yes, thank you, conceptual model (rather than formal dimension attribute)
is where i'm coming from at this point.
----------------------------------------
It would be helpful to distinguish between a formal dimension attribute,
and a (personal) conceptual model of whether or not any particular R
object, or type of object, has dimension. Mention of data frames having
dimension can be found in the help page for the dim() function.
> foo <- 1:10
> is.vector(foo)
[1] TRUE
> dim(foo)
NULL
> attributes(foo)
NULL
> str(foo)
int [1:10] 1 2 3 4 5 6 7 8 9 10
> length(foo)
[1] 10
> bah <- matrix(1:10, nrow=2)
> is.vector(bah)[1] FALSE
> dim(bah)
[1] 2 5
> attributes(bah)
$dim
[1] 2 5
> str(bah)
int [1:2, 1:5] 1 2 3 4 5 6 7 8 9 10
> length(bah)
[1] 10
The vector does not have a formal dimension ("dim") attribute, but the
matrix does.
Regarding data frames and lists:
##
## data frame
##
> junk <- data.frame(a=1:3,b=1:3)
> str(junk)
'data.frame': 3 obs. of 2 variables:
$ a: int 1 2 3
$ b: int 1 2 3
> attributes(junk)
$names
[1] "a" "b"
$row.names
[1] 1 2 3
$class
[1] "data.frame"
> dim(junk)
[1] 3 2
##
## list
##
> glug <- list(a=1, b=letters[3])
>
> str(glug)
List of 2
$ a: num 1
$ b: chr "c"
> attributes(glug)
$names
[1] "a" "b"
> dim(glug)
NULL
> length(glug)
[1] 2
Conceptually, I would consider data frames to have two dimensions (rows
and columns). They do not have a formal "dim" attribute, but the dim()
function does return a value.
I personally do not think of lists as having dimension -- I never ask
myself, what is the dimension of a list? But I do often enquire as to the
length of a list, so might, if forced to, admit that lists have one
dimension, length. But I do not think it is helpful to think of lists as
having dimension. Certainly, lists do not have two dimensions.
--
Don MacQueen
Lawrence Livermore National Laboratory
7000 East Ave., L-627
Livermore, CA 94550
925-423-1062
On 8/16/12 11:49 AM, "Schumacher, Jay S" <j...@neo.tamu.edu> wrote:
>
>
>hi,
> i'm trying to understand r data structures. i see that vectors,
>matrix, factors and arrays have a "dimension."
> there seems to be no mention of dimensionality anywhere for lists or
>dataframes. can i consider lists and frames to be of fixed dimension 2?
>thanks,
> jay s
>
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