Compiling C and calling it from R. 

The base R comes with a C compiler if you have version 2.1.3 or later. 
There are also other C compilers you can use, like XCode, for Mac OSX. 
It includes a C and C++ compiler, among other tools.
Another common one is GCC. 
There are many free ones for PCs.
See for instance https://www.thoughtco.com/list-of-free-c-compilers-958190 . 
But for most of you, if you download R to your computer, it should be fine. 

The first step to writing C code is opening a text editor.
Write your C code, call it something.c, then compile it, to create an
object file called something.so. Then you can load that into R.

Hello world.
Create a C function to print "Hello world!" and call this function n
times in R.

In a text editor, create a C file.  Say it's called hello.c
The file looks like:

	#include <R.h>
	#include <Rmath.h>

	/* Start a comment.
	   Continue your comment.
         */

        void hello (int *n)
           {
	  int i,j;
	  double a2;
	  a2 = 4.5;
	  j = 3;
	  for(i = 0; i < *n; i++)
	     Rprintf("The %d rd integral is %f .\n", j, a2);
	  }

PUT THE FILE HELLO.C IN YOUR WORKING R DIRECTORY, OR 
MAKE YOUR CURRENT R DIRECTORY THE FOLDER CONTAINING HELLO.C. 

In UNIX, in same directory where hello.c is, type

	R CMD SHLIB hello.c

or, in R, do system("R CMD SHLIB hello.c")

In R, in the same directory, do

	dyn.load("hello.so")

	hello2 = function(n){
		.C("hello",as.integer(n))
	}

	y = hello2(10)