[R] Can a matrix have 'list' as rows/columns?

[Worik R]

Thanks.  That is clear.

I need to seperate the concept of Array and Vector in my head.  As
soon as I hit sendon that last post I realised it is a bit silly to
say vectors only contain atomic things.

W

On Thu, Apr 19, 2012 at 12:35 PM, William Dunlap <wdunlap at tibco.com> wrote:
>> -----Original Message-----
>> From: Worik R [mailto:worikr at gmail.com]
>> Sent: Wednesday, April 18, 2012 5:05 PM
>> To: William Dunlap
>> Cc: r-help
>> Subject: Re: [R] Can a matrix have 'list' as rows/columns?
>>
>> [snip]
>> >
>> > sapply.  In this case I would expect M to be a list.  I am gobsmacked that
>> > > a list can be considered a vector.    Is that a bug?  It must be bad design?
>> > >
>> > > I have been using R for a number of years (5?) and heavilly for two years.
>> > > I am still getting bitten by these "features" in R.  To my mind there are
>> > > many places that R violates the *principle of least surprise.  But it may
>> > > be my mind that is at fault!  What are other people's experience?*
>> >
>> > Since a matrix may contain logical, integer, numeric (double precision),
>> > complex, and character data, I would be surprised if it didn't also handle
>> > list data.  I am surprised that a matrix cannot contain factor data.
>>
>>
>> logical, integer, double, complex and character are all atomic.
>> Usually, where I have been (have I been in the wrong places?) a vector
>> is a one dimensional ordered collection of homogeneously typed atomic
>> things.  A matrix an ordered  collection of  vectors, all the same
>> type and length.
>
> It will be more fruitful in R to think of a vector as a thing that can
> be subscripted by one subscript and a matrix as something that
> can be subscripted by two.  (This is not exactly how a matrix is defined,
> but I find it is good way to think about them.)  A matrix should definitely
> not be thought of as "an ordered collection of vectors, all the same type
> and length".  It is just a vector with an attribute called "dims" that
> lets you subscript with 2 indices.
>
> I have used a matrix(list(...),...) to conveniently contain remote
> sensing observations binned by latitude and longitude.  Each bin
> may contain any number of observations.
>
>> A list is R's hold all type.  It has a count of things in it, but each
>> thing is an arbitrary thing in itself.  It could be another list, or
>> whatever.  Elsewhere on this thread there are some helpful examples of
>> how lists must be changed to be forced into a matrix or to be a
>> vector.
>>
>> That is the violation of  the "Least Astonishment Principle".
>>
>> Here is an example I just made up...
>>
>> > L <- list()
>> > L[[1]] <- list()
>> > L[[1]][[1]] <- TRUE
>> > L[[1]][["ABC"]] <- pi
>> > L
>> [[1]]
>> [[1]][[1]]
>> [1] TRUE
>>
>> [[1]]$ABC
>> [1] 3.141593
>>
>>
>> > V <- as.vector(L)
>> > V
>> [[1]]
>> [[1]][[1]]
>> [1] TRUE
>>
>> [[1]]$ABC
>> [1] 3.141593
>
> Note that as.vector just strips the attributes (except for the names)
> from the input list.  Bare lists are vectors.  Hence V and L are identical.
>
>>  M <- matrix(L, ncol=2, nrow=3)
>>  M
>>      [,1]   [,2]
>> [1,] List,2 List,2
>> [2,] List,2 List,2
>> [3,] List,2 List,2
>>
>> > V[2] <- list()
>> Error in V[2] <- list() : replacement has length zero
>
> Compare this to an atomic vector - there is no difference in behavior:
>    > a <- 1 # a length 1 atomic vector
>    > a[2] <- numeric()
>    Error in a[2] <- numeric() : replacement has length zero
> You cannot replace a 1-long part of a vector with a 0-long vector.
> (If you could, I suppose it would be a no-op.)
>
> With a list you can do
>   V[[2]] <- list()
> because V[[2]] refers to the 2nd element of the list, not the
> sublist of length 1 that contains the 2nd element that V[2]
> means.
>
>
> Bill Dunlap
> Spotfire, TIBCO Software
> wdunlap tibco.com
>
>
>
>
>>
>> > L2 <- list()
>> > L2[[1]] <- "asd"
>> > V[2] <- L2
>> > class(V)
>> [1] "list"
>>
>> > V2 <- as.vector(V)
>> > length(V2)
>> [1] 2
>> > V2[1]
>> [[1]]
>> [[1]][[1]]
>> [1] TRUE
>>
>> [[1]]$ABC
>> [1] 3.141593
>>
>>
>> > V2[2]
>> [[1]]
>> [1] "asd"
>>
>>
>> That is astonishing to me!  I had no way to predict what would happen.
>>  5 days ago I would have expected the statement "as.vector(L)" to
>> produce an error.  "V[2] <- list()" did.
>>
>> cheers
>> Worik