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

[Alexandre Bujard]

Hi Joshua,

When you declare "NumericVector xx(x)" xx is a copy of the pointer x. But
they both address the same location in the memory.
What you need here is to do a "deep copy" of the vector x. The best was to
do that in Rcpp is to use the function "clone".
Also declaring your vector xx with "const" will guarantee that your vector
x will not be modified by the function.

So your code should look like this :
## Rcpp function body
src <- '
  const NumericVector xx(x);
  int n = xx.size();
  NumericVector res = clone(xx);
  int toggle = 0;
  int tot = 100;
  int max = 100;
 [...]

On Sun, Jul 8, 2012 at 1:20 AM, Joshua Wiley <jwiley.psych at gmail.com> wrote:

> 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/
> _______________________________________________
> Rcpp-devel mailing list
> Rcpp-devel at lists.r-forge.r-project.org
> https://lists.r-forge.r-project.org/cgi-bin/mailman/listinfo/rcpp-devel
>
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