[R] intersect of list elements

[Sundar Dorai-Raj]

Georg Otto wrote:
> Hi,
> 
> i have a list of several vectors, for example:
> 
> 
>>vectorlist
> 
> $vector.a.1
> [1] "a" "b" "c"
> 
> $vector.a.2
> [1] "a" "b" "d"
> 
> $vector.b.1
> [1] "e" "f" "g"
> 
> 
> I can use intersect to find elements that appear in $vector.a.1 and
> $vector.a.2:
> 
> 
>>intersect(vectorlist[[1]], vectorlist[[2]])
> 
> [1] "a" "b"
> 
> 
> I would like to use grep to get the vectors by their names matching an
> expression and to find the intersects between those vectors. For the
> first step:
> 
> 
>>vectorlist[grep ("vector.a", names(vectorlist))]
> 
> $vector.a.1
> [1] "a" "b" "c"
> 
> $vector.a.2
> [1] "a" "b" "d"
> 
> 
> Unfortunately, I can not pass the two vectors as argument to intersect:
> 
> 
>>intersect(vectorlist[grep ("vector.a", names(vectorlist))])
> 
> Error in unique(y[match(x, y, 0)]) : argument "y" is missing, with no default
> 
> I am running R Version 2.3.1 (2006-06-01) 
> 
> 
> Could somone help me to solve this?
> 
> Cheers,
> 
> Georg
> 
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Will this work for you?

vectorlist <- list(vector.a.1 = c("a", "b", "c"),
                    vector.a.2 = c("a", "b", "d"),
                    vector.b.1 = c("e", "f", "g"))

intersect2 <- function(...) {
   args <- list(...)
   nargs <- length(args)
   if(nargs <= 1) {
     if(nargs == 1 && is.list(args[[1]])) {
       do.call("intersect2", args[[1]])
     } else {
       stop("cannot evaluate intersection fewer than 2 arguments")
     }
   } else if(nargs == 2) {
     intersect(args[[1]], args[[2]])
   } else {
     intersect(args[[1]], intersect2(args[-1]))
   }
}

vector.a <- vectorlist[grep ("vector.a", names(vectorlist))]
intersect2(vector.a)
intersect2(vectorlist)

HTH,

--sundar