[R] dim vs length for vectors

[Uwe Ligges]

Olivia Lau wrote:
>>> IMHO the difference between vectors and 1-dimensional arrays is 
>>> really annoying and I had already several bugs in my code, because I 
>>> mixed these up.
>>> Is it possible in future versions of R that R does not differentiate 
>>> between vectors and 1-dimensional arrays (e.g. by treating all 
>>> vectors as 1-dimensional arrays)?
>>
>>
>> No, that would break huge amounts of code!
>>
>> See ?"[" and learn how to use its argument "drop".
> 
> 
> What I was proposing doesn't require a lot of programming.  Just 
> whenever you call dim(), it does length() if the object is a vector and 
> returns it in the format:
> 
> R> a <- 1:12
> R> dim(a)
>  [1]  12

Dim reports the dimesnion attribute, so why do you want to make it 
inconsistant????



> This means that you can provide a unified introduction to data 
> structures that take only one type of atomic value (and generally call 
> these structures "arrays").  What I call a "one dimensional array" only 
> has one dim attribute and hence would use a[4] for extraction (for 
> example), which is consistent with the usage for "[" (to my 
> understanding).  That way, when you introduce "[" and arrays at the same 
> time, you can tell beginners:
>  1) Run dim()
>  2) If there's one dim, use [  ]; if there are 2 dims, use [ ,  ]; if 
> there are 3 dims, you use [ , , ].
> This is conceptually easy for a beginner and avoids a bit of frustration.


Well, you forgot that even a matrix is represented as a vector, but with 
dimension attributes!
And indexing a matrix vector-like has some benefits in some cases.


> If I need to initiate an empty vector for example I use
> 
> R>  a <- array()
> 
> It just looks like the difference between a 1-d array (which doesn't 
> exist as far as I can tell) and a vector is semantic, and I think that R 


Note the difference:

   x <- 1:5
   dim(x) # NULL
   str(x) # int [1:5] 1 2 3 4 5

   x <- array(1:5, dim=5)
   dim(x) # [1] 5
   str(x) # int [, 1:5] 1 2 3 4 5



> would be more logical if they were treated as the same thing.  This 
> doesn't mean changing the way the is.array(), is.vector(), <-, etc 
> functions work, but just changing dim().

But then, you won't see the difference beween an 1D array (vector with 
dim attribute and a vector without dim attribute any more!

You might want to read the R Language Definition manual.

Uwe Ligges

> Yours,
> 
> Olivia Lau