: > > NetCDF: Not a valid data type or _FillValue type mismatch > > however, NC_NETCDF4 works ok with nc_put_vars_text and fails on > nc_put_vars_uchar and nc_put_vars_uchar with > > NetCDF: Attempt to convert between text & numbers This error message is also what's intended, for the same reason as above. The "uchar" and "schar" in the function name still only refer to the numeric type declarations of the C data to be written, not the netCDF type of the target variable. > not sure from > > http://www.unidata.ucar.edu/software/netcdf/docs/netcdf-c.html#nc_005fput_005fvars_005f-type > > what is expected for char type. Yes, I think better documentation is needed to explain the allowed conversions that take place when you write (or read) data and the in-memory type differs from the netCDF on-disk type. I thought there used to be a section on type conversions, but now I'm not seeing it. All plausible numeric conversions are supported, but no conversion is permitted between text and numeric values. The intent of the type part of function names also needs better explanation, as the original netCDF-4 code actually had a bunch of read and write functions that used a "_ubyte" suffix which were identical to the existing functions that used a "_uchar" suffix, because of a misunderstanding of which type the suffix referred to. NetCDF client code should not have to know what numeric type is used in a file for numeric data. It can read it all as doubles by just using the functions with the "_double" prefix. > 1) what is uchar and schar used for ?? Only for C functions that deal with 8-bit signed or unsigned values in-memory. If no conversion between memory and disk is desired, then uchar corresponds to the netCDF type NC_UBYTE and schar to NC_BYTE. > 2) it looks like _FillValue must be messedd up, but im not sure why that > happens. The default _FillValue for the three netCDF types NC_CHAR, NC_BYTE, and NC_UBYTE are all different (0, -127, and 255 respectively). Applications such as ncdump don't use the default fill value for NC_BYTE or NC_UBYTE data, it must be set explicitly if a fill value is intended. I'd have to know more about types of everything and context to know what caused the error message above that mentioned _FillValue. --Russ Russ Rew UCAR Unidata Program address@hidden http://www.unidata.ucar.edu Ticket Details =================== Ticket ID: DIG-256328 Department: Support netCDF Priority: Normal Status: Closed
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