[R] Extract part of a list based on the value of one of the subsubscript (weird, but with a reproducible example for clarity sake)

[Duncan Murdoch]

On 14-03-17 7:27 AM, sylvain willart wrote:
> Dear R (list)-users
>
> I'm trying to extract part of a list based on the value of one of the
> subsubscript element
>
> As those things are usually quite complicated to explain, let me provide a
> simple reproducible example that closely matches my problem (although my
> actual list has a few thousands elements):
>
> ###### EXAMPLE
> MyFullList<-list()
> v1<-rep(c("A","B","C","D","E"),2)
> v2<-c(rep(1,5),rep(2,5))
> for (i in 1:10){
>    MyFullList[[i]]<-density(runif(10,0,1))
>    MyFullList[[i]][8]<-i
>    MyFullList[[i]][9]<-v1[i]
>    MyFullList[[i]][10]<-v2[i]
> }
> ###### end example
>
> Now, my problem is that I would like to extract, in a new list, a part of
> the full list based on the value of it's 9th subscript, let say, "B". This
> new list has to include S3-densities objects (stored in the first 7
> sub-elements)

You'll need to do this in two steps, you can't do it using only 
indexing.  The problem is that logical indexing requires a logical 
vector, and it's not easy to get one of those when you are starting with 
a list.  So here's how to do it:

First create a new character vector containing the 9th subscript of each 
element, e.g.

ninths <- sapply(MyFullList, function(x) x[[9]])

You use sapply so that the result is coerced to a character vector, and 
x[[9]] rather than x[9] so that each result is a scalar character.

Do your test on that, and use the result to index the full list:

MyFullList[ninths == "B"]

This returns a list containing the cases where the test evaluates to TRUE.

Duncan Murdoch

>
> Here's what I tried (and the errors I got)
>
> ####### TRIALS
> MyList_v1_B<-MyFullList[MyFullList[[]][9]=="B"]
> # error invalid subscript type 'symbol'
> MyList_v1_B<-MyFullList[MyFullList[][9]=="B"]
> # no errors, but returns an empty list ???
> MyList_v1_B<-MyFullList[MyFullList[,9]=="B"]
> # error incorrect number of dimensions
> ######## end trials (for now)
>
> Obviously, I'm missing something,
> And I would appreciate any clue to help me perform this task
>
> # Here is my R.version info, although I'm not sure it's relevant here
>> R.version
> platform       x86_64-unknown-linux-gnu
> arch           x86_64
> os             linux-gnu
> system         x86_64, linux-gnu
> status
> major          2
> minor          15.2
> year           2012
> month          10
> day            26
> svn rev        61015
> language       R
> version.string R version 2.15.2 (2012-10-26)
> nickname       Trick or Treat
>
> thanks
> Sylvain Willart
>
> 	[[alternative HTML version deleted]]
>
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