On Fri, 12 Dec 2008, Duncan Murdoch wrote:
On 11/12/2008 9:45 PM, Mike Rowe 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)]
And if my.vector is of length less than 5?
Also consider
my.vector[-(1:4)]
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.
It doesn't save typing, but a more readable version would be
rows <- nrow(this.matrix)
cols <- ncol(this.matrix)
this.matrix[3:rows, cols]
I would have used
this.matrix[-(1:2), ncol(this.matrix)]
which I find much clearer as to its intentions.
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?
Learning to use the power of R's indexing and functios like head() and
tail() (which are just syntactic sugar) will probably lead you not to miss
this.
I like the general rule that subexpressions have values that can be evaluated
independent of context, so I don't think this is a good idea.
Also, '[' is generic, so it would need to be done in such a way that it
applied to all methods. As arguments other than the first are passed
unevaluated to the methods, I don't think this is really possible (you
don't even know if the third argument to `[` is a dimension for a method).
Also, this would effectively make 'end' a reserved word, or 3:end is
ambiguous (or at best context-dependent).
--
Brian D. Ripley, rip...@stats.ox.ac.uk
Professor of Applied Statistics, http://www.stats.ox.ac.uk/~ripley/
University of Oxford, Tel: +44 1865 272861 (self)
1 South Parks Road, +44 1865 272866 (PA)
Oxford OX1 3TG, UK Fax: +44 1865 272595
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