[Rd] Does anyone use Sweave (RweaveLatex) option "expand=FALSE"?

Charles C. Berry cberry at tajo.ucsd.edu
Fri Aug 20 06:26:19 CEST 2010 

On Thu, 19 Aug 2010, Kevin Coombes wrote:

> I picked the example from segmenting chromosomes for a reason.  I have a fair 
> chunk of code that deals with not quite exceeding the amount of RAM available 
> in the machine sitting on my desktop.  If I use functions, then the 
> pass-by-value semantics of R will push me beyond the limits at some points. 
> (This is an empirical statement, not a theoretical one.  I was bitten by it 
> several times while trying to analyze a couple of these datasets. And, yes, I 
> know I can get around this by buying a bigger and better machine; it's on 
> order...)  The real point is that using functions can be detrimental to the 
> efficiency of the program, in ways that have real world consequences.
>
> I haven't thought about doing the same thing with expressions. Expressions 
> don't have quite the same semantics as chunks, and you'd have to make sure 
> the evaluation was delayed so that you cold use the current values of things 
> that were computed in the meantime.... and I already know how to do this with 
> chunks without having to think so hard.
>
> Using expressons would, however, help with the one difficulty that I have 
> with reusing <<chunks>> (independent of whether or not I use 'expand=FALSE'). 
> I usually work inside emacs, using the emacs-speaks-statistics (ESS) package. 
> ESS doesn't know how to evaluate the <<chunk>> call inside another chunk. so 
> if I want to step through the code during development, I have to jump around 
> myself to locate the source chunks.  With expressions that wouldn't matter.
>

emacs org-mode might help.

You can define a chunk, <<ch1>> (say), then place it in a subsequent 
chunk, <<ch2>> (say), inside an eval() and refer to <<ch2>> inside an 
eval() inside another chunk, <<ch3>>, and so on. The only trick is to 
write the chunk so that the chunk it refers to is on its own line, like 
this:

#+source: ch3
#+begin_src R :session :noweb yes
eval(
<<ch2>>
)
z <- mean(y)+1
#+end_src

as the noweb expansion will repeat any other text on the line with the 
named chunk for each line of code in the referenced chunk.

You can define a library of chunks separately, which might help you if you 
reuse them in different places.

You can use ess-mode to edit the chunks and execute them with the usual 
ess-eval-* commands, or run them in org-mode with the results optionally 
returned to the edit buffer, but outside the code block.

If you want to debug a bunch of nested chunks and 'step through the code 
during development', you can tangle the chunk you wish to execute to 
produce a *.R file. The nested chunks are expanded into R. Open that *.R 
file and debug away.

With a recent emacs, I think this gets you going:

 	C-h i m Org Mode RET C-s source RET RET

or Google 'org-babel R' or some such or just go look here:


 	http://blogisticreflections.wordpress.com/

> As I ramble on about this, it occurs to me that the underlying issue is that 
> <<chunks>> are not first class objects either in the LaTeX world or in the R 
> world part of Sweave.  If there were a way to promote them to first class 
> objects somehow, then it might make my use of ESS easier while simultaneously 
> making it easier for Duncan to figure out how to report the correct line 
> numbers.  But I only have an extremely vague idea of how one might start to 
> do that...

org-mode might be the path of least resistance.

FWIW, the code chunks are objects processed by emacs-lisp before they are 
handed off to R, so there might be a slicker way to handle the nesting 
than dropping chunks inside eval()s in R blocks. If that interests you, 
there is a very active listserv for orgmode, where you might inqure.

HTH,

Chuck




>
>   Kevin
>
> Matt Shotwell wrote:
>>  On Thu, 2010-08-19 at 17:07 -0400, Kevin Coombes wrote:
>> 
>> >  I use it, frequently. The idea for it goes back to some of Knuth's 
>> >  original literate programming ideas for developing weave and tangle when 
>> >  he was writing TeX (the program).  I want to be able to document the 
>> >  pieces of some complex algorithm without having to see all of the gory 
>> >  details.  For instance, I have code that looks like the following. 
>> >  (Note that this is typed on the fly rather than copied from actual 
>> >  source, so there may be typos.)
>> > 
>> >  <<mainloop,keep.source=TRUE,expand=FALSE>>=
>> >  for (i in 1:nSamples) {
>> >  <<getInfoAboutThisSample>>
>> >  for (j in 1:nChromosomes) {
>> >  <<getChromosomeDataForCurrentSample>>
>> >  <<normalizeChromosomeData>>
>> >  <<findSegments>>
>> >  <<computeSignificance>>
>> >  <<writeResults>>
>> >   }
>> >  }
>> >  @
>> > 
>> >  Each of the <<chunks>> is itself a fairly long piece of code defined and 
>> >  documented somewhere else.  (Some of them may themselves be written in 
>> >  the same form to reduce the final size of a chunk to something a human 
>> >  has a chance of understanding. That's the difference between weave and 
>> >  tangle in the original implementation.)   By blocking expansion, I can 
>> >  focus on the main steps without having them lost in pages and pages of 
>> >  code.
>> > 
>> > 
>>
>>  Couldn't you achieve the same amount of abstraction using function
>>  calls, rather than embedded code chunks? The reader can then see real
>>  code, rather than non-code, or meta-code, or whatever. Alternatively,
>>  represent the code chunks as R expressions, then evaluate the
>>  expressions at the appropriate points.
>>
>>  -Matt
>>
>> 
>> >  So I vote strongly for retaining "expand=FALSE".
>> > 
>> >  Best,
>> >      Kevin
>> > 
>> >  Duncan Murdoch wrote:
>> > 
>> > >  On 19/08/2010 4:29 PM, Claudia Beleites wrote:
>> > > 
>> > > >  I never used it.
>> > > > 
>> > > >  I got curious, though. What would be a situation that benefits of 
>> > > >  this option?
>> > > > 
>> > > > 
>> > >  When I put it in, I thought it would be for people who were writing 
>> > >  about Sweave.
>> > > 
>> > >  Duncan Murdoch
>> > > 
>> > > 
>> > > >  Maybe a use case could be found by "brute force" (grep all .Rnw 
>> > > >  files on CRAN for the option?
>> > > > 
>> > > >  Claudia
>> > > > 
>> > > > 
>> > > > 
>> > >  ______________________________________________
>> > >  R-devel at r-project.org mailing list
>> > >  https://stat.ethz.ch/mailman/listinfo/r-devel
>> > > 
>> >  ______________________________________________
>> >  R-devel at r-project.org mailing list
>> >  https://stat.ethz.ch/mailman/listinfo/r-devel
>> > 
>> 
>> 
>
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Charles C. Berry                            (858) 534-2098
                                             Dept of Family/Preventive Medicine
E mailto:cberry at tajo.ucsd.edu	            UC San Diego
http://famprevmed.ucsd.edu/faculty/cberry/  La Jolla, San Diego 92093-0901

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