On 11/7/2007 8:13 AM, Duncan Murdoch wrote:
> On 11/7/2007 7:46 AM, Alexy Khrabrov wrote:
>> Greetings -- coming from Python/Ruby perspective, I'm wondering about
>> certain features of R as a programming language.
>
> Lots of question, I'll intersperse some answers.
>>
>> Say I have a huge table t of the form
>>
>> run ord unit words new
>> 1 1 6939 1013 641
>> 1 2 275 1001 518
>> 1 3 3314 1008 488
>> 1 4 14154 1018 463
>> 1 5 2982 1006 421
>>
>> Alternatively, it may have a part column in front. For each run (in
>> a part if present), I select ord and new columns as x and y and plot
>> their functions in various ways. t is huge. So I want to select the
>> subset to plot, as follows:
>>
>> t.xy <- function(t,part=NA,run=NA) {
>> if (is.na(run)) {
>> # TODO does this entail a full copy -- or how do we do
>> references
>> in R?
>> r <- t
>
> Semantically it acts as a full copy, though there is some internal
> optimization that means the copy won't be made until necessary, i.e. one
> of r or t changes.
>
> There are some kinds of objects in R that are handled as references:
> environments, external pointers, names, NULL. (I may have missed some.)
> There are various kludges to expand this list to other kinds of objects,
> the most common way being to wrap an object in an environment. But
> there is a fond wish that people use R as a functional language and
> avoid doing this.
>
>> } else if (is.na(part)) {
>> r <- t[t$run == run,]
>> } else { # part present too
>> r <- t[t$part == part & t$run == run,]
>> }
>> x <- r$ord
>> y <- r$new
>> xy.coords(x,y)
>> }
>>
>> What I'm wondering about is whether r <-t will copy the complete t,
>> and how do I minimize copying in R. I heard it's a functional
>> language -- is there lazy evaluation in place here?
>
> There is lazy evaluation of function arguments, but assignments trigger
> evaluation of their RHS.
>
>>
>> Additionally, tried to use --args command line arguments, and found a
>> way only due to David Brahm -- who helped with several important R
>> points (thanks Dave!):
>>
>> #!/bin/sh
>> # graph a fertility run
>> tail --lines=+4 "$0" | R --vanilla --slave --args $*; exit
>> args <- commandArgs()[-(1:4)]
>> ...
>>
>> And, still no option processing as in GNU long options, or python or
>> ruby's optparse.
>>
>> What's the semantics of parameter passing -- by value or by reference?
>
> By value.
>
>> Is there anything less ugly than
>>
>> print(paste("x=",x,"y=",y))
>>
>> -- for routine printing? Can [1] be eliminated from such simple
>> printing? What about formatted printing?
>
> You can use cat() instead of print(), and avoid the numbering and
> quoting. Remember to explicitly specify a "\n" newline at the end.
>
> At first I thought you were complaining about the syntax, which I find
> ugly. There was a proposal last year to overload + to do concatenation
> of strings, so you'd type cat("x=" + x + "y=" + y + "\n"), but there was
> substantial resistance, on the grounds that + should be commutative.
>
>> Is there a way to assign all of
>>
>> a <- args[1]
>> b <- args[2]
>> c <- args[3]
>>
>> in one fell swoop, a lá Python's
>>
>> a,b,c = args
>
> No, but you can do
>
> abc <- args[1:3]
> names(abc) <- c('a', 'b', 'c')
Oops, this code assumed that args was a list already, and I think yours
was a character vector. In that case you'd need
abc <- as.list(args[1:3])
on the first line.
>
> and refer to the components as abc$a, etc.
>
>> What's the simplest way to check whether a filename ends in ".rda"?
>
> Probably something like
>
> if (regexpr("\\.rda$", filename) > 0) ...
>
> You double the escape char to get it entered into the RE, and then the
> regexpr function uses it to escape the dot in the RE.
>
> Duncan Murdoch
>
>> Will ask more as I go programming...
>>
>> (Will someone here please write an O'Reilly's "Programming in R"? :)
>>
>> Cheers,
>> Alexy
>> ______________________________________________
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>> https://stat.ethz.ch/mailman/listinfo/r-help
>> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
>> and provide commented, minimal, self-contained, reproducible code.
>
>
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