[Rpy] [Fwd: Re: [Numpy-discussion] Possible example applicati on of the array interface]

[Warnes, Gregory R]

Hi All,

It is possible to establish conversion functions so that R dataframe, lists,
and vector objects are better translated into python equivalents.  I've made
several aborted stabs at this, but my time has been extremely limited.

The basic task is to create a functionally equivalent python class [The
tricky bit here is that R list and vector objects have both order and names.
It is possible to emulate this in python by creating a base object that
maintains a dictionary of names in along side the data vector/matrix data.]

See the example in the rpu documentation at
http://rpy.sourceforge.net/rpy/doc/manual_html/DataFrame-class.html#DataFram
e%20class.

This shouldn't be very hard if someone can dedicate a bit of time to it.

-Greg
(Current RPy maintainer)



> -----Original Message-----
> From: rpy-list-admin at lists.sourceforge.net
> [mailto:rpy-list-admin at lists.sourceforge.net]On Behalf Of Tim Churches
> Sent: Wednesday, April 06, 2005 4:22 PM
> To: rpy-list at lists.sourceforge.net
> Subject: [Rpy] [Fwd: Re: [Numpy-discussion] Possible example 
> application
> of the array interface]
> 
> 
> The following discussion occured on the Numeric Python mailing list.
> Others may wish to enjoin the conversation.
> 
> Tim C
> 
> -------- Original Message --------
> Subject: Re: [Numpy-discussion] Possible example application of the
> array interface
> Date: Thu, 7 Apr 2005 03:10:08 +1000 (EST)
> From: Michael Sorich <mike_lists at yahoo.com.au>
> To: numpy-discussion at lists.sourceforge.net
> 
> I think that this is a great idea! While I have a
> strong preference for python, I generally use R for
> statistical analyses due to the large number of mature
> libraries available. There are also some aspects of
> the R data types (eg data-frames and column/row names
> for 2D arrays) that are really nice for spreadsheet
> like data. I hope that scipy.base record arrays will
> be as easily manipulated as data-frames are.
> 
> While RPy works well for small simple problems, there
> are data conversion limitations between R and Python.
> If one could efficiently convert between the major R
> data types and python scipy.base data types without
> loss of data, it would become possible to do most of
> the data manipulation in python and freely mix in R
> functions when required. This may encourage the use of
> python for the development of statistical routines.
> 
> >From my meager understanding of RPy:
> 
> R vectors are converted to python lists. It may make
> more sense to convert them to an array (either stdlib
> or scipy.base version) - without copying data if
> possible.
> 
> R arrays and matrices are converted to Numeric arrays.
> Eg
> 
> In [8]: r.array([1,2,3,4,5,6],dim=[2,3])
> Out[8]:
> array([[1, 3, 5],
>        [2, 4, 6]])
> 
> However, column and row names (or dimnames for arrays
> with >2 dimensions) are lost in R->Py conversion. I do
> not know whether these conversions require copying of
> the data.
> 
> R data-frames are currently converted to python
> dictionaries and I don’t think that there is any
> simple way to convert a python object to an R data
> frame. This is the biggest limitation of rpy in my
> opinion.
> 
> In [16]:
> r.data_frame(col1=[1,2,3,4],col2=['one','two','three','four'])
> Out[16]: {'col2': ['one', 'two', 'three', 'four'],
> 'col1': [1, 2, 3, 4]}
> 
> If it were possible to convert between an R data-frame
> and a scipy.base record array without copying or
> losing data, RPy would become more useful.
> 
> I wish I understood C, scipy.base and R well enough to
> give this a go. However, this is Way over my head!
> 
> Mike
> 
> --- Magnus Lie Hetland <magnus at hetland.org> wrote:
> > I was just thinking about some experimental designs,
> > and whether I
> > could, perhaps, do the statistics in Python. I
> > remembered having used
> > RPy [1] briefly at some time (there may be other
> > similar bindings out
> > there -- I don't remember) and started thinking
> > about whether I could,
> > perhaps, combine it with numpy in some way. My first
> > thought was to
> > reimplement the relevant statistical functions; then
> > I thought about
> > how to convert data back and forth -- but then it
> > occurred to me that
> > R also uses arrays extensively, and that it could,
> > perhaps, be
> > possible to expose those (through something like
> > RPy) through the
> > array interface/protocol!
> > 
> > This would be (IMO) a good example of the benefits
> > of the array
> > protocol; it's not a matter of "getting yet another
> > array module". RPy
> > is an external library/language with *lots* of
> > features that might be
> > useful to numpy users, many of which aren't likely
> > to be implemented
> > in Python for quite a while, I'd guess (unless,
> > perhaps, someone
> > writes a translator from R, which I'm sure is
> > doable).
> > 
> > I don't know enough (at least yet ;) about the
> > implementation of RPy
> > and the R library to say for sure whether this would
> > even be possible,
> > but it does seem like it could be really useful...
> > 
> > [1] rpy.sf.net
> > 
> > -- 
> > Magnus Lie Hetland                    Fall seven
> > times, stand up eight
> > http://hetland.org                                 
> > [Japanese proverb]
> 
> 
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