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

[Joshua Wiley]

Hi Alexandre,

Thank you so much!  That worked and helped me understand why it was
happening :)  The idea of pointers is new to me so I think I need to
spend more time learning about that.

Thanks again,

Josh


On Sat, Jul 7, 2012 at 4:46 PM, Alexandre Bujard
<alexandre.bujard at gmail.com> wrote:
> 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
>
>



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