Question

I am trying to write some simple Rcpp code examples. This is remarkably easy with the Rcpp and inline packages.

But I am stumped on how to test whether two character elements for equality. The following example compares the first elements of two character vectors. But I can’t get it to compile.

What is the trick?

library(Rcpp)
library(inline)

cCode <- '
    Rcpp::CharacterVector cx(x);
    Rcpp::CharacterVector cy(y);
    Rcpp::LogicalVector r(1);
    r[0] = (cx[0] == cy[0]);
    return(r);
    '

cCharCompare <- cxxfunction(signature(x="character", y="character"), 
                            plugin="Rcpp", body=cCode)
cCharCompare("a", "b")

—

The comparison using == works perfectly fine if one of the two elements is a constant. The following code compiles and gives expected results:

cCode <- '
    Rcpp::CharacterVector cx(x);
    Rcpp::LogicalVector r(1);
    r[0] = (cx[0] == "a");
    return(r);
    '

cCharCompareA <- cxxfunction(signature(x="character"), plugin="Rcpp", body=cCode)

cCharCompareA("a")
[1] TRUE

cCharCompareA("b")
[1] FALSE

Answer 1

Very nice (technical) answer by @kohske, but here is something more C++-ish: just compare strings!

library(inline)      ## implies library(Rcpp) when we use the plugin

cCode <- '
    std::string cx = Rcpp::as<std::string>(x);
    std::string cy = Rcpp::as<std::string>(y);
    bool res = (cx == cy);
    return(Rcpp::wrap(res));
    '

cCharCompare <- cxxfunction(signature(x="character", y="character"),
                            plugin="Rcpp", body=cCode)
cCharCompare("a", "b")

If you really want to compare just the first character of the strings, then you can go from x to x.c_str() and either index its initial element, or just dereference the pointer to the first char.

A more R-ish answer could maybe sweep over actual vectors of strings…