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I'm happy to work on it... though I'm having trouble understanding what the STRING_AS_ARRAY stuff is there for. Is this an attempt to fix the problem? Turning on debugging, I don't see many of its parts being invoked when strings are encountered (such as in NCConnect::parse_array_dims()) -----Original Message----- From: owner-dods-tech@xxxxxxxxxxxxxxxx [mailto:owner-dods-tech@xxxxxxxxxxxxxxxx] On Behalf Of James Gallagher Sent: Thursday, February 23, 2006 4:52 PM To: David Wojtowicz Cc: 'Ethan Davis'; dods-tech@xxxxxxxxxxxxxxxx; support-netcdf-java@xxxxxxxxxxxxxxxx; 'support-thredds' Subject: Re: 20050922:DODS/THREDDS confusion David, It would be fantastic if you could look into fixing this! If you need some help, let me know. After the 6th of March I should be able to really dive into this. James On Feb 23, 2006, at 2:35 PM, David Wojtowicz wrote: > I've pinned the problems down somewhat. > > The actual netCDF file that contains the data on the TDS, has > string data > that looks like this (from dncdump on the local file): > > --------------------------------- > dimensions: > station = 4818; > id_len = 4; > variables: > char station_id(station, id_len) ; > station_id:long_name = "Station id" ; > station_id:reference = "sfmetar_sa.tbl" ; > --------------------------------- > > > > However, the TDS represents it via DAP as (pertinent bits of DDS > and DAS): > > > --------------------------------- > Dataset { > String station_id[station = 4818]; > } > Attributes { > station_id { > String long_name "Station id"; > String reference "sfmetar_sa.tbl"; > DODS { > Int32 strlen 4; > String dimName id_len; > } > } > } > --------------------------------- > > So, the two dimensional character array has been converted to a single > dimensional array of strings, whose length is stored in the nested > attribute > "DODS". > > When you do dncdump on the dods URL, you get: > > --------------------------------- > dimensions: > station = 4818 ; > variables: > char station_id(station) ; > station_id:long_name = "Station id" ; > station_id:reference = "sfmetar_sa.tbl" ; > station_id:DODS > dncdump: Attribute not found (program terminates) > ---------------------------------- > > > Dncdump aborts when it gets to the nested attribute, "DODS" because > a nested > attribute is represented as a container and the Attr_container type is > something that the code in Dattr.cc can't deal with. > > > Now, even if it could deal with nested attributes, there'd still be > the > problem of figuring out what to do with the strings. > > I temporarily hacked around the Attr_container problem and had it > pretend it > was an empty character array....i.e. station_ID:DODS = "" ; > essentially > making it harmless to the rest of the netCDF API. > > dncdump ran through then without aborting, but if I asked it to dump > "station_id" (-v station_id option) it'd get the strings all wrong > because > the second dimension, id_len, isn't there. > > So, there are two problems: > > 1) Can't deal with nested attributes (i.e. containers) > 2) Can't deal with DAP strings > > > Ideally the output from the DAP network version should look just > like the > output from the local file version... the differences become > transparent. > > This would involve fixing the attribute handling code to not pass > through > the nested attribute containers to the netCDF API (like the Java > version > does). > > It would also involve converting the string arrays back to two- > dimensional > character arrays as far as the netCDF API is concerned. > > > I've already been hacking around in the code and thinking about > this for a > while. If you'd like I could take a first shot at it. > > > ----- > David Wojtowicz, Sr. Research Programmer, Sysadmin > Dept of Atmospheric Sciences / Computer Services > University of Illinois at Urbana-Champaign > davidw@xxxxxxxx (217) 333-8390 > > -----Original Message----- > From: James Gallagher [mailto:jhrg@xxxxxxx] > Sent: Thursday, February 23, 2006 10:14 AM > To: Ethan Davis > Cc: David Wojtowicz; dods-tech@xxxxxxxxxxxxxxxx; > support-netcdf-java@xxxxxxxxxxxxxxxx; 'support-thredds' > Subject: Re: 20050922:DODS/THREDDS confusion > > This is in Trac as ticket 320. I'll try to get a fix out by the 6th > of March. > > James > > On Feb 23, 2006, at 7:02 AM, Ethan Davis wrote: > >> Hi David, >> >> Yeah, this has been an issue for some time (seems to have started >> on this list in 2003, http://www.unidata.ucar.edu/support/help/ >> MailArchives/dods-tech/msg01910.html). >> >> The underlying issue is that netCDF-3 doesn't understand String, it >> only knows char and arrays of char, and OPeNDAP knows String but >> not char. So, the mapping between netCDF-3 and OPeNDAP is a bit >> thorny. The current nc servers map char arrays to String arrays (so >> each String contains one character). The TDS maps char arrays into >> Strings. The problem is that the nc client library doesn't know how >> to map Strings back into char arrays just from the DDS/DAS because >> the length of Strings can vary. (The TDS contains some hints in the >> DAS, that is how the netCDF-java library knows how to deal with >> them.) >> >> I don't know if this comes up anywhere else. But it will be an >> issue for any OPeNDAP dataset that contains String arrays. Do any >> of the other servers map things to String arrays? >> >> Sorry I don't actually have any answers but since this is in some >> ways an issue for the TDS, I'll look into this some more. >> >> Ethan > > -- > James Gallagher jgallagher at opendap.org > OPeNDAP, Inc 406.723.8663 > > -- James Gallagher jgallagher at opendap.org OPeNDAP, Inc 406.723.8663
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