[R] lists -- "vector" etc.

[Martin Maechler]

>>>>> "Jim" == Jim Lindsey <jlindsey at alpha.luc.ac.be> writes:

    Jim> I am still struggling with broken libraries after the changes in
    Jim> lists in 0.63 (as is my son who says he has abandoned lists
    Jim> altogether!).  A user can supply a vector (say one time series), a
    Jim> matrix (several series as balanced repeated measures), or a list
    Jim> (unbalanced repeated measures).  I used to check with is.vector,
    Jim> is.matrix, and is.list. This no longer works because a list is now
    Jim> a vector.
This is not different from S...
There really is a terminology problem hidden here which uses to confuse
(almost) everyone from time to time :
	 
 What is a "vector"?   <<-->>  "How should  as.vector() / is.vector() behave?
 ~~~~~~~~~~~~~~~~~~~   

 1) For most people with some math background, it is very natural that
        c(2,3,5) or c("A","BC", "DE") is a vector.

 2) Then one learns that the S language
	 [in the broad ESS sense, R being a dialect, S3 and S4 as well],
    allows for names (often called "tags" R internally],
    one would expect that c(a=2,b=3,c=5) is a vector as well, namely just
    c(2,3,5) with a names attribute.	 

    However, in S 3 and S 4,

 3) a "vector" is a ``simple object'' in the sense of S, i.e.
         is.vector(c(a=2)) #>> FALSE
    whereas it stil gives TRUE in R [0.63.1].  However, we have been having a
    hot and not yet resolved discussion within the R core team on whether
    we want to keep it as it is now...
       [[And yes, there are more problems with as.vector() / is.vector()]]
    Maybe we should discuss this more in the R-devel mailing list.

    Even more `strangely' in the sense of "1)" above,
	 is.vector(list(1, list(3, list(4:6)))) ## >> TRUE
    in S 3 or S 4, and now also in R 0.63 [but not in R 0.62].
  
    Another argument for this behavior is the R (and S?) internal
    implementation of list()s, as so called ``generic vectors''.
    I.e., here "vector" is used as ``generic vector''.

BTW:  ?is.vector  in S-plus  5.0 (rel2) contains

  >> BUGS:
  >>        A list may have names and still test TRUE with  is.vector,
  >>        but atomic objects with names will test FALSE.

What I don't know if this is a bug that anyone [JMC?] plans to fix or one
that has been in place for ever (in S) and will be kept (in S) for backwards
compatibility  ((and I personally would really not want to adopt in R))...

In (partial) summary, we currently have

  ( ll <- list(a=1, list(b=3, list(cc=4:6))) )
  ( vv <- unlist(ll, recursive = TRUE) )
  is.vector(ll) # TRUE
  is.vector(vv) # FALSE in S [incl. S+5 rel 2]   TRUE in R

-----------------------------------

    Jim> The vector must allow calculations so can be integer,
    Jim> double, or logical. So I set the mode in is.vector. Try the
    Jim> following:

    Jim> mode(c(23,42))
       # "numeric"

       typeof(c(23,42))
or     storage.mode(c(23,42))
both give
     "double"

[which is compatible to S 3 (S+ 3.x, 4.x) 
		but not S 4 (S+ 5.0++) which now uses integer for integer
		literals]

    Jim> is.vector(c(23,42),mode="numeric")
      FALSE

This *is* a bug, [and not the only with  is.vector/as.vector, see above].
However,
	> is.vector(c(23,42),mode="numeric")
	TRUE

    Jim> is.vector(c(23,42),mode=c("integer","double","logical"))

    Jim> Of course, in 0.62.4, things were no more coherent than this;

    Jim> is.vector(list(1))
    Jim> is.vector(list(1),mode="list")

---
As a matter of fact, my current recommendation:
Don't use  is.vector() at all at least not until we [and the S authors?]
know what is desired?
Instead, use
		is.list()
and/or		is.atomic()  is.recursive()  is.array()

Martin Maechler <maechler at stat.math.ethz.ch>	http://stat.ethz.ch/~maechler/
Seminar fuer Statistik, ETH-Zentrum SOL G1;	Sonneggstr.33
ETH (Federal Inst. Technology)	8092 Zurich	SWITZERLAND
phone: x-41-1-632-3408		fax: ...-1086			<><
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