Many thanks, that is exactly what I was looking for. I did not know about
bquote, interesting!
MP


On Mon, Feb 3, 2014 at 6:29 PM, William Dunlap <wdunlap@tibco.com> wrote:

> Is this the sort of thing you are looking for?
>    > fm <- y ~ x1 + x2 + x3 + log(x4)
>    > # Use terms() instead of just all.vars() to keep log(x4) as log(x4)
>    > xVars <- with(attributes(terms(fm)),
> as.list(variables)[-c(1,response+1)])
>    > str(xVars)
>    List of 4
>     $ : symbol x1
>     $ : symbol x2
>     $ : symbol x3
>     $ : language log(x4)
>    > # use bquote to make the addition to the formula
>    > update(fm, bquote( ~ . + .(xVars[[1]]) * .(xVars[[length(xVars)]])))
>    y ~ x1 + x2 + x3 + log(x4) + x1:log(x4)
>
> As a function it would be
>    addInteraction <- function(formula){
>       xVars <- with(attributes(terms(formula)),
> as.list(variables)[-c(1,response+1)])
>      update(formula, bquote( ~ . + .(xVars[[1]]) *
> .(xVars[[length(xVars)]])))
>    }
> used as
>    > addInteraction(y~x1+x2+sqrt(x3))
>    y ~ x1 + x2 + sqrt(x3) + x1:sqrt(x3)
>
> If the last 'term' in the formula is a compound like x4:x5 and you
> want x1:x4:x5 added you will need to do more work (look at the
> 'factors' attribute of terms()'s output) - currently it adds x1:x5.
>
>
> Bill Dunlap
> TIBCO Software
> wdunlap tibco.com
>
>
> > -----Original Message-----
> > From: r-help-bounces@r-project.org [mailto:r-help-bounces@r-project.org]
> On Behalf
> > Of Marie-Pierre Sylvestre
> > Sent: Monday, February 03, 2014 12:54 PM
> > To: r-help@r-project.org
> > Subject: [R] update.formula() to add interaction terms
> >
> > Hi,
> > I have a list of formulae that I need to modify. For each formula, I need
> > to add an interaction term between the first independent variable and the
> > last one. I want to write a function to achieve that because the list of
> > formulae to modify is long.
> >
> > For example, if the formula is y ~ x1 + x2 + x3 + x4, then I need to turn
> > it either into
> >
> > y ~ x1*x4 + x2 + x3
> >
> > or
> >
> > y ~ x1 + x2 + x3 + x4 + x4:x1
> >
> > (I know they are the same, but one may be easier to work with than the
> > other).
> >
> >
> > Suppose I have the formula a which is defined as a <- formula(y ~
> > x1+x2+x3+x4)
> >
> > I know how to access to the first and last independent variables of my
> > formula:
> >
> > firstvar <- all.vars(a[[3]])[1]
> > lastvar <- all.vars(a[[3]])[length( all.vars(a[[3]]))]
> >
> >
> > What I can't figure out is how to use update.formula() to include my
> > interaction term in order to get y ~ x1+x2+x3+x4 + x1:x4. Specifically,
> >
> > update.formula(a, ~ . + paste(all.vars(a[[3]])[1],firstvar, lastvar, sep
> =
> > ':'))
> >
> > is not producing y ~ x1+x2+x3+x4 + x1:x4.
> >
> > Any idea?
> >
> > Thanks in advance,
> > MP
> >
> >       [[alternative HTML version deleted]]
> >
> > ______________________________________________
> > R-help@r-project.org mailing list
> > https://stat.ethz.ch/mailman/listinfo/r-help
> > PLEASE do read the posting guide
> http://www.R-project.org/posting-guide.html
> > and provide commented, minimal, self-contained, reproducible code.
>

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