Fastest way to transpose a list in R / Rcpp

Question

I have a list:

ls <- list(c(\"a\", \"b\", \"c\"), c(\"1\", \"2\", \"3\"), c(\"foo\", \"bar\", \"baz\"))
ls

#> [[1]]
#> [1] \"a\" \"b\" \"c\"

#>         


        

Answer 1

In the data.table package, there's a transpose() function which does exactly this. It is implemented in C for speed.

require(data.table) # v1.9.6+
transpose(ls)
# [[1]]
# [1] "a"   "1"   "foo"

# [[2]]
# [1] "b"   "2"   "bar"

# [[3]]
# [1] "c"   "3"   "baz"

It also fills automatically with NA in case the list elements are not of equal lengths, and also coerces automatically to the highest SEXPTYPE. You can provide a different value to the fill argument if necessary. Check ?transpose.