[R] Subsetting a list

[Martin Maechler]

>>>>> "Jose" == José Ernesto Jardim <ernesto at ipimar.pt>
>>>>>     on Tue, 18 Oct 2005 15:30:59 +0100 writes:

    Jose> Dennis Fisher wrote:
    >> Colleagues,
    >> 
    >> I have created a list in the following manner:
    >> TEST    <- list(c("A1", "A2"), c("B1", "B2"), c("C1", "C2"))
    >> 
    >> I now want to delete one element from the list, e.g., the third.  The  
    >> command
    >> TEST[[3]]
    >> yields (as expected):
    >> [1] "C1" "C2"
    >> 
    >> The command
    >> TEST[[-3]]
    >> yields:
    >> Error: attempt to select more than one element
    >> 
    >> How can I accomplish delete one or more elements from this list?
    >> 
    >> I am running R2.2.0 on a Linux platform.
    >> 
    >> Dennis
    >> 

    Jose> TEST[[3]] <- NULL

    Jose> Lists are not subsetable like data.frames or arrays, see the manuals.

yes, they are, almost, see other replies and "the manuals" :

Lists can have 'dim' attributes and hence be treated as arrays;
Note that this is pretty rarely used and not too well supported
by some tools, one could say even 'print()' :

> set.seed(0); L0 <- L <- lapply(rpois(12, lambda=3), seq); dim(L) <- 3:4; L
     [,1]      [,2]      [,3]      [,4]     
[1,] Integer,5 Integer,3 Integer,5 Integer,3
[2,] Integer,2 Integer,5 Integer,6 1        
[3,] Integer,2 Integer,2 Integer,4 Integer,2
> str(L)
List of 12
 $ : int [1:5] 1 2 3 4 5
 $ : int [1:2] 1 2
 $ : int [1:2] 1 2
 $ : int [1:3] 1 2 3
 $ : int [1:5] 1 2 3 4 5
 $ : int [1:2] 1 2
 $ : int [1:5] 1 2 3 4 5
 $ : int [1:6] 1 2 3 4 5 6
 $ : int [1:4] 1 2 3 4
 $ : int [1:3] 1 2 3
 $ : int 1
 $ : int [1:2] 1 2
 - attr(*, "dim")= int [1:2] 3 4
> L[2,3]
[[1]]
[1] 1 2 3 4 5 6

> L[-(1:10)] ## treated matrix ``as vector''
[[1]]
[1] 1

[[2]]
[1] 1 2