Re: interactions

[John Darrington]

I think option 2 is best. 

You will need to use casereader_create_append_numeric to
create space for the interaction's values.

There's a function var_create_internal to create a new variable.
You have to remember to destroy the variable when you've finished with
it.

J'




On Tue, Nov 25, 2008 at 04:28:40PM -0500, Jason Stover wrote:
     I've been thinking about how to code interactions for GLM.
     An "interaction", in terms of a linear model, can be thought
     of as another variable whose values are combinations of values
     of other variables. For example:
     
        var1     var2       interaction
        a        c          ac
        a        d          ad
        b        c          bc
        b        d          bd
     
     An interaction should be represented in the covariance
     matrix. Therefore its values need to be computed during the data pass
     that creates the covariance matrix.
     
     So I'm wondering how to compute the interactions. I thought of three
     approaches:
     
     1. Extending the covariance matrix struct to hold the interaction. In
     this case the interaction may or may not be stored as a pointer to a
     struct variable, I'm not sure. Maybe the best way here is to make an
     interaction structure, and an extension of the covariance matrix
     struct that knows about interactions.
     
     2. Append the variables to "the data" during the data pass, without
     adding the new variables and values to the dictionary. In this
     approach, I was thinking of just creating the new variables and values
     during the data pass, after calling case_data(), then storing them in
     the covariance hash. They would then disappear after the data pass,
     leaving only the needed information in the covariance matrix
     hash. This approach has the advantage of not requiring any big change
     to the covariance matrix struct. It has the possible disadvantage of
     requiring me to make a variable that isn't in the dictionary and
     compute its values. Are there functions to support this kind of thing
     in src/data or src/libpspp (I mean without putting them in any
     permanent place)? I have a vague memory of someone telling me there
     were such functions, but they may be a false memory. One possible
     problem with this approach is that of collisions when computing the
     interaction's values. For example:
     
           var1   var2  interaction
           aa     c     aac
           a      ac    aac /* oops */
     
     Then there's:
     
     3. That much better approach that I didn't think of. 
     
     If anyone can explain number 3 to me, please do. Otherwise, which sounds
     best: 1 or 2?
     
     -Jason
     
     
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