Hi netCDF team
This email is rather long, so please bear with me ...
The short read and main question is:
How to find the full dimension names (paths with groups) for all dimensions
that a variable has?
Example:
Note: Incomplete CDL syntax
group: g16 {
dimensions:
lon1=4; //dimension that has a coordinate variable down in scope at
/g16/g16g1/lon1(lon1)
group: g16g1 {
variables:
float lon1(lon1); //coordinate variable /g16/g16g1/lon1 that has
dimension (/g16/lon1) in scope
float lon1_var(lon1); // variable /g16/g16g1/lon1_var that has dimension
(/g16/lon1) in scope *and* coordinate (/g16/g16g1/lon1) in scope
data:
lon1=0.,1.,2.,3.;
lon1_var=0.,1.,2.,3.;
Note that coordinate variables can share dimensions; here's a case of a
"parallel" group /g16/g16g2/ of /g16/g16g1/
where variables have their own local coordinate variable that share the
ancestor dimension (/g16/lon1)
group: g16g2 {
variables:
//coordinate variable (/g16/lon1)
float lon1(lon1);
float lon1_var(lon1);
It is possible to construct other cases, variables with n dimensions, each one
defined in different groups (and each one of these dimensions can have
coordinate
variables in *other* different groups )
More broadly, I am trying to construct a model for ncks of a netCDF4 file that
includes :
1) A list of all "objects" in the file
I call an "object" what I call an object in HDF5: either a group or a variable
(a variable is commonly called in HDF5 a "dataset" ).
2) netCDF4 has dimensions. HDF5 does not (Let's ignore HDF5 dimension scales
for now, to keep this simple... Coincidently netCDF4 *happens* to use HDF5
dimension scales in its inner model, but my understanding is that it did not
had to be that way... I think. Imagine for example that HDF5 dimension scales
did not exist...
It would be perfectly possible for netCDF4 to use HDF5 as the underlying
format... HDF5 dimension scales are not part of the HDF5 format, they
are just an abstraction layer build above HDF5 with a so called "High Level"
API.... At the time the requirement was for HDF5 to have
the equivalent of HDF(4) "coordinate variables", that could be shared between
HDF5 datasets)
excellent article about dimension scales
http://www.unidata.ucar.edu/blogs/developer/en/entry/netcdf4_shared_dimensions
Let's call these netCDF4 dimensions, "unique dimensions".
These are defined in groups.
3) This model stores *full names* of things: full names, for groups, variables
and unique dimensions. Also, full names for coordinate variables.
4) Coordinate variables.
>From the netCDF manual
"It is legal for a variable to have the same name as a dimension. Such
variables have no
special meaning to the netCDF library. However there is a convention that such
variables
should be treated in a special way by software using this library.
A variable with the same name as a dimension is called a coordinate variable."
Dimensions and coordinate variables are used by variables. So, variables must
know where dimensions and coordinate variables (if existent for that variable)
are.
Example of an output, that prints either a dimension or a coordinate variable
for any variable
/g16/g16g1/lon1 ---> coordinate variable
lon1[0]=0
lon1[1]=1
lon1[2]=2
lon1[3]=3
/g16/g16g1/lon1_var ---> variable with coordinate variable
lon1[0]=0 lon1_var[0]=0
lon1[1]=1 lon1_var[1]=1
lon1[2]=2 lon1_var[2]=2
lon1[3]=3 lon1_var[3]=3
The API function that returns a dimension name for a variable is
>From the netCDF C manual
http://www.unidata.ucar.edu/software/netcdf/docs/netcdf-c/nc_005finq_005fdim-Family.html#nc_005finq_005fdim-Family
int nc_inq_dimname (int ncid, int dimid, char *name);
ncid NetCDF ID, from a previous call to nc_open or nc_create.
dimid Dimension ID, from a previous call to nc_inq dimid or nc_def_dim.
name Returned dimension name.
Note: here "ncid" is actually a "location" ID (either a group or the main
netCDF file ID), so I think you should change this in the documentation
The "dimid" parameter is an ID of a dimension.
This is obtained with the API function
int nc_inq_dimid (int ncid, const char *name, int *dimidp);
ncid NetCDF ID, from a previous call to nc_open or nc_create.
name Dimension name.
dimidp Pointer to location for the returned dimension ID.
>From the manual:
"When searching for a dimension, the specified group is searched, and then its
parent group,
and then its grandparent group, etc., up to the root group."
Ok, great, the dimension ID "dimidp" can be in a ancestor group, but how to
know where?
My understanding is that netCDF4 group IDs are "unique"; dimension IDs are not,
they can have duplicated values in several groups.
In the above call nc_inq_dimid, dimension IDs in ancestor groups are returned,
but duplicates may happen.
I think storing IDs, even unique group IDs, in the model above is a recipe for
disaster.
I see IDs as an equivalent of the paper ticket number I am given when I take
the train and want to keep my luggage at a station for a while.
When I get my bags back, I dispose the ticket number.
That ticket is helpful for the person that has to identify my bags only.
As a developer, for debugging purposes, or even as a netCDF4 user, it is also
much easier to identify something by name than by ID.
Possible ways to solve this (to get full dimension name for a variable):
1) Iterate ancestor groups, get all variables for each group, get variables
dimension IDs, and compare with group dimension Ids ?
2) Iterate ancestor groups, try to construct a possible full dimension name and
match ?
Below is some code sample that tries to solve this using option 2) above,
But as a netCDF API user, I don't think that I should have to do this, mainly
because it could just be wrong (it could not cover all cases, for example).
What I think is needed here is a new API function that returns the *full*
dimension names for all dimensions used by a variable, instead of an ID and
relative name only.
With information if that dimension "name" is a coordinate variable or just a
dimension.
Would it be possible for the netCDF group to supply this function?
There is a similar function for groups:
http://www.unidata.ucar.edu/software/netcdf/docs/netcdf-c/nc_005finq_005fgrpname_005ffull.html#nc_005finq_005fgrpname_005ffull
int nc_inq_grpname_full(int ncid, size_t *lenp, char *full_name);
ncid The group id for this operation.
full_name
Pointer to allocated space of correct length.
That returns the *full name* of the group from the group ID, this is one of the
most helpful functions to construct this "full path" model
Thanks for your help
Pedro
------
Pedro Vicente, Earth System Science
University of California, Irvine
http://www.ess.uci.edu/
PS: here's the code that tries to find full dimension names
/* Loop *object* traversal table */
for(unsigned uidx=0;uidx<trv_tbl->nbr;uidx++){
if(trv_tbl->lst[uidx].nco_typ == nco_obj_typ_var){
trv_sct trv=trv_tbl->lst[uidx];
/* Obtain group ID using full group name */
(void)nco_inq_grp_full_ncid(nc_id,trv.grp_nm_fll,&grp_id);
/* Obtain variable ID using group ID */
(void)nco_inq_varid(grp_id,trv.nm,&var_id);
/* Get number of dimensions for variable */
(void)nco_inq_varndims(grp_id,var_id,&nbr_dmn_var);
/* Get dimension IDs for variable */
(void)nco_inq_vardimid(grp_id,var_id,dmn_id_var);
/* Obtain dimension IDs for group. NB: go to parents */
(void)nco_inq_dimids(grp_id,&nbr_dmn_grp,dmn_id_grp,flg_prn);
/* Loop over dimensions of variable */
for(int dmn_idx_var=0;dmn_idx_var<nbr_dmn_var;dmn_idx_var++){
/* Get dimension name */
(void)nco_inq_dimname(grp_id,dmn_id_var[dmn_idx_var],dmn_nm_var);
/* Now the exciting part; we have to locate where "dmn_var_nm" is
located
1) Dimensions are defined in *groups*: find group where variable resides
2) Most common case is for the dimension to be defined in the same
group where variable is
3) If not, we have to traverse the group back until the dimension name
is found
From: "Dennis Heimbigner" <dmh@xxxxxxxxxxxxxxxx>
Subject: Re: [netcdfgroup] defining dimensions in groups
1. The inner dimension is used. The rule is to look up the group tree
from innermost to root and choose the first one that is found
with a matching name.
2. The fact that it is a dimension for a coordinate variable is not
relevant for the
choice.
However, note that this rule is only used by ncgen when disambiguating
a reference
in the CDL. The issue does not come up in the netcdf API because
you have to specifically supply the dimension id when defining the
dimension
for a variable.
4) Use case example: /g5/g5g1/rz variable and rz(rlev), where dimension
"rlev" resides in /g5/rlev
*/
/* Loop over dimensions of group *and* parents */
for(int dmn_idx_grp=0;dmn_idx_grp<nbr_dmn_grp;dmn_idx_grp++){
/* Get dimension name for group */
(void)nco_inq_dimname(grp_id,dmn_id_grp[dmn_idx_grp],dmn_nm_grp);
/* Does dimension name for *variable* match dimension name for
*group* ? */
if(strcmp(dmn_nm_var,dmn_nm_grp) == 0){
