c++ - Rcpp NumericMatrix - 如何删除行/列？

Question

我学习 Rcpp 类/数据结构时的一个新手问题:是否有一个成员函数可以为 Rcpp::NumericMatrix 类的对象删除行/列？ (或其他类型的 type **Matrix -- 我假设它是一个模板类)？

library(Rcpp)
cppFunction('
  NumericMatrix sub1 {NumericMatrix x, int& rowID, int& colID) {
    // let's assume separate functions for rowID or colID
    // but for the example case here
    x.row(rowID).erase(); // ??? does this type of member function exist?
    x.col(colID).erase(); // ???
    return x;
}')

如果不存在这种类型的成员函数，这个怎么样？

cppFunction('NumericMatrix row_erase (NumericMatrix& x, int& rowID) {
  // a similar function would exist for removing a column.
  NumericMatrix x2(Dimension(x.nrow()-1, x.ncol());
  int iter = 0; // possibly make this a pointer?
  for (int i = 0; i < x.nrow(); i++) {
    if (i != rowID) {
      x2.row(iter) = x.row(i);
      iter++;
    }
  }
  return x2;
}')

或者我们希望删除一组行/列:

cppFunction('NumericMatrix row_erase (NumericMatrix& x, IntegerVector& rowID) {
  // a similar function would exist for removing a column.
  rowID = rowID.sort();

  NumericMatrix x2(Dimension(x.nrow()- rowID.size(), x.ncol());
  int iter = 0; // possibly make this a pointer?
  int del = 1; // to count deleted elements
  for (int i = 0; i < x.nrow(); i++) {
    if (i != rowID[del - 1])
      x2.row(iter) = x.row(i);
      iter++;
    } else {
      del++;
    }
  }
  return x2;
}')

Answer 1

使用 RcppArmadillo 怎么样？我认为代码的意图会更清晰......

#include <RcppArmadillo.h>
// [[Rcpp::depends(RcppArmadillo)]]

using namespace arma;

// [[Rcpp::export]]
mat sub1( mat x, uword e) {
  x.shed_col(e-1);
  x.shed_row(e-1);
  return x;
}

/*** R
sub1( matrix(1:9,3), 2 )
*/

---

> sub1( matrix(1:9,3), 2 )
     [,1] [,2]
[1,]    1    7
[2,]    3    9