R help - Aug 2012 - r data structures

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

Hi,

On Thu, Aug 16, 2012 at 2:49 PM, Schumacher, Jay S <jays at neo.tamu.edu>
wrote:
>
>
> hi,
>   i'm trying to understand r data structures.  i see that vectors,
matrix, factors and arrays have a "dimension."
Out of curiosity, where do you "see" that vectors and factors have a
dimension? I mean -- I guess they're one dimensional, but ...
>   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?
data.frames: sure, I guess
lists: no

What would you consider the dimension of this list to be:

x = list(a=1:10, b='hello', c=matrix(1:100, nrow=10))

-steve

-- 
Steve Lianoglou
Graduate Student: Computational Systems Biology
 | Memorial Sloan-Kettering Cancer Center
 | Weill Medical College of Cornell University
Contact Info: http://cbio.mskcc.org/~lianos/contact

On Aug 16, 2012, at 11:49 AM, Schumacher, Jay S 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?
About half of what you have deduced is wrong. Matrices, arrays, and  
dataframes do have dimensions, at least in technical R parlance,  
namely they have an attribute which can be queried with dim(). By  
definition matrices and dataframes have 2 dimensions. Arrays and  
matrices can be redimensioned, but dataframes cannot.

Factors, lists, and atomic vectors do not have "dimensions", but they
do have "lengths". An appropriately structured list (one with vectors
all the same length) can be coerced to a dataframe with as.data.frame().

-- 

David Winsemius, MD
Alameda, CA, USA

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" <jays at 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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