On 12/12/2008 3:41 AM, Wacek Kusnierczyk wrote:
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?
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]
and if nrow(this.matrix) is less than 3?
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?
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.
but this 'general rule' is not really adhered to in r! one example
already discussed here at length is subset:
subset(data.frame(...), select=a)
what will be selected? column named "a", or columns named by the
components of the vector a? this is an example of how you can't say
what an expression means in a context-independent manner.
From which you might conclude that I don't like the design of subset,
and you'd be right. However, I don't think this is a counterexample to
my general rule. In the subset function, the select argument is treated
as an unevaluated expression, and then there are rules about what to do
with it. (I.e. try to look up name `a` in the data frame, if that
fails, ...)
For the requested behaviour to similarly fall within the general rule,
we'd have to treat all indices to all kinds of things (vectors,
matrices, dataframes, etc.) as unevaluated expressions, with special
handling for the particular symbol `end`. But Mike wanted an End
function, so presumably he wanted the old behaviour of indexing, but to
have a function whose value depended on where it was called from. We do
have those (e.g. the functions for examining the stack that Mike wanted
to make use of), and they're needed for debugging and a few special
cases, but as a general rule they should be avoided.
> and this is
an ubiquitous problem in r.
I don't think so.
Duncan Murdoch
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