[R] ave(x, y, FUN=length) produces character output when x is character

[Duncan Murdoch]

On 25/12/2014 1:57 PM, Mike Miller wrote:
> On Thu, 25 Dec 2014, peter dalgaard wrote:
> 
>>
>>> On 25 Dec 2014, at 08:15 , Mike Miller <mbmiller+l at 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. 

"More common"?  The better way to think of this is as a class hierarchy.
 A matrix is a particular kind of vector (the kind that has a dimension
attribute).  A matrix has all the properties that a vector has, plus
some more.

Would you say a cube contains a polygon, or a cube is a polygon?

Duncan Murdoch

 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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