[R] Subsetting a list of lists using lapply

[Rolf Turner]

On 20/02/15 08:45, Aron Lindberg wrote:
> Hi Everyone,
>
>
> I'm working on a thorny subsetting problem involving list of lists.

If you think this is "thorny" you ain't seen nothin' yet!

But note that you've got a list of lists of lists ... i.e. the nesting 
is at least 3 deep.

> I've put a dput of the data here:
>
> https://gist.githubusercontent.com/aronlindberg/b916dee897d051ac5be5/raw/a78cbf873a7e865c3173f943ff6309ea688c653b/dput
>

Thank you for creating a reproducible example.

> I can get one intense of the element I want this way:
>
> > input[[67]]$content[[1]]$sha [1]
> "58cf43ecdc1beb7e1043e9de612ecc817b090f15"
>
> However, I need to use a lapply function to loop over all of the
> items of the list. I've tried something like this, but it doesn't
> work:
>
> get_shas <- function(input){ x <- sapply(input, "[[", "content") y <-
> sapply(x, "[[", "sha") return(y) }
>
> sha_lists <- lapply(commit_lists, get_shas)
>
> However, this doesn't work. When I run each of the lapply commands
> "manually" it returns NULL for every list, and when I run the whole
> apply function it says:
>
>
> Error in FUN(X[[1L]], ...) : subscript out of bounds
>
>
> I've tried reading the sections on lists and subsetting in Hadley's
> Advanced R, but I still cannot figure it out. Can anyone help or
> offer a pointer?

At least part of the problem is that for some values of "i" 
input[[i]]$content[[1]] is a list (with an entry named "sha") and 
sometimes it is a character vector.

I don't follow your function get_shas() completely, so I started from 
scratch:

foo <- function (x){
sapply(x,function(y){
             z <- y$content[[1]]
             if(is.list(z)) z$sha else NA
          })
}

I find that foo(input) gives a vector of length 100, 81 entries of which 
are NA.  Entry number 67 at least agrees with what was shown in your email.

HTH

cheers,

Rolf Turner

-- 
Rolf Turner
Technical Editor ANZJS
Department of Statistics
University of Auckland
Phone: +64-9-373-7599 ext. 88276
Home phone: +64-9-480-4619