R devel - Jul 2018 - apply with zero-row matrix

Forgive me if this has been asked many times before, but I couldn't find
anything on the mailing lists.

I'd expect apply(m, 1, foo) not to call `foo` if m is a matrix with zero
rows.
In fact:

m <- matrix(NA, 0, 5)
apply(m, 1, function (x) {cat("Called...\n"); print(x)})
## Called...
## [1] FALSE FALSE FALSE FALSE FALSE

Similarly for apply(m, 2,...) if m has no columns.
Is there a reason for this? Could it be documented?

David
-- 
Sent from Gmail Mobile

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>>>>> David Hugh-Jones 
>>>>>     on Mon, 30 Jul 2018 05:33:19 +0100 writes:
    > Forgive me if this has been asked many times before, but I
    > couldn't find anything on the mailing lists.

    > I'd expect apply(m, 1, foo) not to call `foo` if m is a
    > matrix with zero rows.  In fact:

    > m <- matrix(NA, 0, 5)
    > apply(m, 1, function (x) {cat("Called...\n"); print(x)})
    > ## Called...
    > ## [1] FALSE FALSE FALSE FALSE FALSE


    > Similarly for apply(m, 2,...) if m has no columns.  Is
    > there a reason for this?

Yes :

The reverse is really true for almost all basic R functions:

    They *are* called and give an "empty" result automatically
    when the main argument is empty.

What you basicaly propose is to add an extra

     if(<length 0 input>)
     	 return(<correspondingly formatted length-0 output>)

to all R functions.  While that makes sense for high-level R
functions that do a lot of things, this would really be a bad
idea in general :

This would make all of these basic functions larger {more to maintain} and
slightly slower for all non-zero cases just to make them
slightly faster for the rare zero-length case.

Martin Maechler
ETH Zurich and R core Team

Hi Martin,

Fair enough for R functions in general. But the behaviour of apply violates
the expectation that apply(m, 1, fun) calls fun n times when m has n rows.
That seems pretty basic.

Also, I understand from your argument why it makes sense to call apply and
return a special result (presumably NULL) for an empty argument; but why
should apply call fun?

Cheers
David

On Mon, 30 Jul 2018 at 08:41, Martin Maechler <maechler at
stat.math.ethz.ch>
wrote:
> >>>>> David Hugh-Jones
> >>>>>     on Mon, 30 Jul 2018 05:33:19 +0100 writes:
>
>     > Forgive me if this has been asked many times before, but I
>     > couldn't find anything on the mailing lists.
>
>     > I'd expect apply(m, 1, foo) not to call `foo` if m is a
>     > matrix with zero rows.  In fact:
>
>     > m <- matrix(NA, 0, 5)
>     > apply(m, 1, function (x) {cat("Called...\n"); print(x)})
>     > ## Called...
>     > ## [1] FALSE FALSE FALSE FALSE FALSE
>
>
>     > Similarly for apply(m, 2,...) if m has no columns.  Is
>     > there a reason for this?
>
> Yes :
>
> The reverse is really true for almost all basic R functions:
>
>     They *are* called and give an "empty" result automatically
>     when the main argument is empty.
>
> What you basicaly propose is to add an extra
>
>      if(<length 0 input>)
>          return(<correspondingly formatted length-0 output>)
>
> to all R functions.  While that makes sense for high-level R
> functions that do a lot of things, this would really be a bad
> idea in general :
>
> This would make all of these basic functions larger {more to maintain} and
> slightly slower for all non-zero cases just to make them
> slightly faster for the rare zero-length case.
>
> Martin Maechler
> ETH Zurich and R core Team
>
> --
Sent from Gmail Mobile

	[[alternative HTML version deleted]]

Hi David,
    
    Besides Martins point, there is also the issue that for a lot of cases you
would still like to have the right class returned.
    Right now these are returns:
    
    > apply(matrix(NA_integer_,0,5), 1, class)
    character(0)
    > apply(matrix(NA_integer_,0,5), 1, identity)
    integer(0)
    > apply(matrix(NA,0,5), 1, identity)
    logical(0)
    
    In your case, these would all return NULL, so I think there is value in
running FUN at least once (Say if you'd want to check if FUN always returns
the right class).
    And from a philosophical point of view, R is mostly a functional programming
language, I think if you want side-effects a for-loop would look better.
    
    
    Best regards, 
    Emil Bode
     
    Data-analyst
     
    +31 6 43 83 89 33
    emil.bode at dans.knaw.nl
     
    DANS: Netherlands Institute for Permanent Access to Digital Research
Resources
    Anna van Saksenlaan 51 | 2593 HW Den Haag | +31 70 349 44 50 | info at
dans.knaw.nl <mailto:info at dans.kn> | dans.knaw.nl
<applewebdata://71F677F0-6872-45F3-A6C4-4972BF87185B/www.dans.knaw.nl>
    DANS is an institute of the Dutch Academy KNAW <http://knaw.nl/nl> and
funding organisation NWO <http://www.nwo.nl/>.
    
    On 30/07/2018, 11:12, "R-devel on behalf of David Hugh-Jones"
<r-devel-bounces at r-project.org on behalf of davidhughjones at
gmail.com> wrote:
    
        Hi Martin,
        
        Fair enough for R functions in general. But the behaviour of apply
violates
        the expectation that apply(m, 1, fun) calls fun n times when m has n
rows.
        That seems pretty basic.
        
        Also, I understand from your argument why it makes sense to call apply
and
        return a special result (presumably NULL) for an empty argument; but why
        should apply call fun?
        
        Cheers
        David
        
        On Mon, 30 Jul 2018 at 08:41, Martin Maechler <maechler at
stat.math.ethz.ch>
        wrote:
        
        > >>>>> David Hugh-Jones
        > >>>>>     on Mon, 30 Jul 2018 05:33:19 +0100 writes:
        >
        >     > Forgive me if this has been asked many times before, but I
        >     > couldn't find anything on the mailing lists.
        >
        >     > I'd expect apply(m, 1, foo) not to call `foo` if m is
a
        >     > matrix with zero rows.  In fact:
        >
        >     > m <- matrix(NA, 0, 5)
        >     > apply(m, 1, function (x) {cat("Called...\n");
print(x)})
        >     > ## Called...
        >     > ## [1] FALSE FALSE FALSE FALSE FALSE
        >
        >
        >     > Similarly for apply(m, 2,...) if m has no columns.  Is
        >     > there a reason for this?
        >
        > Yes :
        >
        > The reverse is really true for almost all basic R functions:
        >
        >     They *are* called and give an "empty" result
automatically
        >     when the main argument is empty.
        >
        > What you basicaly propose is to add an extra
        >
        >      if(<length 0 input>)
        >          return(<correspondingly formatted length-0 output>)
        >
        > to all R functions.  While that makes sense for high-level R
        > functions that do a lot of things, this would really be a bad
        > idea in general :
        >
        > This would make all of these basic functions larger {more to
maintain} and
        > slightly slower for all non-zero cases just to make them
        > slightly faster for the rare zero-length case.
        >
        > Martin Maechler
        > ETH Zurich and R core Team
        >
        > --
        Sent from Gmail Mobile
        
        	[[alternative HTML version deleted]]
        
        ______________________________________________
        R-devel at r-project.org mailing list
        https://stat.ethz.ch/mailman/listinfo/r-devel

Try pmap and related functions in purrr:

  pmap(as.data.frame(m), ~ { cat("Called...\n"); print(c(...)) })
  ## list()

On Mon, Jul 30, 2018 at 12:33 AM, David Hugh-Jones
<davidhughjones at gmail.com> wrote:
> Forgive me if this has been asked many times before, but I couldn't
find
> anything on the mailing lists.
>
> I'd expect apply(m, 1, foo) not to call `foo` if m is a matrix with
zero
> rows.
> In fact:
>
> m <- matrix(NA, 0, 5)
> apply(m, 1, function (x) {cat("Called...\n"); print(x)})
> ## Called...
> ## [1] FALSE FALSE FALSE FALSE FALSE
>
> Similarly for apply(m, 2,...) if m has no columns.
> Is there a reason for this? Could it be documented?
>
> David
> --
> Sent from Gmail Mobile
>
>         [[alternative HTML version deleted]]
>
> ______________________________________________
> R-devel at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-devel


-- 
Statistics & Software Consulting
GKX Group, GKX Associates Inc.
tel: 1-877-GKX-GROUP
email: ggrothendieck at gmail.com

Interesting discussion. I'm not wholly convinced by Martin's and
Emil's
arguments. The behaviour seems to violate an obvious expectation (fun is
called once per row) to satisfy a subtle one (result has a guaranteed
dimension and type).

In any case, here's a suggested chunk of rd to go at the end of the
"Value":

If \code{dim(X)[MARGIN]} is zero, then \code{FUN} is called once, with an
argument of the appropriate dimensions. The argument's type is the same as
\code{typeof(m)}, and the argument values are those returned by
\code{vector(typeof(m))}. For example, if m is numeric, the argument will
be a vector (or matrix or array) of zeroes. The type and length of the
value returned by \code{FUN} is used to determine the type of the result.

And at the end of "Details":

\code{FUN} is always called at least once, see below.


David


On Mon, 30 Jul 2018 at 15:05, Gabor Grothendieck <ggrothendieck at
gmail.com>
wrote:
> Try pmap and related functions in purrr:
>
>   pmap(as.data.frame(m), ~ { cat("Called...\n"); print(c(...)) })
>   ## list()
>
> On Mon, Jul 30, 2018 at 12:33 AM, David Hugh-Jones
> <davidhughjones at gmail.com> wrote:
> > Forgive me if this has been asked many times before, but I
couldn't find
> > anything on the mailing lists.
> >
> > I'd expect apply(m, 1, foo) not to call `foo` if m is a matrix
with zero
> > rows.
> > In fact:
> >
> > m <- matrix(NA, 0, 5)
> > apply(m, 1, function (x) {cat("Called...\n"); print(x)})
> > ## Called...
> > ## [1] FALSE FALSE FALSE FALSE FALSE
> >
> > Similarly for apply(m, 2,...) if m has no columns.
> > Is there a reason for this? Could it be documented?
> >
> > David
> > --
> > Sent from Gmail Mobile
> >
> >         [[alternative HTML version deleted]]
> >
> > ______________________________________________
> > R-devel at r-project.org mailing list
> > https://stat.ethz.ch/mailman/listinfo/r-devel
>
>
>
> --
> Statistics & Software Consulting
> GKX Group, GKX Associates Inc.
> tel: 1-877-GKX-GROUP
> email: ggrothendieck at gmail.com
>
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