How to call C function from R?

Question

How can you use some function written in C from R level using R data. eg. to use function like:

double* addOneToVector(int n, const double* vector) {
    double* ans = malloc(sizeof(double)*n);
    for (int i = 0; i < n; ++i)
        ans[i] = vector[i] + 1
    return ans;
}

in the context:

x = 1:3
x = addOneToVector(x)
x # 2, 3, 4

Answer 1

I've searched stackoverflow first but I noticed there is no answer for that in here.
The general idea is (commands for linux, but same idea under other OS):

1. Create function that will only take pointers to basic types and do everything by side-effects (returns void). eg in a file called foo.c:

   void addOneToVector(int* n, double* vector) {
       for (int i = 0; i < *n; ++i)
           vector[i] += 1.0;
   }
   

2. Compile file C source as dynamic library, you can use R shortcut to do this:

   $ R CMD SHLIB foo.c
   

   This will then create a file called foo.so on Mac or foo.dll on Windows.

3. Load dynamic library from R

   on Mac:

   dyn.load("foo.so")
   

   or on Windows:

   dyn.load("foo.dll")
   

4. Call C functions using .C R function, IE:

   x = 1:3
   ret_val = .C("addOneToVector", n=length(x), vector=as.double(x))
   

It returns list from which you can get value of inputs after calling functions eg.

ret_val$x # 2, 3, 4

You can now wrap it to be able to use it from R easier.

There is a nice page describing whole process with more details here (also covering Fortran):

http://users.stat.umn.edu/~geyer/rc/