How to make a unique combination of vectors in R?

Question

I have a couple of vectors consisting of three names. I want to get all unique pairwise combinations of these vectors. As an example, with two of those vectors, I can get the non-unique combinations with

sham1 <- c('a', 'b')
sham2 <- c('d', 'e')
shams <- list(sham1, sham2) 
combinations <- apply(expand.grid(shams, shams),1, unname)

which gives the following combinations

> dput(combinations)
list(
list(c("a", "b"), c("a", "b")), 
list(c("d", "e"), c("a", "b")), 
list(c("a", "b"), c("d", "e")), 
list(c("d", "e"), c("d", "e"))
)

I tried using unique(combinations) , but this gives the same result. What I would like to get is

> dput(combinations)
list(
list(c("a", "b"), c("a", "b")), 
list(c("d", "e"), c("a", "b")), 
list(c("d", "e"), c("d", "e"))
)

Because there is already the combination list(c("d", "e"), c("a", "b")) , I don't need the combination list(c("a", "b"), c("d", "e")) How can I get only the unique combination of vectors?

Answer 1

I am also not exactly sure what you want but this function might help:

combn

Here is a simple example:

> combn(letters[1:4], 2)
     [,1] [,2] [,3] [,4] [,5] [,6]
[1,] "a"  "a"  "a"  "b"  "b"  "c" 
[2,] "b"  "c"  "d"  "c"  "d"  "d" 

I don't think this is what you want, but if you clarify perhaps I can edit to get you what you want:

> sham1<-c('a','b')
> sham2<-c('d','e')
> combn(c(sham1,sham2),2)
     [,1] [,2] [,3] [,4] [,5] [,6]
[1,] "a"  "a"  "a"  "b"  "b"  "d" 
[2,] "b"  "d"  "e"  "d"  "e"  "e" 

Answer 2

s <- seq(length(shams))
# get unique pairs of shams indexes, including each index with itself.
uniq.pairs <- unique(as.data.frame(t(apply(expand.grid(s, s), 1, sort))))
#   V1 V2
# 1  1  1
# 2  1  2
# 4  2  2
result <- apply(uniq.pairs, 1, function(x) shams[x])

Answer 3

combn gets you the combinations (so, unique), but not the repeated ones. So combine that with something that gives you the repeated ones and you have it:

c(combn(shams, 2, simplify=FALSE), 
  lapply(shams, function(s) list(s,s)))

Answer 4

No idea what your examples are saying. If you want unique pairwise combinations:

strsplit(levels(interaction(sham1, sham2, sep="*")), "\\*")

Answer 5

I don't understand what do you want. And it seems that you changed the desired output from your other question .
You want your two list nested in a list inside another list??? It is not simpler to just once? Like when you have shams ?

dput(shams)
list(c("Sham1.r1", "Sham1.r2", "Sham1.r3"), c("Sham2.r1", "Sham2.r2", 
"Sham2.r3"))

To create such a nested list you could use that:

combinations <- list(shams, "")
dput(combinations)
list(list(c("Sham1.r1", "Sham1.r2", "Sham1.r3"), c("Sham2.r1", "Sham2.r2", 
"Sham2.r3"), "")

Although it is not exactly what do you said...