[R] R as a programming language

[Duncan Murdoch]

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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> 
>