Removing zero rows from a Matrix (elegant way) - c++

Question

I have a matrix with some zero rows. I would like to remove the zero rows. The matrix is Nx3. What I've done is simple. I create std::vector in which every three elements represent a row then I convert it to Eigen::MatrixXd. Is there an elegant way to remove the zero rows?
#include <iostream>
#include <vector>
#include <Eigen/Dense>
Eigen::MatrixXd VecToMat(const std::vector<double> vec)
{
int rows(vec.size()/3) , cols(3);
Eigen::MatrixXd temp( rows , cols);
int count(0);
for ( int i(0); i < rows; ++i)
{
temp(i,0) = vec[count];
temp(i,1) = vec[count+1];
temp(i,2) = vec[count+2];
count += 3;
}
return temp;
}
Eigen::MatrixXd getNewMat(Eigen::MatrixXd& Z)
{
std::vector<double> vec;
for ( int i(0); i < Z.rows(); ++i)
{
if ( (Z(i,0) && Z(i,1) && Z(i,2)) != 0 ){
vec.push_back(Z(i,0));
vec.push_back(Z(i,1));
vec.push_back(Z(i,2));
}
}
Eigen::MatrixXd temp = VecToMat(vec);
return temp;
}
int main()
{
Eigen::MatrixXd Z(5,3);
Z.setOnes();
Z(0,0) = 0;
Z(0,1) = 0;
Z(0,2) = 0;
Z(1,0) = 0;
Z(1,1) = 0;
Z(1,2) = 0;
Z(2,0) = 0;
Z(2,1) = 0;
Z(2,2) = 0;
std::cout << Z << std::endl << std::endl;
std::cout << getNewMat(Z) << std::endl;
std::cin.get();
return 0;
}

Answer 1

Here is a full implementation I find quite elegant. Note that this one does not preserve order of non-zero rules, which maybe isn't what you want, but is more efficient both in complexity and lines of code:
void removeZeroRows(Eigen::MatrixXd& mat)
{
Matrix<bool, Dynamic, 1> empty = (mat.array() == 0).rowwise().all();
size_t last = mat.rows() - 1;
for (size_t i = 0; i < last + 1;)
{
if (empty(i))
{
mat.row(i).swap(mat.row(last));
empty.segment<1>(i).swap(empty.segment<1>(last));
--last;
}
else
++i;
}
mat.conservativeResize(last + 1, mat.cols());
}

Answer 2

Basically you can follow a pseudocode like this:
get N = rows, M = columns
iterate by each N
if N[0] = 0 iterate on rows exiting on first non-zero
if N[0] = 0 && .. && N[M] = 0
remove row
For removing single row:
void removeRow(Eigen::MatrixXd& matrix, unsigned int rowToRemove) {
unsigned int numRows = matrix.rows() - 1;
unsigned int numCols = matrix.cols();
unsigned int rowPos = numRows - rowToRemove;
if( rowToRemove < numRows ) {
matrix.block(rowToRemove, 0, rowPos, numCols) = matrix.block(rowToRemove + 1, 0, rowPos,numCols);
}
matrix.conservativeResize(numRows, numCols);
}

Answer 3

Save each nonempty row into a vector, then make a new matrix
vector<block> buffer; //not sure of the type name for the rows
VectorXd zero(3); //or appropriate comparable type
for(int i = 0; i < Z.rows(); i++){ //note: possibly a function call each time
if(Z.row(i) != zero)
//if((Z.row(i) != 0).any()) //broadcasting comparison?
buffer.push_back(Z.row(i));
}
MatrixXd return_value(buffer.size(), 3);
for(int i = buffer.size(); i --> 0;)
return_value.row(i) = buffer[i];
return return_value;
Warning: Resizing the old one instead of making a new one may erase the contents before you can save them.
I can't read the docs from here, so you will have to see for yourself what kind of comparison operations you can do on block objects. As a last result, try row.any() (faster?) or row.squaredNorm() == 0.