insert elements in a vector in R

Question

I have a vector in R,

a = c(2,3,4,9,10,2,4,19)

let us say I want to efficiently insert the following vectors, b, and c,

b =         


        

Answer 1

After using Ferdinand's function, I tried to write my own and surprisingly it is far more efficient.
Here's mine :

insertElems = function(vect, pos, elems) {

l = length(vect)
  j = 0
  for (i in 1:length(pos)){
    if (pos[i]==1)
      vect = c(elems[j+1], vect)
    else if (pos[i] == length(vect)+1)
      vect = c(vect, elems[j+1])
    else
      vect = c(vect[1:(pos[i]-1+j)], elems[j+1], vect[(pos[i]+j):(l+j)])
    j = j+1
  }
  return(vect)
}

tmp = c(seq(1:5))
insertElems(tmp, c(2,4,5), c(NA,NA,NA))
# [1]  1 NA  2  3 NA  4 NA  5

insert.at(tmp, c(2,4,5), c(NA,NA,NA))
# [1]  1 NA  2  3 NA  4 NA  5

And there's the benchmark result :

> microbenchmark(insertElems(tmp, c(2,4,5), c(NA,NA,NA)), insert.at(tmp, c(2,4,5), c(NA,NA,NA)), times = 10000)
Unit: microseconds
                                        expr    min     lq     mean median     uq      max neval
 insertElems(tmp, c(2, 4, 5), c(NA, NA, NA))  9.660 11.472 13.44247  12.68 13.585 1630.421 10000
   insert.at(tmp, c(2, 4, 5), c(NA, NA, NA)) 58.866 62.791 70.36281  64.30 67.923 2475.366 10000

my code works even better for some cases :

> insert.at(tmp, c(1,4,5), c(NA,NA,NA))
# [1]  1  2  3 NA  4 NA  5 NA  1  2  3
# Warning message:
# In result[c(TRUE, FALSE)] <- split(a, cumsum(seq_along(a) %in% (pos))) :
#   number of items to replace is not a multiple of replacement length

> insertElems(tmp, c(1,4,5), c(NA,NA,NA))
# [1] NA  1  2  3 NA  4 NA  5