使用Rcpp代码访问和修改类arma :: sp_mat的稀疏矩阵的非零元素

Question

我正在使用RcppArmadillo访问和更新Armadillo中类arma:sp_mat的稀疏矩阵的非零元素。例如，在Matrix R包中，如果B是类dgCMatrix的稀疏矩阵，则可以通过执行以下操作来访问和修改其非零元素：

B@x[] = xx

其中xx是包含实际非零元素的新向量。有人可以帮我用犰狳代码做同样的事情吗？

Answer 1

不幸的是，没有一个很好的访问者可以返回sp_mat中条目的位置。

要获取此信息，我们首先计算列表中的元素数量，创建位置umat，然后构建新的sp_mat using a batch constructor as recommended by the API docs。

进场代码

#include <RcppArmadillo.h>
using namespace Rcpp;

// [[Rcpp::depends(RcppArmadillo)]]


// Obtains a list of coordinates from the sparse matrix by using iterators
// First calculates total number of points and, then, obtains list.
// [[Rcpp::export]]
arma::umat get_locations(arma::sp_mat& B)
{

    // Make const iterator
    arma::sp_mat::const_iterator start = B.begin();
    arma::sp_mat::const_iterator end   = B.end();

    // Calculate number of points
    int n = std::distance(start, end);

    // Kill process if no values are found (very sparse matrix)
    if (n <= 0) { Rcpp::stop("No values found!"); }

    // Build a location storage matrix
    arma::umat locs(2, n);

    // Create a vector to store each row information in. (Row, Col)
    arma::uvec temp(2);

    // Start collecting locations
    arma::sp_mat::const_iterator it = start; 
    for(int i = 0; i < n; ++i)
    {
        temp(0) = it.row();
        temp(1) = it.col();
        locs.col(i) = temp;
        ++it; // increment
    }

    return locs;
}

// Updates the sparse matrix by constructing a new one
// [[Rcpp::export]]
arma::sp_mat update_sp_matrix(arma::sp_mat& B, arma::vec values) 
{

    // Get all the locatoins
    arma::umat locs = get_locations(B);

    // Make sure we have the correct number
    if (locs.n_rows != values.n_elem) { 
        Rcpp::stop("Length mismatch between locations and supplied values!");
    }

    // The documentation recommends using batch constructor to rebuild matrix
    B = arma::sp_mat(locs, values, B.n_rows, B.n_cols);

    return B;
}

测试功能

// Generates a sparse matrix interally to test with
// Dimensions are 10 x 10 with only 2 points filled in.
arma::sp_mat make_test_sp()
{

    // creates a matrix C++98 style
    arma::umat locs;
    locs << 4 << 7 << arma::endr
         << 6 << 7 << arma::endr;

    // creates a vector C++98 style
    arma::vec vals;
    vals << 4.5 << 8.2 << arma::endr;

    arma::sp_mat B(locs, vals, 10, 10);

    return B;
}

// Main runner calls the built in test generation function.
// [[Rcpp::export]]
arma::sp_mat test_me() 
{
    arma::sp_mat B = make_test_sp();
    arma::vec temp = arma::ones<arma::vec>(2);
    return update_sp_matrix(B, temp);
}

修改

更改了get_location()代码，以反映

的sp_mat结构

2 x N

而不是

N x 2

感谢@ EricH的评论