CODEWITHSUNDEEP
c++rcpparmadillorcpparmadillo
Tom Wenseleers
Tom Wenseleers

Reputation: 7989

Applying a function over columns of matrix using RcppArmadillo works but returns error when applied over rows

I coded a function qSelectMbycol in Rcpp that returns the kth largest element of each column in O(n) time. This function works OK. If I try to do the same but work over rows instead of columns (function qSelectMbyrow) it returns the error "error: Mat::init(): requested size is not compatible with column vector layout". Anybody any thoughts what I am doing wrong? I saved this file as "qselect.cpp" :

// [[Rcpp::depends(RcppArmadillo)]]
#define RCPP_ARMADILLO_RETURN_COLVEC_AS_VECTOR
#include <RcppArmadillo.h>
using namespace arma;

// [[Rcpp::export]]
arma::vec qSelectMbycol(arma::mat& M, const int k) {

  // ARGUMENTS
  // M: matrix for which we want to find the k-th largest elements of each column
  // k: k-th statistic to look up

  arma::mat Y(M.memptr(), M.n_rows, M.n_cols);
  // we apply over columns
  int c = M.n_cols;
  arma::vec out(c);
  int i;
  for (i = 0; i < c; i++) {
      arma::vec y = Y.col(i);
      std::nth_element(y.begin(), y.begin() + k - 1, y.end());
      out(i) = y(k-1); // the k-th largest value of each column
  }

  return out;

}

// [[Rcpp::export]]
arma::vec qSelectMbyrow(arma::mat& M, const int k) {

  // ARGUMENTS
  // M: matrix for which we want to find the k-th largest elements of each row
  // k: k-th statistic to look up

  arma::mat Y(M.memptr(), M.n_rows, M.n_cols);
  // we apply over rows
  int r = M.n_rows;
  arma::vec out(r);
  int i;
  for (i = 0; i < r; i++) {
    arma::vec y = Y.row(i); // this line throws the error "error: Mat::init(): requested size is not compatible with column vector layout"
    std::nth_element(y.begin(), y.begin() + k - 1, y.end());
    out(i) = y(k-1); // should give k-th largest value of each row
  }

  return out;

}

Example:

n=500
p=100
set.seed(1)
M=matrix(rnorm(n, mean = 100, sd = 1),n,1)
library(Rcpp)
library(RcppArmadillo)
Rcpp::sourceCpp('qselect.cpp')
qSelectMbycol(M,5) # works OK
qSelectMbyrow(M,5) # throws error "error: Mat::init(): requested size is not compatible with column vector layout"

I also tried inserting

  typedef std::vector<double> stdvec;

and replacing the line setting vector y by

arma::vec y = arma::conv_to<stdvec>::from(Y.row(i));

in my qSelectMbyrow function and although the function then runs, it runs slowly compared to applying over columns, and also crashes my R session if I run it 100 times.

Upvotes: 2

Views: 2861

Answers (1)

duckmayr
duckmayr

Reputation: 16920

The issue is that an arma::vec is actually an arma::colvec (see the docs). So, we can solve this issue by changing

arma::vec y = Y.row(i);

(which is incompatible because it thinks you want a matrix with one column but you're trying to give it a matrix with one row) to

arma::rowvec y = Y.row(i);

Upvotes: 4

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