Re: [patch #6071] Resing/Deleting/Inserting variables in GUI

[John Darrington]

On Tue, Jul 10, 2007 at 07:03:36AM -0700, Ben Pfaff wrote:

     > I don't think so.  axis_get_cnt returns the number of columns, not the
     > highest column number ?
     
     Right.  But 
             datasheet_delete_columns (ds, datasheet_get_column_cnt (ds) - 1, 1)
     and
             datasheet_delete_columns (ds, 0, datasheet_get_column_cnt (ds))
     should be valid, right?


You're probably right.  I'll think about it more carefully.
     
     > The problem with dict_compact is that it can change the order of the
     > case indices, which would put the datasheet and the dictionary out of
     > sync.
     
     Hmm.  One solution to that would be to keep the datasheet columns
     and the dictionary variables in the same order, but I see your
     point.
     
     > I thought about this for a while and concluded that the correct
     > solution would be to use libpspp/tower.c to maintain the case indices
     > for dictionary.c  Then dict_compact becomes obsolete.  I just wasn't
     > sure if it was worth the effort.   What do you think ?
     
     That's an interesting idea but I don't think it pays off for two
     reasons.  First, the cost of getting the level (case_index) of a
     tower node is O(lg n) not O(1).

On the other hand, dict_compact_values is O(n) ...

                             Second, in the datasheet
     case we do want deletions and resizings, etc., to have an
     immediate effect on each case_index, but not in many other
     cases.  For example if I read a system file dictionary with
     sfm_open_reader, and delete some variables from that dictionary,
     I don't want the dictionary's case_indexes to be adjusted because
     the cases coming from the system file would then have their
     variables in positions that I don't expect them to be in.

I see.  So unless we're going to somehow make the dictionary aware of
the capabilities of the data source with which it's associated (which
sounds like a nightmare to me) then we have to compromise.  However,
there may other advantages to keeping the case indices monotonic with
respect to the dict indices, and accepting the O(n) overhead of
dict_compact_values (or dict_pad_values) with every
insertion/deletion.

I'll see if I can come up with a better patch.

J'

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