pairlist objects

?list has a little bit of information. As far as I know, historically,
the more inefficient one (pairlist()) came first, where R inherits its
structure and implementation from LISP ; list() came later as a new 
implementation of a list-like object which is more efficient and faster
in various manner (e.g. addressing the (n)th elements in the middle,
and overall storage size). So these days most list-like stuff within R
is done as list()'s rather than pairlist()'s.

Internally, a pairlist() in R is implemented as a recursive binary
tree (LISTSXP), where one branch of the first node consists of the
first elements, its
attributes, and the other branch consists of a daughter node which
consists of the 2nd element as its one branch, etc. Walking such a tree
is slow and its storage requirement is a bit larger than list().

Internally, a list() in R is a VECSXP, which is a one-dimensional 
structure, plus some attributes storing the names of the elements, etc.
It is a bit more efficient in terms of storage (a 1-D structure vs a 
recursive binary tree), and also in random addressing of its elements -
e.g. you can jump to the (n)th element without walking the 1st to the 
(n-1)th elements.

This is my understanding, no doubt the R core team has more and better 
way to say about this.

a list() is not of class vector (despite the implementation in C being a 
VECSXP) - a vector in R is a 1-D structure where all the elements are of 
the same type/mode, which a list() is not.

Hi,

?pairlist gives no explanation about what exactly is the difference
between a pairlist and a list (except that a pairlist of length 0
is 'NULL'). So, what's a pairlist?

class(.Options)
[1] "pairlist"

Some strange things about the "pairlist" type:

  > showClass("pairlist")

  Error in getClass(Class) : "pairlist" is not a defined class

Why the above doesn't work? It works for "list":

  > showClass("list")

  No Slots, prototype of class "list"

  Extends: "vector"

  > is.list(.Options)

  [1] TRUE

  > is.vector(.Options)

  [1] FALSE

This doesn't make sense! If 'x' is a list, then it should be considered
a vector too.

Subsetting a pairlist with [] doesn't produce a pairlist: 

  > class(.Options[1:3])

  [1] "list"

Yes, this one is documented, but still...


Cheers,
H.

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?pairlist gives no explanation about what exactly is the difference  
between a pairlist and a list (except that a pairlist of length 0  
is 'NULL'). So, what's a pairlist?

I read "traditional _dotted pair_ lists (as in LISP)" there - c.f. also
http://cran.r-project.org/doc/manuals/R-lang.html#Pairlist-objects

class(.Options)
[1] "pairlist"

Some strange things about the "pairlist" type:

showClass("pairlist")

  Error in getClass(Class) : "pairlist" is not a defined class

Why the above doesn't work?

Because "pairlist" is not a formal class. Why should it?

It works for "list":

showClass("list")

  No Slots, prototype of class "list"

  Extends: "vector"

is.list(.Options)

  [1] TRUE

is.vector(.Options)

  [1] FALSE

This doesn't make sense! If 'x' is a list, then it should be  
considered a vector too.

Why? They are completely different objects. lists are generic  
vectors, pairlists are not vectors (c.f. the docs above).

Subsetting a pairlist with [] doesn't produce a pairlist:

class(.Options[1:3])

  [1] "list"

Yes, this one is documented, but still...

As the docs say, on R level pairlists are usually converted to  
vectors as the use of pairlists is deprecated.

Cheers,
Simon