# [R] Surprising Symbolic Model Formula Evaluations

William Dunlap wdun|@p @end|ng |rom t|bco@com
Fri Oct 11 20:17:15 CEST 2019

```Look at what terms() (hence lm()) does with such formulae:
> str(terms(y~1*x))
Classes 'terms', 'formula'  language y ~ 1 * x
..- attr(*, "variables")= language list(y, x)
..- attr(*, "factors")= int(0)
..- attr(*, "term.labels")= chr(0)
..- attr(*, "order")= int(0)
..- attr(*, "intercept")= int 1
..- attr(*, "response")= int 1
..- attr(*, ".Environment")=<environment: R_GlobalEnv>
> coef(lm(y~1*x, data=data.frame(y=1:10,x=log2(1:10))))
(Intercept)
5.5

Bill Dunlap
TIBCO Software
wdunlap tibco.com

On Fri, Oct 11, 2019 at 10:59 AM Richard M. Heiberger <rmh using temple.edu>
wrote:

> I can't duplicate your examples
> This is what I see
> > y ~ 1:x
> y ~ 1:x
>
> Please try again in a vanilla R session, and send a reproducible example.
> vanilla means start R from the operating system command line with
> R --vanilla
>
> this prevents any of your initialization files from being loaded.  See
> ?Startup
> for details.
>
> On Fri, Oct 11, 2019 at 1:05 PM Emi Tanaka <dr.emi.tanaka using gmail.com>
> wrote:
> >
> > Hi,
> >
> > I'm wondering about some logics behind the following simplifications:
> >
> > y ~ 1:x simplifies to y ~ 1
> > y ~ x:1 simplifies to y ~ 1
> > y ~ x*1 simplifies to y ~ x
> > y ~ 1*x simplifies to y ~ 1
> >
> > Mainly I would have expected y ~ 1:x to simplify to y ~ x and the cross
> > operator to be invariant to order.
> >
> > I have some further surprising cases below that I'd also like to know
> more
> > about but just above will also be great.
> >
> > https://rpubs.com/emitanaka/unexpected-formula-eval
> >
> > Best,
> >
> > Emi
> >
> > *Dr. Emi Tanaka* | Lecturer in Statistics
> >
> > Faculty of Science, School of Mathematics and Statistics
> >
> >
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