[R] using the .C interface to call compiled C code

[Faheem Mitha]

Dear People,

I have been trying to learn how to use the .C interface. A friend gave me
a toy example, and I have been playing with it. It is 


(C code)

#ifdef USING_R
  typedef int Sint;
#else
  typedef long Sint;
#endif

void kosum(double *a, double *b, Sint *na) 
{
  int i, j, nab;
  double temp;
  nab=*na;
  temp=0.0;
  for(i=0;i < nab; i++)
  {
    temp=temp+a[i];
  }
   *b=temp;
}


And the calling code in R is 

test1<-
function(a, b)
{
        .C("kosum",
                as.double(a),
                b = double(1),
                as.integer(length(a)))[[2]]
}

Now, I noticed that if I change

for(i=0;i < nab; i++) to say for(i=0;i <= nab; i++) or even  
for(i=0;i <= nab + 1; i++)

everything seems happy, and I get the same answers. But I don't understand
why this is so. I am surely trying to read from nonexistent memory in this
case, and I should have some problems.

I may be missing the obvious, but I am neither very experienced in R or in
C.

Also, if anyone has a simple example of using .C that they would like to
share with me, or if they can point to me to (very) simple examples in the
source code that would be suitable for an beginner, I would be extremely
grateful.

                           Sincerely, Faheem Mitha.

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