[R] The end of Matlab

[Gabor Grothendieck]

Here is how to emulate matlab end in R in the case of matrices.

Rather than redefine the matrix class (which would be a bit intrusive) we
just define a subclass of matrix called matrix2.  Note in the examples that
matrix2 survives some operations such as + but not others such as crossprod
so in those one would have to coerce back to matrix2 using as.matrix2.


as.matrix2 <- function(x, ...) UseMethod("as.matrix2")

as.matrix2.default <- function(x, ...) {
   do.call(structure, list(x, ...,
	   class = c("matrix2", setdiff(class(x), "matrix2"))))
}

matrix2 <- function(data, ...) as.matrix2(matrix(data, ...))

"[.matrix2" <- function(x, i, j, ...) {
    i <- if (missing(i)) TRUE
	else eval.parent(do.call(substitute, list(substitute(i), list(end = nrow(x)))))
    j <- if (missing(j)) TRUE
	else eval.parent(do.call(substitute, list(substitute(j), list(end = ncol(x)))))
	.subset(x, i, j, ...)
}


> # test
> m <- matrix2(1:12, 3, 4)
> # matrix2 survives the + operation
> class(m+2)
[1] "matrix2" "matrix"

> # but not crossprod
> class(crossprod(m))
[1] "matrix"

> # coercing back
> as.matrix2(crossprod(m))
     [,1] [,2] [,3] [,4]
[1,]   14   32   50   68
[2,]   32   77  122  167
[3,]   50  122  194  266
[4,]   68  167  266  365
attr(,"class")
[1] "matrix2" "matrix"
>
> # example of using end
> m[2:end, 2:end]
     [,1] [,2] [,3]
[1,]    5    8   11
[2,]    6    9   12



On Thu, Dec 11, 2008 at 9:45 PM, Mike Rowe <mwrowe at gmail.com> wrote:
> Greetings!
>
> I come to R by way of Matlab.  One feature in Matlab I miss is its
> "end" keyword.  When you put "end" inside an indexing expression, it
> is interpreted as the length of the variable along the dimension being
> indexed.  For example, if the same feature were implemented in R:
>
> my.vector[5:end]
>
> would be equivalent to:
>
> my.vector[5:length(my.vector)]
>
> or:
>
> this.matrix[3:end,end]
>
> would be equivalent to:
>
> this.matrix[3:nrow(this.matrix),ncol(this.matrix)]   # or
> this.matrix[3:dim(this.matrix)[1],dim(this.matrix)[2]]
>
> As you can see, the R version requires more typing, and I am a lousy
> typist.
>
> With this in mind, I wanted to try to implement something like this in
> R.  It seems like that in order to be able to do this, I would have to
> be able to access the parse tree of the expression currently being
> evaluated by the interpreter from within my End function-- is this
> possible?  Since the "[" and "[[" operators are primitive I can't see
> their arguments via the call stack functions...
>
> Anyone got a workaround?  Would anybody else like to see this feature
> added to R?
>
> Thanks,
> Mike
>
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