[Rd] outer not applying a constant function
wdunlap at tibco.com
Mon Mar 20 18:20:11 CET 2017
> Or is this a bad idea?
I don't like the proposal. I have seen code like the following (in
fact, I have written such code, where I had forgotten a function was
not vectorized) where the error would have been discovered much later
if outer() didn't catch it.
> outer(1:3, 11:13, sum)
Error in outer(1:3, 11:13, sum) :
dims [product 9] do not match the length of object 
On Mon, Mar 20, 2017 at 6:36 AM, Martin Maechler
<maechler at stat.math.ethz.ch> wrote:
>>>>>> Gebhardt, Albrecht <Albrecht.Gebhardt at aau.at>
>>>>>> on Sun, 19 Mar 2017 09:14:56 +0000 writes:
> > Hi,
> > the function outer can not apply a constant function as in the last line of the following example:
> >> xg <- 1:4
> >> yg <- 1:4
> >> fxyg <- outer(xg, yg, function(x,y) x*y)
> >> fconstg <- outer(xg, yg, function(x,y) 1.0)
> > Error in outer(xg, yg, function(x, y) 1) :
> > dims [product 16] do not match the length of object 
> > Of course there are simpler ways to construct a constant matrix, that is not my point.
> > It happens for me in the context of generating matrices of partial derivatives, and if on of these partial derivatives happens to be constant it fails.
> > So e.g this works:
> > library(Deriv)
> > f <- function(x,y) (x-1.5)*(y-1)*(x-1.8)+(y-1.9)^2*(x-1.1)^3
> > fx <- Deriv(f,"x")
> > fy <- Deriv(f,"y")
> > fxy <- Deriv(Deriv(f,"y"),"x")
> > fxx <- Deriv(Deriv(f,"x"),"x")
> > fyy <- Deriv(Deriv(f,"y"),"y")
> > fg <- outer(xg,yg,f)
> > fxg <- outer(xg,yg,fx)
> > fyg <- outer(xg,yg,fy)
> > fxyg <- outer(xg,yg,fxy)
> > fxxg <- outer(xg,yg,fxx)
> > fyyg <- outer(xg,yg,fyy)
> > And with
> > f <- function(x,y) x+y
> > it stops working. Of course I can manually fix this for that special case, but thats not my point. I simply thought "outer" should be able to handle constant functions.
> ?outer clearly states that FUN needs to be vectorized
> but function(x,y) 1 is not.
> It is easy to solve by wrapping the function in Vectorize(.):
>> x <- 1:3; y <- 1:4
>> outer(x,y, function(x,y) 1)
> Error in dim(robj) <- c(dX, dY) :
> dims [product 12] do not match the length of object 
>> outer(x,y, Vectorize(function(x,y) 1))
> [,1] [,2] [,3] [,4]
> [1,] 1 1 1 1
> [2,] 1 1 1 1
> [3,] 1 1 1 1
> So, your "should" above must be read in the sense
> "It really would be convenient here and
> correspond to other "recycling" behavior of R"
> and I agree with that, having experienced the same inconvenience
> as you several times in the past.
> outer() being a nice R-level function (i.e., no C speed up)
> makes it easy to improve:
> Adding something like the line
> if(length(robj) == 1L) robj <- rep.int(robj, dX*dY)
> before dim(robj) <- c(dX, dY) [which gave the error]
> would solve the issue and not cost much (in the cases it is unneeded).
> Or is this a bad idea?
> R-devel at r-project.org mailing list
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