- Reference Read the web page <a href="http://cran.r-project.org/manuals.html" target="_blank">The R Manuals</a> PDF document Writing R Extensions (Chapter 5) in The R Manuals (my mirror <a href="./reference/R-exts.pdf" target="_blank">here</a>). - <a href="http://www.sfu.ca/~sblay/present.html" target="_blank"> Sigal Blay's web space</a> (my mirror <a href="./reference/R-C-interface.ppt" target="_blank">here</a>). - Steps 1. In R, there are two methods to directly pass the R objects to C, .Call() and .External(). 2. In C, there are two methods to handle R objects by including "Rdefines.h" and "Rinternals.h" as the header files. 3. R objects use a structure type SEXP as a pointer in C. 1. Allocate R objects. 2. Protect R objects. 3. Duplicate Objects passed from R if any modification is required. Any objects passed from R should be read-only. 4. After any computing, copy the results to R objects. 5. Unprotect R objects. 4. Return R objects to R from C. At the following, only .Call() and "Rinternals.h" are demonstrated and R's list and array will be passed to C. - Example 1 Available C file: "<a href="./example/Call_Robj/callRobj.c" target="_blank">callRobj.c</a>". This is a poisoned vitamin C example discussed with <a href="http://rumi.gdcb.iastate.edu/" target="_blank">Dr. Karin S. Dorman</a> for modifying a C pointer permanently inside an internal function. Also, an R "list" object is allocated in C. This object store and return a modified version of input to R. ``` #include <R.h> #include <Rinternals.h> void vitamin_c(SEXP R_n, int **C_n){ *C_n = INTEGER(R_n); } SEXP poison(SEXP R_n){ int i, *C_n_1, *C_n_2; double *C_new_1, *C_new_2; SEXP new_1, new_2, ret, ret_names; char *names[2] = {"new.1", "new.2"}; /* Both have the same results. */ vitamin_c(R_n, &C_n_1); C_n_2 = INTEGER(R_n); /* protect R objects in C. */ PROTECT(new_1 = allocVector(REALSXP, 1)); PROTECT(new_2 = allocVector(REALSXP, 1)); PROTECT(ret = allocVector(VECSXP, 2)); PROTECT(ret_names = allocVector(STRSXP, 2)); /* set a list object for R. */ SET_VECTOR_ELT(ret, 0, new_1); SET_VECTOR_ELT(ret, 1, new_2); /* set list's names for R. */ for(i = 0; i < 2; i++){ SET_STRING_ELT(ret_names, i, mkChar(names[i])); } setAttrib(ret, R_NamesSymbol, ret_names); /* assign points to R objects. */ C_new_1 = REAL(new_1); C_new_2 = REAL(new_2); /* update for return to R. */ *C_new_1 = (double) (*C_n_1 + 1); *C_new_2 = (double) (*C_n_2 + 1); /* unprotect for R. */ UNPROTECT(4); return(ret); } ``` - Outputs ``` > a <- 1 > ret <- .Call("poison", as.integer(a)) > ret $new.1 [1] 2 $new.2 [1] 2 ``` - Conclusion - Call by address in C. - Call by address in R when <code>.Call()</code> and <code>.External()</code> are used. --- <div w3-include-html="../preamble_tail_date.html"></div>