# [Rcpp-devel] Strange behavior of NumericMatrix

Romain François romain at r-enthusiasts.com
Tue Sep 20 10:28:38 CEST 2011

```Hello,

On a more cosmetic level, you could use the pow function from sugar:

require( Rcpp )
require( inline )

fx<- cxxfunction(
signature(),
plugin="Rcpp",
body="
Rcpp::NumericMatrix out_xx(10, 4);
for(int i=0; i<4; i++)
out_xx(_,i) = pow( seq(0, 9), i ) ;
return out_xx;
")
fx()

Apart from the use of the sugar version of pow, also note that you do
not need to wrap out_xx, as this implicit (Rcpp::NumericMatrix converts
itself to a SEXP).

Romain

Le 03/09/11 08:51, Steve Lianoglou a écrit :
> Hi,
>
> On Sat, Sep 3, 2011 at 12:54 AM, Noah Silverman<noahsilverman at ucla.edu>  wrote:
>> Hi,
>>
>> Just starting to learn about Rcpp tonight (Using it through the inline library)
>>
>> I'm attempting to construct a matrix and then fill it with values as I iterate through my function.  The results are wrong.  Am I accessing the cells of the matrix incorrectly?
>
> You are accessing the cells just fine, but you are using the wrong operator.
>
> "^" is a bitwise OR in C++
>
> You are in need of the `pow` function, ie:
>
> R>  library(inline)
> R>  fx<- cxxfunction(
>    signature(),
>    plugin="Rcpp",
>    body="
>      Rcpp::NumericMatrix out_xx(10, 4);
>      for(int i = 1; i != 10; i++){
>        out_xx(i,0) = i;
>        out_xx(i,1) = pow(i,2);
>        out_xx(i,2) = pow(i,3);
>        out_xx(i,3) = pow(i,4);
>      }
>      return Rcpp::wrap(out_xx);
>    ")
>
> R>  fx()
>
>        [,1] [,2] [,3] [,4]
>   [1,]    0    0    0    0
>   [2,]    1    1    1    1
>   [3,]    2    4    8   16
>   [4,]    3    9   27   81
>   [5,]    4   16   64  256
>   [6,]    5   25  125  625
>   [7,]    6   36  216 1296
>   [8,]    7   49  343 2401
>   [9,]    8   64  512 4096
> [10,]    9   81  729 6561
>
> -steve
>
>> The idea was to have an integer in the first position of each row, and then the polynomials in the subsequent positions.
>>
>> Any suggestions?
>>
>> ----------------------------
>>
>> Here is my test code:
>>
>> Test2<- cxxfunction(
>>         signature(),
>>         plugin="Rcpp",
>>         body="
>>
>>                 Rcpp::NumericMatrix out_xx(10, 4);
>>                 for(int i = 1; i != 10; i++){
>>                         out_xx(i,0) = i;
>>                         out_xx(i,1) = i^2;
>>                         out_xx(i,2) = i^3;
>>                         out_xx(i,3) = i^4;
>>
>>                 }
>>                 return Rcpp::wrap(out_xx);
>>
>>         "
>> )
>>
>> -----------------------------
>>> Test2()
>>       [,1] [,2] [,3] [,4]
>>   [1,]    0    0    0    0
>>   [2,]    1    3    2    5
>>   [3,]    2    0    1    6
>>   [4,]    3    1    0    7
>>   [5,]    4    6    7    0
>>   [6,]    5    7    6    1
>>   [7,]    6    4    5    2
>>   [8,]    7    5    4    3
>>   [9,]    8   10   11   12
>> [10,]    9   11   10   13
>>
>>
>> --
>> Noah Silverman
>> UCLA Department of Statistics
>> 8117 Math Sciences Building #8208
>> Los Angeles, CA 90095
>>
>> _______________________________________________
>> 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
>>
>
>
>

--
Romain François
Professional R Enthusiast
+33(0) 6 28 91 30 30
http://romainfrancois.blog.free.fr
```