使用特征消除零列或行 [英] Removing zero columns or rows using eigen

问题描述

我想知道是否有一种更有效的方法来删除全部为零元素的列或行。我确定本征库中正在使用这些函数，但我不知道该怎么做。

I was wondering if there is a more efficient way to remove columns or rows that are all zero elements. I am sure there is using the functions in the eigen library but I do not know how.

现在，我正在这样做，并带有while循环的想法在有多个行/列加起来为零的情况下使用，我不想超出范围限制或不传递任何零行。

Right now I am doing it like so, with the idea of the while loop being used in case there are multiple rows/columns that sum to zero I dont want to exceed range limits or pass any zero rows.

void removeZeroRows() {
    int16_t index = 0;
    int16_t num_rows = rows();

    while (index < num_rows) {
        double sum = row(index).sum();
        // I use a better test if zero but use this for demonstration purposes 
        if (sum == 0.0) {
            removeRow(index);
        }
        else {
            index++;
        }

        num_rows = rows();
    }
}

推荐答案

当前（Eigen 3.3），对此没有直接功能（尽管计划用于Eigen 3.4）。

Currently (Eigen 3.3), there is no direct functionality for this (though it is planned for Eigen 3.4).

同时，可以使用类似的功能（当然，行和 col 可以互换，并且输出仅用于说明）：

Meanwhile can use something like this (of course, row and col can be interchanged, and output is just for illustration):

Eigen::MatrixXd A;
A.setRandom(4,4);
A.col(2).setZero();

// find non-zero columns:
Eigen::Matrix<bool, 1, Eigen::Dynamic> non_zeros = A.cast<bool>().colwise().any();

std::cout << "A:\n" << A << "\nnon_zeros:\n" << non_zeros << "\n\n";

// allocate result matrix:
Eigen::MatrixXd res(A.rows(), non_zeros.count());

// fill result matrix:
Eigen::Index j=0;
for(Eigen::Index i=0; i<A.cols(); ++i)
{
    if(non_zeros(i))
        res.col(j++) = A.col(i);
}

std::cout << "res:\n" << res << "\n\n";

通常，您应避免在每次迭代时调整矩阵大小，但应尽快将其调整为最终大小

Generally, you should avoid resizing a matrix at every iteration, but resize it to the final size as soon as possible.

使用Eigen 3.4可能会出现类似的情况（语法尚未最终确定）：

With Eigen 3.4 something similar to this will be possible (syntax is not final yet):

Eigen::MatrixXd res = A("", A.cast<bool>().colwise().any());

与Matlab / Octave等效：

Which would be equivalent to Matlab/Octave:

res = A(:, any(A));

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