[R] Concepts question: environment, frame, search path

Duncan Murdoch murdoch at stats.uwo.ca
Wed May 2 12:56:34 CEST 2007

On 01/05/2007 11:34 AM, Prof Brian Ripley wrote:
> On Tue, 1 May 2007, Duncan Murdoch wrote:
>> On 01/05/2007 12:29 AM, Graham Wideman wrote:
>>> Refman p122:  "Environments consist of a frame, or collection of named
>>> objects, and a pointer to an enclosing environment."
>>> Is the "or" here explaining parenthetically that a frame is a collection of
>>> named objects, or is separating too alternative structures for an
>>> environment?
>> The former.
>>> If the former, does this imply that a frame can contain arbitrary variables?
>> Yes, but a frame isn't an R object, it's a concept that appears in
>> descriptions, e.g. part of an environment, or the local variables
>> created during function evaluation, etc.
>>> And "pointer"? Is that a type of thing in R?
>> No, there are no pointers in R.  There are a couple of tricks to fake
>> them (e.g. environment objects aren't copied when assigned, you just get
>> a new reference to the same environment; this allows you to construct
>> something like a pointer by wrapping an object in an environment), but I
>> don't recommend using these routinely.
> Nevertheless, the statement is true.  R is implemented using pointers.

Yes, definitely.  I misread the original posting, and didn't notice that 
the "pointer" question was connected to the previous quote.

Duncan Murdoch

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