[R] The assign function in R

[Peter Dalgaard]

Prof Brian Ripley wrote:
> For the record, frame=1 and where=1 are not the same thing (whereas 
> frame=0 and where=0 are both the session database): assign() in S has 
> both 'frame' and 'where' arguments.
>
Drats... I stand corrected. Thanks to Brian for clearing this up.

    -p
> where=1 is the first database on the search() path, and is roughly 
> equivalent to the workspace (although you can attach databases ahead 
> of the working database in S(-PLUS), people who write code like 
> where=1 often do not know that).
>
> In S(-PLUS) there is a working database aka chapter, which is the 
> nearest equivalent to the R workspace but is (conceptually) stored on 
> disc and so permanent. It is like running R with --restore --save.  
> Unless some other database is attached at pos=1, the working database 
> is at where=1.
>
> The Blue Book contains the help page for assign() in S ca 1988, and 
> apart from what where=0 means (which may well be a later addition) the 
> information is all there.
>
> On Thu, 19 Jun 2008, Peter Dalgaard wrote:
>
>> Duncan Murdoch wrote:
>>> Applejus wrote:
>>>> Hello,
>>>>
>>>> I want to convert assign("a", b, where =1 ) from SPLUS to R.
>>>> Is it safe to assume that the equivalent of where=1 is pos=1  in R?
>>>>
>>>> Thanks for help! 
>>> Only a limited number of users here know what "where=1" means in 
>>> S-PLUS. If one of those doesn't answer, you might want to explain 
>>> that for those of us who have forgotten or who never knew.
>>>
>> Even some of those who should know may have forgotten by now... It is 
>> detailed in the Blue Book somewhere. As I recall it, the story is (or 
>> was?) that in S-PLUS objects are mostly on disk and the search path 
>> is a list of directories. Then there is a stack of evaluation frames 
>> plus two special frames, frame 0 and frame1. Frame 0 is permanent in 
>> the sense that it exists for the duration of the session, frame 1 
>> exists during evaluation of expressions. Assignments to frame 1 were 
>> mostly done to overcome communication difficulties caused by the lack 
>> of lexical scope in S -- two different functions needing to work with 
>> the same temporary object would be the typical case.
>>
>> I would conjecture that once the purpose of the assignment to frame 1 
>> is understood, you can redesign the code so that it becomes much 
>> cleaner, possibly by using "<<-" semantics. For a quick port, what 
>> you need is some sort of temporary environment. Assigning to pos=1 is 
>> NOT the solution, because that is the global environment and you 
>> will  (A) risk clobbering an existing variable of the same name and 
>> (B) litter your workspace with intermediate results.
>>
>> -- 
>>  O__  ---- Peter Dalgaard             Øster Farimagsgade 5, Entr.B
>> c/ /'_ --- Dept. of Biostatistics     PO Box 2099, 1014 Cph. K
>> (*) \(*) -- University of Copenhagen   Denmark      Ph:  (+45) 35327918
>> ~~~~~~~~~~ - (p.dalgaard at biostat.ku.dk)              FAX: (+45) 35327907
>>
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>>
>


-- 
   O__  ---- Peter Dalgaard             Øster Farimagsgade 5, Entr.B
  c/ /'_ --- Dept. of Biostatistics     PO Box 2099, 1014 Cph. K
 (*) \(*) -- University of Copenhagen   Denmark      Ph:  (+45) 35327918
~~~~~~~~~~ - (p.dalgaard at biostat.ku.dk)              FAX: (+45) 35327907