r.turner at auckland.ac.nz
Tue Mar 4 02:47:18 CET 2014
In spatial point pattern analysis an important component of a point
pattern object is the *observation window*. In the spatstat package we
have the function owin() to *construct* such a window. It creates an
object of class "owin" which can happily exist separately from any point
pattern object (object of class "ppp").
However we have at the moment no convenient function to *extract* the
observation component of a ppp object, nor have we a really convenient
means of (re-) assigning the window component of such an object. We can
extract the component via X$window or more suavely via as.owin(X) ---
but the latter is rather counter-intuitive and lacks parallelism with
other usage. Re-assignment of the observation window component of X can
be done either via X$window <- W or X <- X[W]. These are not quite the
same. The former may leaves points which are outside of W hanging
around. The latter does not suffer from this defect, and is more suave,
but is not as intuitive as it might be.
I suggested that we could make the owin() function generic, make the
current owin() creator function into owin.default(), and build an
extractor function owin.ppp() (and an assignment function owin<-.ppp ()
to assign an observation window to a ppp object).
It was pointed out to me that this would make for an unorthodox syntax
It is at the very least *unusual* if not unheard of for a function to be
used both for *creation* of objects and for *extraction* of components
from other objects.
My questions to R-devel are:
(1) Are there any other functions in R that serve this dual role of
constructor and extractor?
(2) Even if there are no such functions, is there anything intrinsically
*wrong* with having a function possessing this somewhat schizophrenic
nature? Is it likely to cause confusion, induce syntactical mistakes,
create errors, or produce wrong results?
Any thoughts, comments, insights or suggestions gratefully received.
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