suggestion for "sets" tools upgrade

On 14-02-06 8:31 PM, Carl Witthoft wrote:

My idea is to provide an upgrade to all the "sets" tools (intersect,
union, setdiff, setequal) that allows the user to apply them in a
strictly algebraic style.

The current tools, as well documented, remove duplicate values in the
input vectors.  This can be helpful in stats work, but is inconsistent
with the mathematical concept of sets and set measure.

I understand what you are asking for, but I think this justification for
it is just wrong.  Sets don't have duplicated elements:  an element is
in a set, or it is not.  It can't be in the set more than once.



What I propose

is that all these functions be given an additional argument with a
default value:  "multiple=FALSE" .  When called this way, the functions
remain as at present.  When called with "multiple=TRUE,"  they treat the
input vectors as true 'sets' of elements.

Here's an example of the new code:

intersect<-function (x, y,multiple=FALSE)
{
      y <- as.vector(y)
    trueint <- y[match(as.vector(x), y, 0L)]
      if(!multiple) trueint <- unique(trueint)
    return(trueint)
}

This is not symmetric.  I'd like intersect(x,y,TRUE) to be the same as
intersect(y,x,TRUE), up to re-ordering.  That's not true of your function:

 > x <- c(1,1,2,3)
 > y <- c(1,1,1,4)
 > intersect(x,y,multiple=TRUE)

[1] 1 1

 > intersect(y,x,multiple=TRUE)

[1] 1 1 1

I'd suggest that you clearly define what you mean by your functions, and
put them in a package, along with examples where they give more useful
results than the standard definitions.  I think the current base package
functions match the mathematical definitions better.

Duncan Murdoch