[Rd] NULL assignment will change the expression's class into list
Duncan Murdoch
murdoch.duncan at gmail.com
Sat Oct 9 01:26:05 CEST 2010
Vitalie Spinu wrote:
> On Fri, Oct 8, 2010 at 7:49 PM, Duncan Murdoch <murdoch.duncan at gmail.com>wrote:
>
>
>> On 08/10/2010 12:24 PM, Vitalie Spinu wrote:
>>
>>
>>> On Fri, Oct 8, 2010 at 12:14 PM, Duncan Murdoch<murdoch.duncan at gmail.com
>>>
>>>> wrote:
>>>>
>>>> Vitalie Spinu wrote:
>>>>
>>>>
>>>>> Hello Everyone!
>>>>>
>>>>> NULL replacement will change expression object into list:
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>> te<- expression(a=23*4, b=33-2)
>>>>>> te
>>>>>>
>>>>>>
>>>>>>
>>>>> expression(a = 23 * 4, b = 33 - 2)
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>> te[["a"]]<- quote(blabla) #ok
>>>>>> te
>>>>>>
>>>>>>
>>>>>>
>>>>> expression(a = blabla, b = 33 - 2)
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>> te[["a"]]<- NULL #change to list
>>>>>> te
>>>>>>
>>>>>>
>>>>>>
>>>>> $b
>>>>> 33 - 2
>>>>>
>>>>> I am on w32, version 2.11.1 (2010-05-31)
>>>>>
>>>>>
>>>>>
>>>> That's certainly an inconsistency, still present in a recent R-devel
>>>>
>>> (but I
>>>
>>>> haven't checked the latest beta). I don't know if it's a bug: NULL
>>>> assignments are handled specially in other situations (e.g. if te was a
>>>>
>>> list
>>>
>>>> to start, the NULL assignment would remove the "a" entry).
>>>>
>>>> A simple workaround is to use
>>>>
>>>> te["a"]<- expression(NULL)
>>>>
>>>> or te<- te[-1]
>>>>
>>>> instead, depending on what you expected to happen.
>>>>
>>>>
>>> As ussual with NULL assignment in recursive structures, I would expect to
>>> remove the elements altogether. And this is exactly what I need.
>>>
>>> I would say it's a bug, because NULL assignment in data.frames would not
>>> convert them to lists, for example.
>>>
>>>
>> I think you're probably right.
>>
>> Thanks for looking into it. It's quite inconvenient when you have to
>>
>>> manipulate named expression. Have to use constructs like
>>> et<-et[!names(et)%in%"a"].
>>>
>>>
>> Or simply follow te["a"] <- NULL
>>
>> with
>>
>> te <- as.expression(te)
>>
>> This is a pretty fast operation if te is an expression or a list formed by
>> mistaken conversion from one.
>>
>> D
>>
>
> But is this a reliable way to do it? I have been struggling some time ago
> with this type of conversion.
> Can not thing of an example now. But as far as I could remember, the
> conversion to list of an expression is not always reversible with
> as.expression. Am I wrong?
>
I don't know of any examples, but this is your construction. I think
it's very unlikely this will be fixed for 2.12.0, but it will probably
be fixed for 2.12.1, if
1. There is a 2.12.1
and
2. It really isn't intentional behaviour.
Duncan Murdoch
>
>
>> uncan Murdoch
>>
>>
>> Vitally.
>>
>>>> Duncan Murdoch
>>>>
>>>> Regards,
>>>>
>>>>> Vitally.
>>>>>
>>>>> [[alternative HTML version deleted]]
>>>>>
>>>>> ______________________________________________
>>>>> R-devel at r-project.org mailing list
>>>>> https://stat.ethz.ch/mailman/listinfo/r-devel
>>>>>
>>>>>
>>>>>
>>>>
>>>
>
>
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