What is the most efficient way to cast a list as a data frame?

Question

Very often I want to convert a list wherein each index has identical element types to a data frame. For example, I may have a list:

> my.list
[[1]]
[[1]]         


        

Answer 1

This

f = function(x) function(i) sapply(x, `[[`, i)

is a function that returns a function that extracts the i'th element of x. So

Map(f(mylist), names(mylist[[1]]))

gets a named (thanks Map!) list of vectors that can be made into a data frame

as.data.frame(Map(f(mylist), names(mylist[[1]])))

For speed it's usually faster to use unlist(lapply(...), use.names=FALSE) as

f = function(x) function(i) unlist(lapply(x, `[[`, i), use.names=FALSE)

A more general variant is

f = function(X, FUN) function(...) sapply(X, FUN, ...)

When do the list-of-lists structures come up? Maybe there's an earlier step where an iteration could be replaced by something more vectorized?