[Rd] named arguments in formula and terms

[Martin Maechler]

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")
    > [1] "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")
    > [1] "f(x, a = z)" "f(x, b = zz)"
    R> attr(terms(y ~ f(x, a = z) + f(b = z, x)), "term.labels")
    > [1] "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