convert a vector to a list

Question

I have a vector like this

 c(\"1\", \"a\",\"b\")

and I\'d like to create this list

list(\"a\"=1,\"b\"=1)

is

Answer 1

It isn't an apply style, but a simple function to wrap the required commands is:

makeList <- function(vec) {
    len <- length(vec[-1])
    out <- as.list(rep(as.numeric(vec[1]), len))
    names(out) <- as.character(vec[-1])
    out
}

Using your vector, it gives:

> vec <- c("1", "a","b")
> makeList(vec)
$a
[1] 1

$b
[1] 1

Answer 2

Using as.list and setNames:

x = c("1", "a","b")
as.list(setNames(rep(as.numeric(x[1]), length(x) - 1), x[-1]))

Answer 3

Like this?

R> kn <- c("1", "a", "b")
R> nl <- vector(mode="list", length=length(kn)-1)
R> names(nl) <- kn[-1]
R> nl <- lapply(nl, function(x) kn[1])
R> nl
$a
[1] "1"

$b
[1] "1"

R> 

With kudos to Gavin for spotting an earlier error.

Answer 4

For completeness, there is a simpler one-liner to do it in an "apply" style as requested:

as.list(sapply(x[-1],function(y) as.double(x[1])))

While not the fastest option, it is surely neat enough to deserve its place as an answer to the question. A significant speed-up is possible by not unnecessarily simplifying to a vector:

    library(microbenchmark)
    microbenchmark(times=20, 
               Charles=as.list(setNames(rep(as.numeric(x[1]), length(x) - 1), x[-1])),
               Gavin=makeList(x),
               Anon=sapply(x[-1],function(y) as.double(x[1]),simplify=FALSE)
)

Unit: microseconds
    expr    min      lq median      uq    max neval
 Charles 10.868 11.7735 11.774 12.3775 55.848    20
   Gavin 12.075 12.6795 13.132 13.8870 26.867    20
    Anon  6.643  7.0950  7.548  8.1520 17.811    20