challenge: optimize unlisting [easy]

list, optimization, r, vector

Solution

If you don't need names and your list is one level deep, then if you can beat

.Internal(unlist(your_list, FALSE, FALSE))

I will vote up everything you do on SO for the next 1 year!!!

[Update: if non-unique names are needed and the list is not recursive, here is a version which improves over the unlist 100 times

 myunlist <- function(l){
    names <- names(l)
    vec <- unlist(l, F, F)
    reps <- unlist(lapply(l, length), F, F)
    names(vec) <- rep(names, reps)
    vec
    }

 myunlist(list(a=1:3, b=2))
 a a a b 
 1 2 3 2 

 > tl <- list(a = 1:20000, b = 1:5000, c = 2:30)
 > system.time(for(i in 1:200) unlist(tl))
 user  system elapsed 
 22.97    0.00   23.00 

 > system.time(for(i in 1:200) myunlist(tl))
 user  system elapsed 
 0.2     0.0     0.2 

 > system.time(for(i in 1:200) unlist(tl, F, F))
 user  system elapsed 
 0.02    0.00    0.02

]

[Update2: Responce to challenge Nr3 from Richie Cotton.

bigList3 <- replicate(500, rnorm(1e3), simplify = F)

unlist_vit <- function(l){
    names(l) <- NULL
    do.call(c, l)
    }

library(rbenchmark)

benchmark(unlist = unlist(bigList3, FALSE, FALSE),
          rjc    = unlist_rjc(bigList3),
          vit    = unlist_vit(bigList3),
          order  = "elapsed",
          replications = 100,
          columns = c("test", "relative", "elapsed")
          )

    test  relative elapsed
1 unlist   1.0000    2.06
3    vit   1.4369    2.96
2    rjc   3.5146    7.24

]

PS: I assume a "big fish" is the one with more reputation than you. So I am pretty much small here :).

Problem

Because SO is a bit slow lately, I'm posting an easy question. I would appreciate it if big fishes stayed on the bench for this one and give rookies a chance to respond. Sometimes we have objects that have a ridiculous amount of large list elements (vectors). How would you "unlist" this object into a single vector. Show proof that your method is faster than `unlist()`.

Original source