Alternative in R to cbind loop when final size unknown

Question

I need to calculate for each element of my list all the combination of the elements inside and stock all the combination ( b in the example). I am currently doing like in my example but for a bigger list it is really slow because of the use of cbind in the loop. as i dont know the total final size of my vector b, i cant allocate an object at first. i am looking for other alternatives to make it more efficient.

b=0
a = list(id1=c(1,2,3,4,5,6),id2=c(10,11,12))
for(i in 1:length(a)){
   temp=combn(a[[i]],2) 
   b=cbind(b,temp)
}

Answer 1

We can do this with base R

do.call(cbind, lapply(a, combn, 2))
#[,1] [,2] [,3] [,4] [,5] [,6] [,7] [,8] [,9] [,10] [,11] [,12] [,13] [,14]
#[1,]    1    1    1    1    1    2    2    2    2     3     3     3     4     4
#[2,]    2    3    4    5    6    3    4    5    6     4     5     6     5     6
#     [,15] [,16] [,17] [,18]
#[1,]     5    10    10    11
#[2,]     6    11    12    12

Answer 2

Here is an alternative to Arun's solution from the comments, using purrr::map

data.frame(purrr::map(a, combn, 2))
#  id1.1 id1.2 id1.3 id1.4 id1.5 id1.6 id1.7 id1.8 id1.9 id1.10 id1.11 id1.12
#1     1     1     1     1     1     2     2     2     2      3      3      3
#2     2     3     4     5     6     3     4     5     6      4      5      6
#  id1.13 id1.14 id1.15 id2.1 id2.2 id2.3
#1      4      4      5    10    10    11
#2      5      6      6    11    12    12

Or

map_dfc(a, ~data.frame(combn(.x, 2)))
#  X1 X2 X3 X4 X5 X6 X7 X8 X9 X10 X11 X12 X13 X14 X15 X16 X21 X31
#1  1  1  1  1  1  2  2  2  2   3   3   3   4   4   5  10  10  11
#2  2  3  4  5  6  3  4  5  6   4   5   6   5   6   6  11  12  12