c++ - Rcpp 通过 NumericVector 选择/子集 NumericMatrix 列

Question

我可以选择矩阵的所有行和矩阵的一系列列，如下所示:

library(Rcpp)
cppFunction('
NumericMatrix subset(NumericMatrix x){
  return x(_, Range(0, 1));
}
')

但是，我想根据 NumericVector y 选择列例如，它可能类似于 c(0, 1, 0, 0, 1) .我试过这个:

library(Rcpp)
cppFunction('
NumericMatrix subset(NumericMatrix x, NumericVector y){
  return x(_, y);
}
')

但它不编译。我该怎么做？

Answer 1

唉，Rcpp 对非连续 View 或仅在单个语句中选择第 1 和第 4 列没有很好的支持。如您所见，可以通过 Rcpp::Range() 访问选择连续 View 或选择所有列。 .您可能希望升级到 RcppArmadillo 以更好地控制 matrix subsets .

RcppArmadillo 子集示例

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

// [[Rcpp::export]]
arma::mat matrix_subset_idx(const arma::mat& x,
                            const arma::uvec& y) { 

    // y must be an integer between 0 and columns - 1
    // Allows for repeated draws from same columns.
    return x.cols( y );
}


// [[Rcpp::export]]
arma::mat matrix_subset_logical(const arma::mat& x,
                                const arma::vec& y) { 
    // Assumes that y is 0/1 coded.
    // find() retrieves the integer index when y is equivalent 1. 
    return x.cols( arma::find(y == 1) );
}

测试

# Sample data
x = matrix(1:15, ncol = 5)
x
#      [,1] [,2] [,3] [,4] [,5]
# [1,]    1    4    7   10   13
# [2,]    2    5    8   11   14
# [3,]    3    6    9   12   15

# Subset only when 1 (TRUE) is found:
matrix_subset_logical(x, c(0, 1, 0, 0, 1))
#      [,1] [,2]
# [1,]    4   13
# [2,]    5   14
# [3,]    6   15

# Subset with an index representing the location
# Note: C++ indices start at 0 not 1!
matrix_subset_idx(x, c(1, 3))
#      [,1] [,2]
# [1,]    4   13
# [2,]    5   14
# [3,]    6   15

纯 Rcpp 逻辑

如果你不想承担 Armadillo 的依赖，那么 Rcpp 中矩阵子集的等价物是:

#include <Rcpp.h>

// [[Rcpp::export]]
Rcpp::NumericMatrix matrix_subset_idx_rcpp(
        Rcpp::NumericMatrix x, Rcpp::IntegerVector y) { 

    // Determine the number of observations
    int n_cols_out = y.size();

    // Create an output matrix
    Rcpp::NumericMatrix out = Rcpp::no_init(x.nrow(), n_cols_out);

    // Loop through each column and copy the data. 
    for(unsigned int z = 0; z < n_cols_out; ++z) {
        out(Rcpp::_, z) = x(Rcpp::_, y[z]);
    }

    return out;
}