[External] Two ALTREP questions

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.)

   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.

'Reporting Bugs' link on the R home page https://www.r-project.org/

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

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--
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 at uiowa.edu
Iowa City, IA 52242                 WWW:  http://www.stat.uiowa.edu