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.
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