[R] Can a matrix have 'list' as rows/columns?

[William Dunlap]

> From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org] On Behalf
> Of Worik R
> Sent: Tuesday, April 17, 2012 4:28 PM
> To: r-help
> Subject: Re: [R] Can a matrix have 'list' as rows/columns?
> 
> On Tue, Apr 17, 2012 at 11:52 PM, David Winsemius <dwinsemius at comcast.net>wrote:
> 
> >
> > On Apr 17, 2012, at 12:13 AM, Worik R wrote:
> >
> >  After a lot of processing I get a matrix into M.  I expected each row and
> >> column to be a vector.  But it is a list.
> >>
> >
> > This behavior is not the result of limitation in how R's sapply might have
> > processed a purely numeric set of results, but is because you (probably)
> > returned a hetergeneous set of classes rom you inner function. Assuming
> > that "last" is actually function(x){tail,1}, then the structure of M is
> >
> > str(M)
> > List of 6
> > [snip]
> >  ..$ : chr [1:3] "aaa" "bbb" "ccc"
> >
> > Had the result been a more homogeneous collection, I sapply would have
> > returned an array of atomic numeric vectors. Try just returning a number:
> >
> > > M2 <- sapply(Qm, function(nm, DF){last(DF[DF[, "Name"]==nm,"Value"])},
> > DF)
> >
> 
> Yes that returns a vector.  I want a matrix.
> 
> I see that my problem is that the columns of DF are not all the same type.
> Once I did that (made Value character) I get my matrix just as I need.  SO
> it was I passed *in* that was the problem  Not what I did with it inside
> sapply.  In this case I would expect M to be a list.  I am gobsmacked that
> a list can be considered a vector.    Is that a bug?  It must be bad design?
> 
> I have been using R for a number of years (5?) and heavilly for two years.
> I am still getting bitten by these "features" in R.  To my mind there are
> many places that R violates the *principle of least surprise.  But it may
> be my mind that is at fault!  What are other people's experience?*

Since a matrix may contain logical, integer, numeric (double precision),
complex, and character data, I would be surprised if it didn't also handle
list data.  I am surprised that a matrix cannot contain factor data.
  > z <- matrix(factor(letters[1:6], levels=letters[1:10]), nrow=2)
  > z
       [,1] [,2] [,3]
  [1,] "a"  "c"  "e" 
  [2,] "b"  "d"  "f" 
  > str(z)
   chr [1:2, 1:3] "a" "b" "c" "d" "e" "f"

Different things surprise different people.

You might try using vapply() instead of sapply().  It forces you
to supply what you expect the output of FUN(X[[i]]) to look like
and will stop with an explicit error message if it doesn't match
your expectation on any iteration. 

Bill Dunlap
Spotfire, TIBCO Software
wdunlap tibco.com




> 
> Worik
> 
> 
> > > class(M)
> > [1] "numeric"
> > > str(M2)
> >  Named num [1:3] 0.6184 0.0446 0.3605
> >  - attr(*, "names")= chr [1:3] "aaa" "bbb" "ccc"
> >
> > --
> > David.
> >
> >>
> >> R-Inferno says...
> >>
> >> "Arrays (including matrices) can be subscripted with a matrix of positive
> >> numbers. The subscripting matrix has as many columns as there are
> >> dimensions
> >> in the array-so two columns for a matrix. The result is a vector (not an
> >> array)
> >> containing the selected items."
> >>
> >> My version of R:
> >> version.string R version 2.12.1 (2010-12-16)
> >>
> >> Here is an example...
> >>
> >>  Qm <- c("aaa", "bbb", "ccc")
> >>> DF <- data.frame(Name=sample(Qm, replace=TRUE, size=22), Value=runif(22),
> >>>
> >> stringsAsFactors=FALSE)
> >>
> >>> M <- sapply(Qm, function(nm, DF){last(DF[DF[, "Name"]==nm,])}, DF)
> >>> class(M)
> >>>
> >> [1] "matrix"
> >>
> >>> class(M[,1])
> >>>
> >> [1] "list"
> >>
> >>> class(M[1,])
> >>>
> >> [1] "list"
> >>
> >>> M
> >>>
> >>     aaa       bbb      ccc
> >> Name  "aaa"     "bbb"    "ccc"
> >> Value 0.4702648 0.274498 0.5529691
> >>
> >>> DF
> >>>
> >>  Name      Value
> >> 1   ccc 0.99948920
> >> 2   aaa 0.51921281
> >> 3   aaa 0.10803943
> >> 4   aaa 0.82265847
> >> 5   ccc 0.83237260
> >> 6   bbb 0.88250933
> >> 7   aaa 0.41836131
> >> 8   aaa 0.66197290
> >> 9   ccc 0.01911771
> >> 10  ccc 0.99994699
> >> 11  bbb 0.35719884
> >> 12  ccc 0.86274858
> >> 13  bbb 0.57528579
> >> 14  aaa 0.12452158
> >> 15  aaa 0.44167731
> >> 16  aaa 0.11660019
> >> 17  ccc 0.55296911
> >> 18  aaa 0.12796890
> >> 19  bbb 0.44595741
> >> 20  bbb 0.93024768
> >> 21  aaa 0.47026475
> >> 22  bbb 0.27449801
> >>
> >>>
> >>>
> >>        [[alternative HTML version deleted]]
> >>
> >> ______________________________**________________
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> help<https://stat.ethz.ch/mailman/listinfo/r-help>
> >> PLEASE do read the posting guide http://www.R-project.org/**
> >> posting-guide.html <http://www.R-project.org/posting-guide.html>
> >> and provide commented, minimal, self-contained, reproducible code.
> >>
> >
> > David Winsemius, MD
> > West Hartford, CT
> >
> >
> 
> 	[[alternative HTML version deleted]]