Le 10/27/2010 15:45, Gabor Grothendieck a écrit :
On Wed, Oct 27, 2010 at 8:38 AM, Ivan Calandra
<ivan.calan...@uni-hamburg.de> wrote:
What I don't understand is why vectors (with more than one value) don't have
dimensions. They look like they do have 1 dimension. For me no dimension
would be a scalar. Like in geometry: a point has no dimension, a line has 1,
a square has 2, a cube 3 and so on. Is it because of some internal process?
The intuitive geometry way of thinking is not programmatically relevant?
Maybe you used APL before. The basic structure in that language is
an array but that is not the case for R. The basic structure for data
is a vector and more complex data objects are build from that. An
array is a more complex object than a vector. A 1d array is not the
same as a vector. Dimensions are an additional concept unlike APL.
I've never used any other language before. It's just that I compare the
printing of an object to a geometric object, which means that a vector
of length > 1 has 1 dimension. In my mind, as I said, a point has no
dimension, a line has 1, a square has 2, a cube 3 and so on. But now, I
kind of understand that in R, dimensions do not really correspond to the
"shape" of an object, it's just an attribute. I'll then just accept that
1d array are not vectors and that vectors have NULL dimension (which is
not zero I guess!)
I would also add these:
- the components of a vector have to be of the same mode (character,
numeric, integer...)
however, a list with no attributes is a vector too so this is a vector:
> vl<- list(sin, 3, "a")
> is.vector(vl)
[1] TRUE
A vector may not have attributes so arrays and factors are not vectors
although they are composed from vectors.
That's also completely unexpected for me! What is then a vector?! And then
the difference between a vector and a list?! I mean, in practice, it's not
so important, my understanding is probably enough for what I'm doing in R,
but I'd like to understand how it works.
A list is really a vector of pointers so the components are of the
same type. Its just that you can't access the pointer nature of the
components. For example, you can have a matrix based on a list. We
have added a dimension to the list so it becomes an array even though
its based on a list:
m<- matrix(list(sin, "a", 1, list(1:3)), 2, 2)
dput(m)
structure(list(.Primitive("sin"), "a", 1, list(1:3)), .Dim = c(2L,
2L))
m
[,1] [,2]
[1,] ? 1
[2,] "a" List,1
is.array(m)
[1] TRUE
class(m)
[1] "matrix"
And even though each element has a different mode! I expected all
elements to be converted to a single character vector with 2d.
Good that I never have such extreme cases in my data! But it's something
to keep in mind to ensure that every object I create is what I want it to be
Also you wrote that a vector may not have attributes. I might be wrong (and
certainly am), but aren't names attributes? So with is a named list still a
vector:
my.list<- list(num=1:3, let=LETTERS[1:2])
names(my.list)
[1] "num" "let"
is.vector(my.list)
[1] TRUE
names don't count. Neither does class.
Thanks again!
Ivan
--
Ivan CALANDRA
PhD Student
University of Hamburg
Biozentrum Grindel und Zoologisches Museum
Abt. Säugetiere
Martin-Luther-King-Platz 3
D-20146 Hamburg, GERMANY
+49(0)40 42838 6231
ivan.calan...@uni-hamburg.de
**********
http://www.for771.uni-bonn.de
http://webapp5.rrz.uni-hamburg.de/mammals/eng/mitarbeiter.php
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