[Rd] [External] Two ALTREP questions

[Jiefei Wang]

Thank Dirk and Luke for the answers!

(That's C code. The confusion here is partly our fault. When Romain and I
> extended the inline package with 'cxxfunction' to support the then-young
> but
> active Rcpp package, we picked C++. Strictly speaking that isn't required;
> you are only in C++ here because ... "it made sense to us 10 years ago" and
> it could generalized to C++ or C.  All ALTREP is, of course, purely C as it
> is an R API.)


Sometimes I forget to distinguish C/C++ code. Yes, this should be C code
and it is just C++ compatible.
Anyway, `inline` is a great package, `cxxfunction` makes life much easier
for reporting the low-level problem to the devel team.

- Try with 1:6 replaced by as.character(1:6), and look at the REF
>    values in both cases.
> - In particular, look at what this gives you:
>      x <- as.character(1:6)
>      attr(x, "dim") <- c(2, 3)
> - Things can be a little different with larger vectors; try variants
>    of your examples for more than 64 elements.


I see why we cannot change the attribute of the compact sequence(It is
shared, or at least marked as shared)
and the use of the wrapper for the large vector. Only the matrix function
needs to be patched.

The generally recommended way is via a bug report at bugs.r-project.org


You can find the suggested process for contributing described in the
> 'Reporting Bugs' link on the R home page https://www.r-project.org/


Bugzilla sounds like a good place to start, I will send an email to acquire
an account.

Best,
Jiefei


On Sun, Nov 22, 2020 at 6:57 AM <luke-tierney using uiowa.edu> wrote:

> On Sat, 21 Nov 2020, Jiefei Wang wrote:
>
> > Hello,
> >
> > I have two related ALTREP questions. It seems like there is no way to
> > assign attributes to an ALTREP vector without using C++ code. To be more
> > specifically, I want to make an ALTREP matrix, I have tried the
> following R
> > code but none of them work.
> > ```
> > .Internal(inspect(1:6))
> > .Internal(inspect(matrix(1:6, 2,3)))
> > .Internal(inspect(as.matrix(1:6)))
> > .Internal(inspect(structure(1:6, dim = c(2L,3L))))
> > .Internal(inspect({x <- 1:6;attr(x, "dim") <- c(2L,3L);x}))
> > .Internal(inspect({x <- 1:6;attributes(x)<- list(dim = c(2L,3L));x}))
> > ```
>
> Some things that my help you:
>
> - Try with 1:6 replaced by as.character(1:6), and look at the REF
>    values in both cases.
>
> - In particular, look at what this gives you:
>
>      x <- as.character(1:6)
>      attr(x, "dim") <- c(2, 3)
>
> - Things can be a little different with larger vectors; try variants
>    of your examples for more than 64 elements.
>
> > This also brings
> > my second question, it seems like the ALTREP coercion function does not
> > handle attributes correctly.  After the coercion, the ALTREP object will
> > lose its attributes.
> > ```
> > coerceFunc <- inline::cxxfunction( signature(x = "SEXP", attr = "SEXP" )
> , '
> > SET_ATTRIB(x,attr);
> > return(Rf_coerceVector(x, REALSXP));
> > ')
> >> coerceFunc(1:6, pairlist(dim = c(2L, 3L)))
> > [1] 1 2 3 4 5 6
> >> coerceFunc(1:6 + 0L, pairlist(dim = c(2L, 3L)))
> >     [,1] [,2] [,3]
> > [1,]    1    3    5
> > [2,]    2    4    6
> > ```
> > The problem is that the coercion function is directly dispatched to the
> > user-defined ALTREP coercion function, so the user is responsible to
> attach
> > the attributes after the coercion. If he forgets to do so, then the
> result
> > is a plain vector. Similar to the `Duplicate` and `DuplicateEX` functions
> > where the former one will attach the attributes by default, I feel that
> the
> > `Coerce` function should only return a plain vector and there should be a
> > `CoerceEx` function to do the attribute assignment, so the logic in the
> > no-EX ALTREP functions can be consistent. I do not know how dramastic the
> > change would be, so maybe this is too hard to do.
>
> Since you raised this earlier I have been looking at it and also think
> that this needs to he handled along the lines of
> Duplicate/DuplicateEx. I need to find some time to think that through
> and implement it; hopefully I'll get to it before the end of the year.
>
> > BTW, is there any way to contribute to the R source? I know R has a
> limited
> > resouces, so if possible, I will be happy to fix the matrix issue myself
> > and make some minor contributions to the R community.
>
> You can find the suggested process for contributing described in the
> 'Reporting Bugs' link on the R home page https://www.r-project.org/
>
> Best,
>
> luke
>
> > Best,
> > Jiefei
> >
> >       [[alternative HTML version deleted]]
> >
> > ______________________________________________
> > R-devel using r-project.org mailing list
> > https://stat.ethz.ch/mailman/listinfo/r-devel
> >
>
> --
> Luke Tierney
> Ralph E. Wareham Professor of Mathematical Sciences
> University of Iowa                  Phone:             319-335-3386
> Department of Statistics and        Fax:               319-335-3017
>     Actuarial Science
> 241 Schaeffer Hall                  email:   luke-tierney using uiowa.edu
> Iowa City, IA 52242                 WWW:  http://www.stat.uiowa.edu
>

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