[R] Base within reverses column order

[Dan Murphy]

Thanks, Duncan. Using names is certainly the most reliable solution,
but requires remembering to modify the "surrounding code" when
enhancing what's within -- a bug risk source when passing on the code.
Are you saying that an automatic reversal as follows may break in the
future because R-devel may change the current behavior of 'within'?
(If so, then that's the greater source of bug risk .. so back to
'names'.)

ln <- length(foo)
foo <- within(foo, {
  bar <- whatever
  other()
  })
foo <- foo[c(1:ln, length(foo):(ln+1))] # to reverse within's assumed
backwards column order

Is there a way to capture the names of new objects created within?


On Fri, Apr 4, 2014 at 10:55 AM, Duncan Murdoch
<murdoch.duncan at gmail.com> wrote:
> On 04/04/2014 1:32 PM, Dan Murphy wrote:
>>
>> I just noticed this annoyance, but I'm not the first one, apparently
>> -- see
>> http://lists.r-forge.r-project.org/pipermail/datatable-help/2012-May/001176.html
>>
>> The thread never answered the OP's question "Is this a bug?" so I
>> assume the answer, unfortunately, is No.
>>
>> If not a bug, do users of within have a workaround to produce a result
>> with columns as ordered within 'within'? I can think of a way using
>> names and subset-with-select, but that seems unduly kludgy.
>
>
> I wouldn't be surprised if it is not consistent about that.  It uses as.list
> to convert an environment to a list, and that's where the reversal occurs:
> but since environments are unordered collections of objects, you just happen
> to be seeing an undocumented and unpromised property of the internal
> implementation.
>
> If the order matters to you, then create your initial dataframe with the new
> variables (set to NA, for example), or reorder it afterwards.  But generally
> speaking even in a dataframe (which is an ordered collection of objects),
> it's better to program in a way that doesn't make assumptions about the
> order.  Columns have names, and you should use those.
>
> Duncan Murdoch