# [Rd] named arguments in formula and terms

Martin Maechler maechler at stat.math.ethz.ch
Mon Mar 13 10:16:25 CET 2017

```Dear Achim,

>>>>> Achim Zeileis <Achim.Zeileis at r-project.org>
>>>>>     on Fri, 10 Mar 2017 15:02:38 +0100 writes:

> Hi, we came across the following unexpected (for us)
> behavior in terms.formula: When determining whether a term
> is duplicated, only the order of the arguments in function
> calls seems to be checked but not their names. Thus the
> terms f(x, a = z) and f(x, b = z) are deemed to be
> duplicated and one of the terms is thus dropped.

R> attr(terms(y ~ f(x, a = z) + f(x, b = z)), "term.labels")
>  "f(x, a = z)"

> However, changing the arguments or the order of arguments
> keeps both terms:

R> attr(terms(y ~ f(x, a = z) + f(x, b = zz)), "term.labels")
>  "f(x, a = z)" "f(x, b = zz)"
R> attr(terms(y ~ f(x, a = z) + f(b = z, x)), "term.labels")
>  "f(x, a = z)" "f(b = z, x)"

> Is this intended behavior or needed for certain terms?

> We came across this problem when setting up certain smooth
> regressors with different kinds of patterns. As a trivial
> simplified example we can generate the same kind of
> problem with rep(). Consider the two dummy variables rep(x
> = 0:1, each = 4) and rep(x = 0:1, times = 4). With the
> response y = 1:8 I get:

R> lm((1:8) ~ rep(x = 0:1, each = 4) + rep(x = 0:1, times = 4))

> Call: lm(formula = (1:8) ~ rep(x = 0:1, each = 4) + rep(x
> = 0:1, times = 4))

> Coefficients: (Intercept) rep(x = 0:1, each = 4) 2.5 4.0

> So while the model is identified because the two
> regressors are not the same, terms.fomula does not
> recognize this and drops the second regressor.  What I
> would have wanted can be obtained by switching the
> arguments:

R> lm((1:8) ~ rep(each = 4, x = 0:1) + rep(x = 0:1, times =4))

> Call: lm(formula = (1:8) ~ rep(each = 4, x = 0:1) + rep(x
> = 0:1, times = 4))

> Coefficients: (Intercept) rep(each = 4, x = 0:1) rep(x =
> 0:1, times = 4) 2 4 1

> Of course, here I could avoid the problem by setting up
> proper factors etc. But to me this looks a potential bug
> in terms.formula...

I agree that there is a bug.
According to https://www.r-project.org/bugs.html
I have generated an R bugzilla account for you so you can report
it there (for "book keeping", posteriority, etc).

> Thanks in advance for any insights, Z

and thank *you* (and Nikolaus ?) for the report!

Best regards,
Martin

```

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