On Thu, 25 Dec 2014, peter dalgaard wrote:
On 25 Dec 2014, at 08:15 , Mike Miller <mbmille...@gmail.com> wrote:
"is.vector returns TRUE if x is a vector of the specified mode having
no attributes other than names. It returns FALSE otherwise."
So that means that a vector in R has no attributes other than names.
Wrong. Read carefully. There are
- vectors
- vectors having no attributes other than names
You are right. I was being difficult about the meaning of "is.vector()".
But would you also say that a matrix is a vector?
I was going to ask a question about it how to test that an object is a
vector, but then I found this:
"is.vector() does not test if an object is a vector. Instead it returns
TRUE only if the object is a vector with no attributes apart from names.
Use is.atomic(x) || is.list(x) to test if an object is actually a vector."
From here:
http://adv-r.had.co.nz/Data-structures.html#vectors
a <- c(1,2,3,4)
names(a) <- LETTERS[1:4]
attr(a, "vecinfo") <- "yes, I'm a vector"
a
A B C D
1 2 3 4
attr(,"vecinfo")
[1] "yes, I'm a vector"
attributes(a)
$names
[1] "A" "B" "C" "D"
$vecinfo
[1] "yes, I'm a vector"
is.vector(a)
[1] FALSE
is.atomic(a) || is.list(a)
[1] TRUE
But then we also see this:
b <- matrix(1:4, 2,2)
is.atomic(b) || is.list(b)
[1] TRUE
"It is common to call the atomic types ‘atomic vectors’, but note that
is.vector imposes further restrictions: an object can be atomic but not a
vector (in that sense)."
https://stat.ethz.ch/R-manual/R-devel/library/base/html/is.recursive.html
I think a matrix is always atomic. So a matrix is "not a vector (in that
sense)," but "is.matrix returns TRUE if x is a vector and has a 'dim'
attribute of length 2."
I do think I get what is going on with this, but why should I buy into
this conceptualization? Why is it better to say that a matrix *is* a
vector than to say that a matrix *contains* a vector? The latter seems to
be the more common way of thinking but such things. Even in R you've had
to construct two different definitions of "vector" to deal with the
inconsistency created by the "matrix is a vector" way of thinking. So
there must be something really good about it that I am not understanding
(and I'm not being facetious or ironic!)
Mike
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