R devel - Jan 2013 - Suggestions for 'diff.default'

I have suggestions for function 'diff.default' in R.


Suggestion 1: If the input is matrix, always return matrix, even if empty.

What happens in R 2.15.2:
> rbind(1:2)    # matrix
     [,1] [,2]
[1,]    1    2
> diff(rbind(1:2))   # not matrix
integer(0)
> sessionInfo()
R version 2.15.2 (2012-10-26)
Platform: i386-w64-mingw32/i386 (32-bit)

locale:
[1] LC_COLLATE=English_United States.1252
[2] LC_CTYPE=English_United States.1252
[3] LC_MONETARY=English_United States.1252
[4] LC_NUMERIC=C
[5] LC_TIME=English_United States.1252

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base


The documentation for 'diff' says, "If 'x' is a matrix then
the difference operations are carried out on each column separately."
If the result is empty, I expect that the result still has as many columns as
the input.


Suggestion 2: Make 'diff.default' applicable more generally by
(a) not performing 'unclass';
(b) generalizing (changing)
ismat <- is.matrix(x)
to become
ismat <- length(dim(x)) == 2L


If suggestion 1 is to be applied, if 'unclass' is not wanted (point (a)
in suggestion 2 is also to be applied),

    if (lag * differences >= xlen)
	return(x[0L])

can be changed to

    if (lag * differences >= xlen)
	return(
            if (ismat) x[0L, , drop = FALSE] - x[0L, , drop = FALSE] else
            x[0L] - x[0L])

It will handle class where subtraction (minus) operation change class.

Otherwise, if 'unclass' is wanted, maybe the handling of empty result
can be moved to be after 'unclass', to be consistent with non-empty
result.


If point (a) in suggestion 2 is applied, 'diff.default' can handle input
of class "Date" and "POSIXt". If, in addition, point (b) in
suggestion 2 is also applied, 'diff.default' can handle data frame as
input.

--- On Mon, 28/1/13, Suharto Anggono Suharto Anggono <suharto_anggono at
yahoo.com> wrote:
> From: Suharto Anggono Suharto Anggono <suharto_anggono at yahoo.com>
> Subject: Suggestions for 'diff.default'
> To: R-devel at lists.R-project.org
> Date: Monday, 28 January, 2013, 5:31 PM
> I have suggestions for function
> 'diff.default' in R.
> 
> 
> Suggestion 1: If the input is matrix, always return matrix,
> even if empty.
> 
> What happens in R 2.15.2:
> 
> > rbind(1:2)? ? # matrix
> ? ???[,1] [,2]
> [1,]? ? 1? ? 2
> > diff(rbind(1:2))???# not matrix
> integer(0)
> > sessionInfo()
> R version 2.15.2 (2012-10-26)
> Platform: i386-w64-mingw32/i386 (32-bit)
> 
> locale:
> [1] LC_COLLATE=English_United States.1252
> [2] LC_CTYPE=English_United States.1252
> [3] LC_MONETARY=English_United States.1252
> [4] LC_NUMERIC=C
> [5] LC_TIME=English_United States.1252
> 
> attached base packages:
> [1] stats? ???graphics? grDevices
> utils? ???datasets?
> methods???base
> 
> 
> The documentation for 'diff' says, "If 'x' is a matrix
then
> the difference operations are carried out on each column
> separately."
> If the result is empty, I expect that the result still has
> as many columns as the input.
> 
> 
> Suggestion 2: Make 'diff.default' applicable more generally
> by
> (a) not performing 'unclass';
> (b) generalizing (changing)
> ismat <- is.matrix(x)
> to become
> ismat <- length(dim(x)) == 2L
> 
> 
> If suggestion 1 is to be applied, if 'unclass' is not wanted
> (point (a) in suggestion 2 is also to be applied),
> 
> ? ? if (lag * differences >= xlen)
> ??? return(x[0L])
> 
> can be changed to
> 
> ? ? if (lag * differences >= xlen)
> ??? return(
> ? ? ? ? ? ? if (ismat) x[0L, ,
> drop = FALSE] - x[0L, , drop = FALSE] else
> ? ? ? ? ? ? x[0L] - x[0L])
> 
> It will handle class where subtraction (minus) operation
> change class.
Sorry, I wasn't careful enough. To obtain the correct class for the result,
differencing should be done as many times as specified by argument
'differences'.

I consider the case of
diff(as.POSIXct(c("2012-01-01", "2012-02-01"),
tz="UTC"), d=2)
versus
diff(diff(as.POSIXct(c("2012-01-01", "2012-02-01"),
tz="UTC")))
To be safe, maybe just compute as usual, even when it is known that the end
result will be empty. It can be done like this.

    empty <- integer()
    if (ismat)
	for (i in seq_len(differences))
	    r <- if (lag >= nrow(r))
                r[empty, , drop = FALSE] - r[empty, , drop = FALSE] else
                ...
    else
        for (i in seq_len(differences))
            r <- if (lag >= length(r))
                r[empty] - r[empty] else
                ...

If that way is used, 'xlen' is no longer needed.
> 
> Otherwise, if 'unclass' is wanted, maybe the handling of
> empty result can be moved to be after 'unclass', to be
> consistent with non-empty result.
> 
> 
> If point (a) in suggestion 2 is applied, 'diff.default' can
> handle input of class "Date" and "POSIXt". If, in
addition,
> point (b) in suggestion 2 is also applied, 'diff.default'
> can handle data frame as input.
>