[Rd] Expected behaviour of is.unsorted?

[Duncan Murdoch]

On 12-05-23 4:37 AM, Matthew Dowle wrote:
>
> Hi,
>
> I've read ?is.unsorted and searched. Have found a few items but nothing
> close, yet. Is the following expected?
>
>> is.unsorted(data.frame(1:2))
> [1] FALSE
>> is.unsorted(data.frame(2:1))
> [1] FALSE
>> is.unsorted(data.frame(1:2,3:4))
> [1] TRUE
>> is.unsorted(data.frame(2:1,4:3))
> [1] TRUE
>
> IIUC, is.unsorted is intended for atomic vectors only (description of x in
> ?is.unsorted). Indeed the C source (src/main/sort.c) contains an error
> message "only atomic vectors can be tested to be sorted". So that is the
> error message I expected to see in all cases above, since I know that
> data.frame is not an atomic vector. But there is also this in
> ?is.unsorted: "except for atomic vectors and objects with a class (where
> the>= or>  method is used)" which I don't understand. Where>= or>  is
> used by what, and where?

If you look at the source, you will see that the basic test for classed 
objects is

all(x[-1L] >= x[-length(x)])

(in the function base:::.gtn).

This comparison doesn't really makes sense for dataframes, but it does 
seem to be backwards:  that tests that x[2] >= x[1], x[3] >= x[2], etc., 
returning TRUE if all comparisons are TRUE:  but that sounds like it 
should be is.sorted(), not is.unsorted().  Or is it my brain that is 
backwards?

Duncan Murdoch


>
> I understand why the first two are FALSE (1 item of anything must be
> sorted). I don't understand the 3rd and 4th cases where length is 2:
> do_isunsorted seems to call lang3(install(".gtn"), x, CADR(args))). Does
> that fall back to TRUE for some reason?
>
> Matthew
>
>> sessionInfo()
> R version 2.15.0 (2012-03-30)
> Platform: x86_64-pc-mingw32/x64 (64-bit)
>
> locale:
> [1] LC_COLLATE=English_United Kingdom.1252  LC_CTYPE=English_United
> Kingdom.1252
> [3] LC_MONETARY=English_United Kingdom.1252 LC_NUMERIC=C
> [5] LC_TIME=English_United Kingdom.1252
>
> attached base packages:
> [1] stats     graphics  grDevices utils     datasets  methods   base
>
> other attached packages:
> [1] data.table_1.8.0
>
> loaded via a namespace (and not attached):
> [1] tools_2.15.0
>
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