[Rcpp-devel] function created from cxxfunction modifies input?

[Joshua Wiley]

Hi,

I am familiar with R but quite new to C++.  I am reading a C++
textbook, so if this is an obvious question just resulting in my
ignorance of C++, please let me know.

I tried to translate someone else's R loop that was slow into c++ to
use with Rcpp and the inline package:

require(Rcpp)
require(inline)

## Rcpp function body
src <- '
  NumericVector xx(x);
  int n = xx.size();
  NumericVector res(xx);
  int toggle = 0;
  int tot = 100;
  int max = 100;

  typedef NumericVector::iterator vec_iterator;
  vec_iterator ixx = xx.begin();
  vec_iterator ires = res.begin();
  for (int i = 0; i < n; i++) {
    if (ixx[i] != 0) {
      if (toggle == 1) {
        ires[i] = 0;
      }
      if (toggle != 1) {
        tot += ixx[i];
        if (tot > max) {
          max = tot;
        }
        if (tot <= max) {
          if (.98 * max > tot) {
            toggle = 1;
          }
        }
      }
   }
   if (ixx[i] == 0) {
     tot = 100;
     max = 100;
     toggle = 0;
   }
  }
  return res;
'

## compile the function
foo.rcpp <- cxxfunction(signature(x = "numeric"), src, plugin = "Rcpp")

## here is a little input vector, the function should
## output the same but with a 0 instead of 8
> d
 [1]  0  0  0  1  3  4  5 -1  2  3 -5  8
> foo.rcpp(d) # great it works!
 [1]  0  0  0  1  3  4  5 -1  2  3 -5  0
> d # but now d itself has changed?
 [1]  0  0  0  1  3  4  5 -1  2  3 -5  0

It seems particularly surprising to me, because I do not think I even
modify the input vector, I copy it into the numeric vector res and
alter that.

Thanks,

Josh


-- 
Joshua Wiley
Ph.D. Student, Health Psychology
Programmer Analyst II, Statistical Consulting Group
University of California, Los Angeles
https://joshuawiley.com/