如何在向量列表上应用索引向量？

Question

I want to apply a long index vector (50+ non-sequential integers) to a long list of vectors (50+ character vectors containing 100+ names) in order to retrieve specific values (as a list, vector, or data frame).

A simplified example is below:

> my.list <- list(c("a","b","c"),c("d","e","f"))
> my.index <- 2:3

Desired Output

[[1]]
[1] "b"
[[2]]
[1] "f"
##or
[1] "b"
[1] "f"
##or
[1] "b" "f"

I know I can get the same value from each element using:

> lapply(my.list, function(x) x[2])
##or
> lapply(my.list,'[', 2)

I can pull the second and third values from each element by:

> lapply(my.list,'[', my.index)
[[1]]
[1] "b" "c"

[[2]]
[1] "e" "f"

##or
> for(j in my.index) for(i in seq_along(my.list)) print(my.list[[i]][[j]])
[1] "b"
[1] "e"
[1] "c"
[1] "f"

I don't know how to pull just the one value from each element.

I've been looking for a few days and haven't found any examples of this being done, but it seems fairly straight forward. Am I missing something obvious here?

Thank you, Scott

Answer 1

Whenever you have a problem that is like lapply but involves multiple parallel lists/vectors, consider Map or mapply ( Map simply being a wrapper around mapply with SIMPLIFY=FALSE hardcoded).

Try this:

Map("[",my.list,my.index)
#[[1]]
#[1] "b"
#
#[[2]]
#[1] "f"

..or:

mapply("[",my.list,my.index)
#[1] "b" "f"