loki loki - 3 months ago 37
R Question

Sequence of Integers in Rcpp

I want to create a sequence of integer numbers for indexing within a matrix. The

R
pendant would be:

indexRow <- max(0,1):min(2,12)
matrix1[indexRow, ]


This is what i have tried in
Rcpp
to create the sequence of integers:

#include <Rcpp.h>
#include <algorithm>
#include <vector>
#include <numeric>
using namespace Rcpp;
using namespace std;


// [[Rcpp::export]]
NumericVector test(NumericVector x) {
IntegerVector indexRow = Rcpp::seq_along(max(0, 1), min(1, 12));
}


however I get the Error message:

no matching function for call to 'seq_along(const int&, const int&)'


how can I create a sequence of integers in Rcpp?

Answer

Quite clearly, R and C++ are different programming languages. The R function seq_along is not valid C++ code. It therefore cannot be used within an Rcpp code.

Here is a possible Rcpp implementation :

library(Rcpp)
cppFunction(plugins='cpp11','NumericVector myseq(int &first, int &last) {
NumericVector y(abs(last - first) + 1);
if (first < last) 
   std::iota(y.begin(), y.end(), first);
else {
   std::iota(y.begin(), y.end(), last);
   std::reverse(y.begin(), y.end());
 }
return y;
}')
#> myseq(max(0,1), min(13,17))
#[1]  1  2  3  4  5  6  7  8  9 10 11 12 13

This code generates a function myseq which takes two arguments: The first and the last number in an integer series. It corresponds to R's seq function called with two arguments seq(first, last).

A documentation on the C++11 function std::iota is given here.

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