interactions

[Jason Stover]

To have a glm procedure, pspp needs a data structure to handle
interactions. An interaction can be thought of as another variable
which is a function of two or more variables, usually categorical,
like this:

     Variable 1         Variable 2        Interaction
         A                 B                 AB
         E                 B                 EB
         A                 C                 AC
         E                 C                 EC
         

...etc. The interaction term could be created in one of two ways:
Either 1) create a new variable in the dictionary that corresponds to
the interaction, or 2) create a new 'interaction' data structure
that contains all necessary mappings between existing variables and
the value of the interaction.

Approach 1 would add a variable to the dictionary, but would not
create any more observations in the data set. It would make coding any
procedures that use interactions easier than approach 2, because doing
so would mean the procedure doesn't need to know about much special
code to handle interactions. It would also prevent the need for having
any more obscure string-values-to-binary-vector code like that in
category.[ch]. Approach 1 would still require the creation of some
code to create the interaction, though it may not require the creation
of a specialized "interaction" data structure to be available for use
by all procedures.

Approach 2 doesn't require adding anything to the dictionary, but it
does mean that any procedures that need to use interactions would have
to create those interactions themselves. These interactions would
therefore be lost after the procedure exits, meaning that any other
procedure that needs interactions would have to recreate
them. Approach 2 also means writing more code that partly duplicates
the code already in category.[ch].

I favor approach number 1, but before I fiddle with the
dictionary, I thought I should ask.

-Jason