Update a variable within a loop

Question

I am trying to update a variable within a loop but I am receiving the error

static assertion failed: cannot convert type to SEXP

I am trying to reproduce the following R code in Rcpp:

> v = rep(1, 5)
> for(k in 0:3){
+   v = cumsum(v)
+ }
> print(v)
[1]  1  5 15 35 70

I have gone through the following attempts (uncommenting / commenting the relevant chunks of code) but all give the same error. How can I do this and what am I doing wrong please?

#include <Rcpp.h>
using namespace Rcpp;

// [[Rcpp::export]]
IntegerVector fun() {

  IntegerVector v = rep(1, 5);

  // Attempt 1. 
  for(int k = 0; k < 4; ++k){
    v = cumsum(v);
  }

  // Attempt 2.
  // IntegerVector tempv;
  // for(int k = 0; k < 4; ++k){
  //   tempv = cumsum(v);
  //   v = tempv;
  // }

  // can reproduce error more simply with the following: 
  // so issue is assigning back to variable or change of class?
  // v = cumsum(v);

  // Attempt 3.
  // IntegerVector tempv;
  // for(int k = 0; k < 4; ++k){
  //   tempv = cumsum(v);
  //   v = as<IntegerVector>(tempv);
  // }  

  return v;
}

EDIT:

Okay, so I have something working (thanks to this)

#include <Rcpp.h>
using namespace Rcpp;

// [[Rcpp::export]]
IntegerVector fun() {

  IntegerVector v = rep(1, 5);
     for(int k = 0; k < 4; ++k){
        std::partial_sum(v.begin(), v.end(), v.begin());
     }
   return v;
}

So I suppose my question is now what I was doing wrong previously? Thanks

Looks like a bug. If you check the unit tests for the package, you see that cumsum() is used each time---but only on a NumericVector. So if you swap your IntegerVector for NumericVector, it works. It should of course also work for IntegerVector. — Dirk is no longer here
– Dirk is no longer here, Commented Jan 27, 2020 at 19:22
Thanks for your comment Dirk. Apologies, I should of checked that but still trying to find my feet. — user2957945
– user2957945, Commented Jan 27, 2020 at 19:29
No apologies needed--it is a bug. That should probably have worked. — Dirk is no longer here
– Dirk is no longer here, Commented Jan 27, 2020 at 19:31

Dirk is no longer here · Accepted Answer · 2020-01-27 19:37:25Z

2

As I hinted in my earlier comment, that is supposed to work. As it is not, you found a bug.

Whether it is worth fixing it is another manner. Whenvever I compute on or with vectors, I typically reach for RcppArmadillo. So here is a minimal (working) version of your first attempt, in RcppArmadillo.

Code

#include <RcppArmadillo.h>
// [[Rcpp::depends(RcppArmadillo)]]

// [[Rcpp::export]]
arma::ivec fun() {
  arma::ivec v(5, arma::fill::ones);

  for (int k=0; k<3; k++) {
    v = arma::cumsum(v);
  }

  return(v);
}

/*** R
fun()
*/

Output

R> sourceCpp("~/git/stackoverflow/59936632/answer.cpp")

R> fun()
     [,1]
[1,]    1
[2,]    4
[3,]   10
[4,]   20
[5,]   35
R>

Edit

Made one smal fix and replace the earlier C++11 curly-init with a call to ones to replicate rep(1,5).

edited Jan 27, 2020 at 19:37

answered Jan 27, 2020 at 19:30

Dirk is no longer here

370k60 gold badges668 silver badges742 bronze badges

Sign up to request clarification or add additional context in comments.

Collectives™ on Stack Overflow

Update a variable within a loop

1 Answer 1

Code

Output

Edit

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Code

Output

Edit

Comments

Your Answer

Sign up or log in

Post as a guest

Related