[R] do.call("+", ...)

[Robin Hankin]

Thanks for this Duncan

>>


>> Why is Peter Dalgaard's suggestion necessary?  Why can't  "+"
>> take more than two arguments?
>
> One reason is that it's a binary operator, so that's all it needs to
> take.  We have the sum function for multiple operands.
>
> I would guess the historical reason is so that it can share code with
> other binary operators.  For example, + currently shares code with the
> other binary operators -, *, /, ^, %%, %/%, but the grouping needed
> varies between them:  a-b-c == (a-b)-c, but a^b^c == a^(b^c).  R lets
> the parser handle operator binding.
>


OK, I see.  But in algebra the  "+" symbol is special: it is reserved
exclusively for associative and commutative operations [thus a+b+c is
always well-defined]; perhaps the parser could fall in with this  
convention?


> By the way, another complaint is that sum() is supposed to be generic,
> but you can't define a sum.matrix() method so that sum(a,b,c) does the
> same as a+b+c when a is a matrix.  (This would probably be a bad idea
> because people may be relying on the current behaviour, but R tries  
> not
> to prevent people from testing out bad ideas.)  ...


Can  just clarify this?   I can see that it's a bad idea, but I don't  
quite
see why one *can't* do it.  sum() is generic, and the manpage says
that methods can be defined for it directly.

OK, so I define a function sum.matrix()
[for the moment just taking two arguments]:


  R> sum.matrix <- function(a,b){matrix(apply(matrix(c(a,b),ncol=2), 
1,sum),dim(a))}

R> a <- matrix(1:6,2,3)

R> sum(a,a)
[1] 42
  R> sum.matrix(a,a)
      [,1] [,2] [,3]
[1,]    2    6   10
[2,]    4    8   12
R>


So why doesn't  sum(a,a) dispatch to  sum.matrix()?



best wishes


rksh


> You need to apply your
> own class to the matrices (e.g. mymatrix), then it all works:



>> f  <- function(i){m <- matrix((1:6)^i,2,3); class(m) <-  
>> "mymatrix"; m}
>>
>> sum.mymatrix <- function(...) {
> +     x <- list(...)
> +     return(x[[1]] + do.call(sum, x[-1]))
> + }
>>
>> do.call("sum",sapply(1:4,f,simplify=FALSE))
>       [,1] [,2] [,3]
> [1,]    4  120  780
> [2,]   30  340 1554
> attr(,"class")
> [1] "mymatrix"
>
> Duncan Murdoch
>
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> and provide commented, minimal, self-contained, reproducible code.

--
Robin Hankin
Uncertainty Analyst
National Oceanography Centre, Southampton
European Way, Southampton SO14 3ZH, UK
  tel  023-8059-7743