[Rd] RFC: sapply() limitation from vector to matrix, but not further

[Hadley Wickham]

I think an even better approach would be to extract the
"simplification" component out of sapply, so that could write

sapply <- function(...) simplify(lapply(...))

(although obviously some arguments would go to lapply and some to simplify).

The advantage of this would be that you could use the same
simplification algorithm in other places.

Hadley

On Wed, Dec 1, 2010 at 8:39 AM, Martin Maechler
<maechler at stat.math.ethz.ch> wrote:
> sapply() stems from S / S+ times and hence has a long tradition.
> In spite of that I think that it should be enhanced...
>
> As the subject mentions, sapply() produces a matrix in cases
> where the list components of the lapply(.) results are of the
> same length (and ...).
> However, it unfortunately "stops there".
> E.g., if you *nest* two sapply() calls where the inner one
> produces a matrix, very often the logical behavior would be for
> the outer sapply() to stack these matrices into an array of
> rank 3 ["array rank"(x) := length(dim(x))].
> However it does not do that, e.g., an artifical example
>
> p0 <- function(...) paste(..., sep="")
> myF <- function(x,y) {
>    stopifnot(length(x) <= 3)
>    x <- rep(x, length.out=3)
>    ny <- length(y)
>    r <- outer(x,y)
>    dimnames(r) <- list(p0("r",1:3), p0("C", seq_len(ny)))
>    r
> }
>
> and
>
>> (v <- structure(10*(5:8), names=LETTERS[1:4]))
>  A  B  C  D
> 50 60 70 80
>
> if we let sapply() not simplify, we see the list of same size
> matrices it produes:
>
>> sapply(v, myF, y = 2*(1:5), simplify=FALSE)
> $A
>    C1  C2  C3  C4  C5
> r1 100 200 300 400 500
> r2 100 200 300 400 500
> r3 100 200 300 400 500
>
> $B
>    C1  C2  C3  C4  C5
> r1 120 240 360 480 600
> r2 120 240 360 480 600
> r3 120 240 360 480 600
>
> $C
>    C1  C2  C3  C4  C5
> r1 140 280 420 560 700
> r2 140 280 420 560 700
> r3 140 280 420 560 700
>
> $D
>    C1  C2  C3  C4  C5
> r1 160 320 480 640 800
> r2 160 320 480 640 800
> r3 160 320 480 640 800
>
> However, quite deceptively
>
>> sapply(v, myF, y = 2*(1:5))
>        A   B   C   D
>  [1,] 100 120 140 160
>  [2,] 100 120 140 160
>  [3,] 100 120 140 160
>  [4,] 200 240 280 320
>  [5,] 200 240 280 320
>  [6,] 200 240 280 320
>  [7,] 300 360 420 480
>  [8,] 300 360 420 480
>  [9,] 300 360 420 480
> [10,] 400 480 560 640
> [11,] 400 480 560 640
> [12,] 400 480 560 640
> [13,] 500 600 700 800
> [14,] 500 600 700 800
> [15,] 500 600 700 800
>
>
> My proposal -- implemented and "make check" tested --
> is to add an optional argument  'ARRAY'
> which allows
>
>> sapply(v, myF, y = 2*(1:5), ARRAY=TRUE)
> , , A
>
>    C1  C2  C3  C4  C5
> r1 100 200 300 400 500
> r2 100 200 300 400 500
> r3 100 200 300 400 500
>
> , , B
>
>    C1  C2  C3  C4  C5
> r1 120 240 360 480 600
> r2 120 240 360 480 600
> r3 120 240 360 480 600
>
> , , C
>
>    C1  C2  C3  C4  C5
> r1 140 280 420 560 700
> r2 140 280 420 560 700
> r3 140 280 420 560 700
>
> , , D
>
>    C1  C2  C3  C4  C5
> r1 160 320 480 640 800
> r2 160 320 480 640 800
> r3 160 320 480 640 800
>
>>
> -----------
>
> In the best of all worlds, the default would be 'ARRAY = TRUE',
> but of course, given the long-standing different behavior,
> it seem much too "risky", and my proposal includes remaining
> back-compatible with default ARRAY = FALSE.
>
> Martin Maechler,
> ETH Zurich
>
> ______________________________________________
> R-devel at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-devel
>



-- 
Assistant Professor / Dobelman Family Junior Chair
Department of Statistics / Rice University
http://had.co.nz/