Rcpp-which.R

library("Rcpp")

cppFunction("std::vector<int> c_which(NumericVector x, Function f, List args) {
    std::vector<int> ind = as< std::vector<int> >(f(x, args));
    std::vector<int> out(ind);
    std::vector<int>::iterator it;
    int j = 0;
    it = std::find(ind.begin(), ind.end(), 1);
    while(it++ != ind.end()){
      out[j++] = it - ind.begin();
      it = std::find(it, ind.end(), 1);
    }
    out.resize(j);
    return out;
}")

foo1 <- function(x, a) x[which(x > a)]
# use as.integer to avoid coerceToInteger errors
foo2 <- function(x, a) x[c_which(x, function(x, bla) as.integer(x > bla), a)]

library("rbenchmark")
set.seed(1001)
x <- rnorm(1e+7)

all.equal(foo1(x, 0.1), foo2(x, 0.1))
# [1] TRUE

benchmark(foo1(x, 0.1), foo2(x, 0.1), order = "relative")[, 1:4]
#           test replications elapsed relative
# 1 foo1(x, 0.1)          100   8.971    1.000
# 2 foo2(x, 0.1)          100  33.778    3.765