On Fri, 20 Oct 2000, Bill Simpson wrote:
> I have read over Venables and Ripley and am still confused about [] vs
> [[]]. I had thought it was simple: use [] for accessing items in a vector,
> and [[]] for items in a list.
>
> Vector
> > x<-1:10
> > x[4]
> [1] 4
> > x[[4]]
> [1] 4
> doesn't seem to matter if you use [] or [[]]
In this special case yes (IMHO)
>
> List
> > x<-list(1,2,3,4)
> > x[2]
> [[1]]
> [1] 2
>
> > x[[2]]
> [1] 2
> doesn't seem to matter if you use [] or [[]]
No, it does matter:
> x<-list(1,2,3,4)
> is.list(x[2])
[1] TRUE
> is.list(x[[2]])
[1] FALSE
> is.vector(x[[2]])
[1] TRUE
Using [] with a list you get a list, using [[]] you get
the item of these "sublist".
But be aware off:
> testlist<- list(list(cbind(1:4,1:4),1:5,"any.item"))
> testlist[1]
[[1]]
[[1]][[1]]
[,1] [,2]
[1,] 1 1
[2,] 2 2
[3,] 3 3
[4,] 4 4
[[1]][[2]]
[1] 1 2 3 4 5
[[1]][[3]]
[1] "any.item"
>>> returns a list, as usual
> testlist[[1]]
[[1]]
[,1] [,2]
[1,] 1 1
[2,] 2 2
[3,] 3 3
[4,] 4 4
[[2]]
[1] 1 2 3 4 5
[[3]]
[1] "any.item"
>>> returns also a list!
> testlist[[1]][[1]]
[,1] [,2]
[1,] 1 1
[2,] 2 2
[3,] 3 3
[4,] 4 4
returns the matrix!
Hope there are no more obscurity than before.
jan
--
-----------------------------------
Jan Goebel (mailto:jgoebel at diw.de)
DIW German Institute for Economic Research
SOEP
Königin-Luise-Str. 5
D-14195 Berlin
Germany
phone: 49 30 89789-377
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