Rcpp Loop and Subset Numeric Matrix

Question

Is it possible to loop through a matrix and do some analysis of some subset?

In R:

for(i in 10:nrow(mat)){
   hist = mat[(i-5):(i),]
   // Do something
}

In the above R example, I am looping through the mat matrix from 10th row to its last row. On each iteration I subset the most recent 5 obseravations and do something.

Is this possible in Rcpp? The following example is something I tried..

  int n_col = sample_data.ncol();
  int n_row= sample_data.nrow();
  int max_lb = 10;
  for( int i=(max_lb+1); i<n_row; i++) {

    SubMatrix<REALSXP> res = sample_data(Range(i-max_lb,i),Range(0,n_col));
    //NumericMatrix hist = res;  //If this is uncommented, it fails when I run it...it pretty much just freezes after some iteration...
    Rcpp::Rcout << "-----------------------------" <<std::endl;
    Rcpp::Rcout << i << "\n" <<std::endl;
    Rcpp::Rcout << res .nrow() <<std::endl; // Dimensions do not match what I have
    Rcpp::Rcout << res .ncol() <<std::endl;
  }

In the line //NumericMatrix hist = res;, i try to convert it back to type NumericMatrix but it fails.

Please go to the Rcpp Gallery and look at the subset example to answer this question. In short: sure! — Dirk is no longer here
– Dirk is no longer here, Commented Dec 21, 2016 at 22:53

nrussell · Accepted Answer · 2016-12-21 23:28:58Z

There's no reason to use SubMat<> here, the result of the subset operation will convert to NumericMatrix directly:

#include <Rcpp.h>

// [[Rcpp::export]]
void submat(Rcpp::NumericMatrix x, int max_lb = 10) {
    int n_col = x.ncol();
    int n_row = x.nrow();

    for (int i = max_lb + 1; i < n_row; i++) {
        Rcpp::NumericMatrix res =
            x(Rcpp::Range(i - max_lb, i), Rcpp::Range(0, n_col - 1));

        std::printf(
            "i = %d: [%d x %d]\n",
            i, res.nrow(), res.ncol()
        );
    }
}

submat(matrix(1:40, 20))
# i = 11: [11 x 2]
# i = 12: [11 x 2]
# i = 13: [11 x 2]
# i = 14: [11 x 2]
# i = 15: [11 x 2]
# i = 16: [11 x 2]
# i = 17: [11 x 2]
# i = 18: [11 x 2]
# i = 19: [11 x 2]

As for why

it pretty much just freezes after some iteration

was happening, you have an out of bounds access here

sample_data(Range(i-max_lb,i),Range(0,n_col));
//                            ^^^^^^^^^^^^^^^^

which is undefined behavior. And you might say, "yes but it was the next line that caused my program to freeze", but that's not really true. You did something illegal in the previous line and for whatever reason the commented-out line is where you "paid for it".

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