CODEWITHSUNDEEP
c++armadillo
DHJ
DHJ

Reputation: 621

replacing matrix of indices with corresponding vector with armadillo

I have an arma::umat matrix containing indices corresponding to an arma::vec vector containing either 1 or -1:

arma::umat A = { {8,9,7,10,6}, {5,3,1,2,4}};
arma::vec v = {-1, 1, 1, 1, -1, -1, 1, -1, -1 ,1};

I would like to replace each element in the matrix with the corresponding value in the vector, so the output look like this:

A = {{-1,-1,1,1,-1},{-1,1,-1,1,1,1}}

Any suggestions? Thanks

Upvotes: 0

Views: 428

Answers (1)

darcamo
darcamo

Reputation: 3493

Saving the result into A is not an option, since A contains unsigned integers, and your v vector has doubles. Just create an arma::mat to contain the result and loop for each row to index v accordingly. One way to do this is using .each_row member.

#include <armadillo>

int main(int argc, char *argv[]) {
    arma::umat A = {{7, 8, 6, 9, 5}, {4, 2, 0, 1, 3}};
    arma::vec v  = {-1, 1, 1, 1, -1, -1, 1, -1, -1, 1};

    arma::mat result(A.n_rows, A.n_cols);

    auto lineIdx = 0u;
    // We capture everything by reference and increase the line index after usage.
    // The `.st()` is necessary because the result of indexing `v` is
    // a "column vector" and we need a "row vector".
    A.each_row([&](auto row) { result.row(lineIdx++) = v(row).st(); });

    result.print("result");

    return 0;
}

This code prints

result
  -1.0000  -1.0000   1.0000   1.0000  -1.0000
  -1.0000   1.0000  -1.0000   1.0000   1.0000

Upvotes: 1

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