[Rd] Getting started with .C

[Sharpie]

Jeff Brown wrote:
> 
> Hi,
> 
> I'm trying to learn to use .C, which lets one invoke compiled C code from
> within R.  To do that, one has to first get the C code into R as a shared
> object, which (I think) means first compiling it (with COMPILE or SHLIB)
> and then loading it (with dyn.load()).  
> 

I would suggest taking it a step further and building an R package to hold
your compiled code.  The pros are:

  *  It keeps the R wrapper scripts and other things you will end up
creating packaged together with your code.

  *  It handles compilation automagically during installation.

  *  It handles loading the dylib for you.

The only con I can think of is:

  *  It takes ~2 extra minutes of your time to set up.  But compared to
other languages I have used this is a ridiculously small price to pay for
the portability and organization offered by packages.

I wrote a post that goes through step-by-step how to do this for the .Call()
interface, including example code.  You can find it at:

 
http://n4.nabble.com/Writing-own-simulation-function-in-C-td1580190.html#a1580423



In "Writing R Extensions", p. 79, they give the following example of a C
program for convolution of two vectors.  (The details aren't important; it's
just a function that does something to some stuff.)

void convolve (double *a, int *na, double *b, int *nb, double *ab) {
	int i, j, nab = *na + *nb - 1;
	for(i = 0; i < nab; i++)
		ab[i] = 0.0;
	for(i = 0; i < *na; i++)
		for(j = 0; j < *nb; j++)
			ab[i + j] += a[i] * b[j]
}



Jeff Brown wrote:
> 
> The document suggests calling it from R like this (again the details
> aren't important):
> 
> conv <- function(a, b) 
> 	.C("convolve",
> 		as.double(a), 
> 		as.integer(length(a)), 
> 		as.double(b), 
> 		as.integer(length(b)), 
> 		ab = double(length(a) + length(b) - 1))$ab
> 
> I wrote a file, "convolve.c", with nothing but the above C code in it.  I
> can't figure out how to compile it.  I don't understand the syntax (no
> parentheses?) and I always get the same information-free error message:
> 
>> list.files()
> [1] "AER"        "convolve.c" "sendmailR" 
>> R CMD SHLIB "compile.c"
> Error: syntax error
>> COMPILE "compile.c"
> Error: syntax error
>> R CMD SHLIB "compile"
> Error: syntax error
>> COMPILE "compile"
> Error: syntax error
>> R CMD SHLIB compile.c
> Error: syntax error
>> COMPILE compile.c
> Error: syntax error
>> R CMD SHLIB compile
> Error: syntax error
>> COMPILE compile
> Error: syntax error
> 
> I'm using an Intel MacBook Pro running Leopard.  At a console, typing "gcc
> --version" yields 4.2.1.  I know I'm supposed to be using version 4.2; I
> thought 4.2.1 would qualify, but please let me know if I'm wrong about
> that.
> 
> For guidance I've been relying on "Writing R Extensions", "R Installatino
> and Administration", the "R for Mac OS X Developer's Page", and the
> built-in help.  Please let me know if there are other important resources
> I've missed.
> 
> Many thanks,
> Jeff
> 

All R CMD commands must be executed at the command line- i.e. in a Windows
CMD shell or Unix/Linux bash shell.  They are not meant for use inside the R
interpreter.

Hope this helps!

-Charlie

-----
Charlie Sharpsteen
Undergraduate-- Environmental Resources Engineering
Humboldt State University
-- 
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