R from C — Simplest Possible Helloworld

Question

What is the simplest possible C function for starting the R interpreter, passing in a small expression (eg, 2+2), and getting out the result? I\'m trying to compile with Mi

Answer 1

I don't think any of the above has answered the question - which was to evaluate 2 + 2 ;). To use a string expression would be something like:

#include 
#include 
#include 

int main(int argc, char **argv) {
    SEXP x;
    ParseStatus status;
    const char* expr = "2 + 2";

    Rf_initEmbeddedR(argc, argv);

    x = R_ParseVector(mkString(expr), 1, &status, R_NilValue);
    if (TYPEOF(x) == EXPRSXP) { /* parse returns an expr vector, you want the first */
        x = eval(VECTOR_ELT(x, 0), R_GlobalEnv);
        PrintValue(x);
    }

    Rf_endEmbeddedR(0);

    return 0;
}

This lacks error checking, obviously, but works:

Z:\>gcc -o e.exe e.c -IC:/PROGRA~1/R/R-213~1.0/include -LC:/PROGRA~1/R/R-213~1.0/bin/i386 -lR
Z:\>R CMD e.exe
[1] 4

(To get the proper commands for your R use R CMD SHLIB e.c which gives you the relevant compiler flags)

You can also construct the expression by hand if it's simple enough - e.g., for rnorm(10) you would use

SEXP rnorm = install("rnorm");
SEXP x = eval(lang2(rnorm, ScalarInteger(10)), R_GlobalEnv);