[Rd] head.matrix can return 1000s of columns -- limit to n or add new argument?

[Gabriel Becker]

Hi Martin,


On Wed, Oct 30, 2019 at 4:30 AM Martin Maechler <maechler using stat.math.ethz.ch>
wrote:

> >>>>> Gabriel Becker
> >>>>>     on Tue, 29 Oct 2019 12:43:15 -0700 writes:
>
>     > Hi all,
>     > So I've started working on this and I ran into something that I
> didn't
>     > know, namely that for x a multi-dimensional (2+) array, head(x) and
> tail(x)
>     > ignore dimension completely, treat x as an atomic vector, and return
> an
>     > (unclassed) atomic vector:
>
> Well, that's  (3+), not "2+" .
>

You're correct, of course. Apologies for that.

>
> But I did write (on Sep 17 in this thread!)
>
>   > The current source for head() and tail() and all their methods
>   > in utils is just 83 lines of code  {file utils/R/head.R minus
>   > the initial mostly copyright comments}.
>
> and if've ever looked at these few dozen of R code lines, you'll
> have seen that we just added two simple utilities with a few
> reasonable simple methods.  To treat non-matrix (i.e. non-2d)
> arrays as vectors, is typically not unreasonable in R, but
> indeed with your proposals (in this thread), such non-2d arrays
> should be treated differently either via new  head.array() /
> tail.array() methods ((or -- only if it can be done more nicely -- by
> the default method)).
>

I hope you didn't construe my describing surprise (which was honest)  as a
criticism. It just quite literally not what I thought head(array(100, c(25,
2, 2))) would have done based on what head.matrix does is all.


>
> Note however the following  historical quirk :
>
> > sapply(setNames(,1:5), function(K) inherits(array(pi, dim=1:K), "array"))
>     1     2     3     4     5
>  TRUE FALSE  TRUE  TRUE  TRUE
>
> (Is this something we should consider changing for R 4.0.0 -- to
>  have it TRUE also for 2d-arrays aka matrix objects ??)
>

That is pretty odd. IMHO It would be quite nice from a design perspective
to fix that, but I do wonder, as I infer you do as well, how much code it
would break.

Changing this would cause problems in any case where a generic has an array
method but no matrix method, as well as any code that explicitly checks for
inherits from "array" assuming matrices won't return true, correct? My
intuition is that the former would be pretty rare, though it might be a fun
little problem to figure it out.  The latter is ...probably also fairly
rare? My intuition on that one is less strong though.


>
> The consequence of that is that
> currently, "often"   foo.matrix is just a copy of foo.array  in
> the case the latter exists:
> "base" examples: foo in {unique, duplicated, anyDuplicated}.
>
> So I propose you change current  head.matrix and tail.matrix  to
> head.array and tail.array
> (and then have   head.matrix <- head.array  etc, at least if the
>  above quirk must remain, or remains (which I currently guess to
>  be the case)).
>
>

Absolutely, will do. I'm gratified we're going after the more general
approach. Thanks for working with us on this.

Best,
~G


>
>     >> x = array(100, c(4, 5, 5))
>
>     >> dim(x)
>
>     > [1] 4 5 5
>
>     >> head(x, 1)
>
>     > [1] 100
>
>     >> class(head(x))
>
>     > [1] "numeric"
>
>
>     > (For a 1d array, it does return another 1d array).
>
>     > When extending head/tail to understand multiple dimensions as
> discussed in
>     > this thread, then, should the behavior for 2+d arrays be explicitly
>     > retained, or should head and tail do the analogous thing (with a
> head(<2d
>     array> ) behaving the same as head(<matrix>), which honestly is what I
>     > expected to already be happening)?
>
>     > Are people using/relying on this behavior in their code, and if so,
> why/for
>     > what?
>
>     > Even more generally, one way forward is to have the default methods
> check
>     > for dimensions, and use length if it is null:
>
>     > tail.default <- tail.data.frame <- function(x, n = 6L, ...)
>     > {
>     > if(any(n == 0))
>     > stop("n must be non-zero or unspecified for all dimensions")
>     > if(!is.null(dim(x)))
>     > dimsx <- dim(x)
>     > else
>     > dimsx <- length(x)
>
>     > ## this returns a list of vectors of indices in each
>     > ## dimension, regardless of length of the the n
>     > ## argument
>     > sel <- lapply(seq_along(dimsx), function(i) {
>     > dxi <- dimsx[i]
>     > ## select all indices (full dim) if not specified
>     > ni <- if(length(n) >= i) n[i] else dxi
>     > ## handle negative ns
>     > ni <- if (ni < 0L) max(dxi + ni, 0L) else min(ni, dxi)
>     > seq.int(to = dxi, length.out = ni)
>     > })
>     > args <- c(list(x), sel, drop = FALSE)
>     > do.call("[", args)
>     > }
>
>
>     > I think this precludes the need for a separate data.frame method at
> all,
>     > actually, though (I would think) tail.data.frame would still be
> defined and
>     > exported for backwards compatibility. (the matrix method has some
> extra
>     > bits so my current conception of it is still separate, though it
> might not
>     > NEED to be).
>
>     > The question then becomes, should head/tail always return something
> with
>     > the same dimensionally (number of dims) it got, or should data.frame
> and
>     > matrix be special cased in this regard, as they are now?
>
>     > What are people's thoughts?
>     > ~G
>
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>
>

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