[Rd] outer not applying a constant function

[Martin Maechler]

>>>>> William Dunlap <wdunlap at tibco.com>
>>>>>     on Mon, 20 Mar 2017 10:20:11 -0700 writes:

    >> 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 [1]

    > Bill Dunlap
    > TIBCO Software
    > wdunlap tibco.com

You are right, thank you!
Such a "convenience change" would not be a good idea.

Martin Maechler
ETH Zurich




    > 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 [1]
    >> 
    >> > 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 [1]
    >> 
    >>> 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?
    >> 
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