I suspect this is about your float[] not being initialized. I would recommend you to have a look at containers such as std::vector<> from the STL.

But this is a version that only uses Rcpp classes:


#include <Rcpp.h>
using namespace Rcpp ;

// [[Rcpp::export]]
NumericVector test_func(NumericMatrix my_matrix){
    int num_cols = my_matrix.ncol(); // extract matrix dimensions

    NumericVector my_expr(num_cols);  // output array

    for(int j = 0; j < num_cols; j++) {
        double sum_ = sum( my_matrix(_,j) ) ;
        my_expr[j] = sum_ / ( sum_ + j ) ;
    }
    return my_expr;
}

Put this in a file (foo.cpp) and then do:

> sourceCpp( "foo.cpp" )
> test_func( my_matrix)
[1] 1.0000000 0.6666667 0.0000000 0.4000000 0.5000000 0.2857143 0.3333333
[8] 0.3000000

This uses the Rcpp sugar sum function to sum a column: my_matrix(_,j) is a view on the j^th column.


Romain

Le 19/12/12 19:45, Jeff Stevens a écrit :
Hi,

I'm trying to use Rcpp to speed up some nasty for loops in my R code
(Ubuntu 12.04, R 2.15.2, Rcpp 0.10.1, gcc 4.6.3).  However, I'm a newbie
to C and to Rcpp, and I'm having trouble returning the correct output.
Basically, I input an R matrix to an inline (using Rcpp plugin) C
function that runs through each element of the matrix and performs some
calculations to generate an array (float).  When I run the program in C,
it calculates the correct values.  However, when I run it in R, it give
erratic results.  Here is example code that results in the same problem:
test_src <- '
Rcpp::NumericMatrix my_matrix(a);   // input matrix
int num_rows = my_matrix.nrow(), num_cols = my_matrix.ncol(); // extract
matrix dimensions
float my_sum[50];
float my_sum2[50];
Rcpp::NumericVector my_expr(num_cols);  // output array

     for(int i = 0; i < num_rows; i++) {
       for(int j = 0; j < num_cols; j++) {
       my_sum[j] = my_sum[j] + my_matrix(i,j);   // sum columns
       my_sum2[j] = (my_sum[j] + j);             // sum columns plus the
column index
       }
     }
     for(int j = 0; j < num_cols; j++) {
       my_expr[j] = my_sum[j] / my_sum2[j];      // divide sum by sum2
     }
return my_expr;
'
test_func <- cxxfunction(signature(a = "numeric"), test_src, plugin =
"Rcpp")

my_matrix <- matrix(c(0, 1, 1, 0, 1, 0, 0, 1, 1, 0, 0, 0, 0, 0, 0, 1, 0,
0, 1, 0, 1, 1, 1, 1, 0, 1, 1, 0, 0, 0, 1, 1, 0, 0, 1, 0, 1, 1, 1, 0),
nrow = 5, ncol = 8)

test_func(my_matrix)

When I run this, I get:
1.0000000  0.6666667       NaN       NaN 1.0000000 0.2857143 0.3333333
0.3000000

If I run the following in C:
int num_rows = 5, num_cols = 8;
int my_matrix[5][8];
float my_sum[50];
float my_sum2[50];
float my_expr[50];
int my_matrix[5][8] = {
     {0, 1, 1, 0, 1, 0, 0, 1},
     {1, 0, 0, 0, 0, 0, 0, 1},
     {0, 0, 1, 0, 1, 1, 1, 1},
     {0, 1, 1, 0, 0, 0, 1, 1},
     {0, 0, 1, 0, 1, 1, 1, 0}
};

int main () {
     for(int i = 0; i < num_rows; i++) {
       for(int j = 0; j < num_cols; j++) {
       my_sum[j] = my_sum[j] + my_matrix[i][j];  // sum columns
       my_sum2[j] = (my_sum[j] + j);             // sum columns plus the
column index
       }
     }
     for(int j = 0; j < num_cols; j++) {
       my_expr[j] = my_sum[j] / my_sum2[j];      // divide sum by sum2
       printf("%f\t", my_expr[j]);
     }
}

I get
1.000000  0.666667   0.666667   0.000000   0.428571   0.285714
0.333333   0.363636

I'm sure that there is something simple that I am doing wrong.  Any
suggestions?

Thanks,
Jeff


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