[Rd] Suggestions for 'diff.default'

[Suharto Anggono Suharto Anggono]

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.