Sorting a list of unequal-size vectors in r

Question

Suppose I have several vectors - maybe they're stored in a list, but if there's a better data structure that's fine too:

ll <- list(c(1,3,2),
           c(1,2),
           c(2,1),
           c(1,3,1))

And I want to sort them, using the first number, then the second number to resolve ties, then the third number to resolve remaining ties, etc.:

c(1,2)
c(1,3,1)
c(1,3,2)
c(2,1)

Are there any built in functions that will allow me to do this or do I need to roll my own solution?

(For those who know Python, what I'm after is something that mimics the behavior of sort in Python)

Answer 1

ll <- list(c(1,3,2),
           c(1,2),
           c(2,1),
           c(1,3,1))

I'd prefer using NA for missing values and using rbind.data.frame instead of paste:

sortfun <- function(l) {
  l1 <- lapply(l, function(x, n) {
    length(x) <- n
    x
  }, n = max(lengths(l)))
  l1 <- do.call(rbind.data.frame, l1)
  l[do.call(order, l1)] #order's default is na.last = TRUE
}

sortfun(ll)

#[[1]]
#[1] 1 2
#
#[[2]]
#[1] 1 3 1
#
#[[3]]
#[1] 1 3 2
#
#[[4]]
#[1] 2 1

Answer 2

Here's an approach that uses data.table.

The result is a rectangular data.table with the rows ordered in the form you described. NA values are filled in where the list item was a different length.

library(data.table)
setorderv(data.table(do.call(cbind, transpose(l))), paste0("V", 1:max(lengths(l))))[]
#    V1 V2 V3
# 1:  1  2 NA
# 2:  1  3  1
# 3:  1  3  2
# 4:  2  1 NA

---

This is ugly, but you can use the result on your list with something like:

l[setorderv(
  data.table(
    do.call(cbind, transpose(l)))[
      , ind := seq_along(l)][], 
  paste0("V", seq_len(max(lengths(l)))))$ind]