Accessing same named list elements of the list of lists in R

Frequently I encounter situations where I need to create a lot of similar models for different v开发者_高级运维ariables. Usually I dump them into the list. Here is the example of dummy code:

modlist <- lapply(1:10,function(l) {
   data <- data.frame(Y=rnorm(10),X=rnorm(10))
   lm(Y~.,data=data)
})

Now getting the fit for example is very easy:

lapply(modlist,predict)

What I want to do sometimes is to extract one element from the list. The obvious way is

sapply(modlist,function(l)l$rank)

This does what I want, but I wonder if there is a shorter way to get the same result?

probably these are a little bit simple:

> z <- list(list(a=1, b=2), list(a=3, b=4))
> sapply(z, `[[`, "b")
[1] 2 4
> sapply(z, get, x="b")
[1] 2 4

and you can define a function like:

> `%c%` <- function(x, n)sapply(x, `[[`, n)
> z %c% "b"
[1] 2 4

and also this looks like an extension of $:

> `%$%` <- function(x, n) sapply(x, `[[`, as.character(as.list(match.call())$n))
> z%$%b
[1] 2 4

I usually use kohske way, but here is another trick:

 sapply(modlist, with, rank)

It is more useful when you need more elements, e.g.:

 sapply(modlist, with, c(rank, df.residual))

As I remember I stole it from hadley (from plyr documentation I think).

Main difference between [[ and with solutions is in case missing elements. [[ returns NULL when element is missing. with throw an error unless there exist an object in global workspace having same name as searched element. So e.g.:

dah <- 1
lapply(modlist, with, dah)

returns list of ones when modlist don't have any dah element.

With Hadley's new lowliner package you can supply map() with a numeric index or an element name to elegantly pluck components out of a list. map() is the equivalent of lapply() with some extra tricks.

library("lowliner")

l <- list(
  list(a = 1, b = 2),
  list(a = 3, b = 4)
)

map(l, "b")
map(l, 2)

There is also a version that simplifies the result to a vector

map_v(l, "a")
map_v(l, 1)