An equivalent which() function in Rcpp

c_which.R

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)return(x[which(x > a)])
foo2 = function(x, a)return(x[c_which(x, function(x, bla){as.integer(x > bla)},a)]) ## 加上as.integer防止coerceToInteger报错。
require(rbenchmark)
set.seed(1001)
x = rnorm(1e+7)
benchmark(foo1(x, 0.1), foo2(x, 0.1), order = 'relative')[, 1:4]
##           test replications elapsed relative
## 1 foo1(x, 0.1)          100   22.60    1.000
## 2 foo2(x, 0.1)          100   35.59    1.575