[R] sapply returning list instead of matrix

[William Dunlap]

> I can read the documentation, I see why it happens, but who in their right
> mind would design a function this way?  Can you imagine how many bugs are
> lurking because people haven't yet hit the right set of input that is going
> to cause sapply() to return a list instead of a matrix().

If you always want a list output use lapply().  If you want the simplification
that sapply does, but with sanity checks, use vapply().

vapply() lets you assert the type and size of FUN's return value.  If all goes
well it returns what sapply() would return but it throws an error if any call
to FUN returns something unexpected.  (Also, if length(X) is 0, vapply
makes the output be a zero-length object of the appropriate type.)
  > vapply(1:3, FUN=seq_along, FUN.VALUE=1L)
  [1] 1 1 1
  > vapply(1:3, FUN=range, FUN.VALUE=c(0,0))
       [,1] [,2] [,3]
  [1,]    1    2    3
  [2,]    1    2    3
  > vapply(1:3, FUN=seq, FUN.VALUE=1L)
  Error in vapply(1:3, FUN = seq, FUN.VALUE = 1L) :
    values must be length 1,
   but FUN(X[[2]]) result is length 2
  > vapply(numeric(0), FUN=range, FUN.VALUE=c(0,0)) # returns 2 by 0 numeric matrix
  
  [1,]
  [2,]

Bill Dunlap
TIBCO Software
wdunlap tibco.com


> -----Original Message-----
> From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org] On Behalf
> Of chris warth
> Sent: Friday, January 31, 2014 2:22 PM
> To: r-help at r-project.org
> Subject: Re: [R] sapply returning list instead of matrix
> 
> Hey thanks for the helpful snark, Bert.
> To everyone else, I apologize for neglecting to actually include the
> examples.
> 
> a <- function(i) { list(1) }
> b <- function(i) { list(1,2) }
> ll <- sapply(seq(3), a, simplfy="list")
> mm <- sapply(seq(3), b)
> class(ll)
> class(mm)
> > class(ll)
> [1] "list"
> > class(mm)
> [1] "matrix"
> 
> I can read the documentation, I see why it happens, but who in their right
> mind would design a function this way?  Can you imagine how many bugs are
> lurking because people haven't yet hit the right set of input that is going
> to cause sapply() to return a list instead of a matrix().
> 
> The point is that having the type of return value depend on the length of
> output from the applied function is simply madness.   It is a terrible
> design decision.  What is to be gained from the fact that I have to test
> the type of value returned from sapply()?   I was hoping plyr::laply()
> would be better but it perpetuates the same bad interface.
> 
> [so sorry for sending html, if that is what's happening.   I guess gmail
> send html by default? ]
> 
> 
> On Fri, Jan 31, 2014 at 1:44 PM, Bert Gunter <gunter.berton at gene.com> wrote:
> 
> > As you ignored the posting guide and posted in HTML, your "below"
> > didn't get through. So one can only guess that it has something to do
> > with (see ?sapply)
> >
> > "Simplification in sapply is only attempted if X has length greater
> > than zero and if the return values from all elements of X are all of
> > the same (positive) length. If the common length is one the result is
> > a vector, and if greater than one is a matrix with a column
> > corresponding to each element of X. "
> >
> > Return values most also be of the same type, also, obviously.
> >
> > Cheers,
> > Bert
> >
> > Bert Gunter
> > Genentech Nonclinical Biostatistics
> > (650) 467-7374
> >
> > "Data is not information. Information is not knowledge. And knowledge
> > is certainly not wisdom."
> > H. Gilbert Welch
> >
> >
> >
> >
> > On Fri, Jan 31, 2014 at 1:36 PM, chris warth <cswarth at gmail.com> wrote:
> > > Can anyone suggest a rationale for why sapply() returns different types
> > > (list and matrix) in the two examples below?   Is there any way to get
> > > sapply() or any other apply() function to return a matrix in both cases?
> > > simplify=TRUE doesn't change the outcome.
> > >
> > > I understand why it is happening, I just can't understand why such
> > > unpredictable behavior makes sense.
> > >
> > >         [[alternative HTML version deleted]]
> > >
> > > ______________________________________________
> > > R-help at r-project.org mailing list
> > > https://stat.ethz.ch/mailman/listinfo/r-help
> > > PLEASE do read the posting guide
> > http://www.R-project.org/posting-guide.html
> > > and provide commented, minimal, self-contained, reproducible code.
> >
> 
> 	[[alternative HTML version deleted]]
> 
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.