Rcpp How to convert IntegerVector to NumericVector

Question

I was wondering how to convert Rcpp IntegerVector to NumericVetortor to sample three times without replacement of the numbers 1 to 5. seq_len outputs an IntegerVector and sample sample only takes an NumericVector

// [[Rcpp::depends(RcppArmadillo)]]
#include <RcppArmadilloExtensions/sample.h>
#include <Rcpp.h>
using namespace Rcpp;

// [[Rcpp::export]]
NumericVector follow_path(NumericMatrix X, NumericVector y) {
IntegerVector i = seq_len(5)*1.0;
NumericVector n = i; //how to convert i?
return sample(cols_int,3); //sample only takes n input
}

why was I down voted for asking this question? – Soren Havelund Welling May 26 '15 at 16:01 — Soren Havelund Welling, May 26 '15 at 16:01

score 11 · Accepted Answer · answered May 24 '15 at 16:42

You are having a few things wrong here, or maybe I grossly misunderstand the question.

First off, sample() does take integer vectors, in fact it is templated.

Second, you were not using your arguments at all.

Here is a repaired version:

// [[Rcpp::depends(RcppArmadillo)]]
#include <RcppArmadilloExtensions/sample.h>
#include <Rcpp.h>
using namespace Rcpp;

// [[Rcpp::export]]
IntegerVector sampleDemo(IntegerVector iv) {   // removed unused arguments
  IntegerVector is = RcppArmadillo::sample<IntegerVector>(iv, 3, false); 
  return is;
}

/*** R
set.seed(42)
sampleDemo(c(42L, 7L, 23L, 1007L))
*/

and this is its output:

R> sourceCpp("/tmp/soren.cpp")

R> set.seed(42)

R> sampleDemo(c(42L, 7L, 23L, 1007L))
[1] 1007   23   42
R>

Edit: And while I wrote this, you answered yourself...

score 9 · Answer 2 · answered May 24 '15 at 16:40

9

I learned from http://adv-r.had.co.nz/Rcpp.html#rcpp-classes to use

NumericVector cols_num = as<NumericVector>(someIntegerVector)

.

// [[Rcpp::depends(RcppArmadillo)]]
#include <RcppArmadilloExtensions/sample.h>
#include <Rcpp.h>
using namespace Rcpp;
using namespace RcppArmadillo;

// [[Rcpp::export]]
NumericVector follow_path(NumericMatrix X, IntegerVector y) {
 IntegerVector cols_int = seq_len(X.ncol());
 NumericVector cols_num = as<NumericVector>(cols_int);
 return sample(cols_num,3,false);
}

answered May 24 '15 at 16:40

Soren Havelund Welling

1,823
1
16
23

1

You are still casting unnecessarily. If you wamt to sample an integer vector, go and sample an integer vector -- see my answer. – Dirk Eddelbuettel May 24 '15 at 16:43
1

If my answer helped, please consider "accepting" (click on tick-mark) and/or "upvoting" (click on upward-triangle) both of which are common at Stack Overflow. – Dirk Eddelbuettel May 24 '15 at 19:45

Rcpp How to convert IntegerVector to NumericVector

2 Answers2