Repeating each elements of an Eigen Matrix both row wise and column wise

Question

Given an Eigen Matrix, say

Eigen::Matrix<float, Eigen::Dynamic, Eigen::Dynamic> A(2, 2);
A << 11, 12, 21, 22;

what is the most elegant way to obtain a matrix B of dimensions 4x4 from A, where each element of A is repeated once both row wise and column wise.

11 11 12 12
11 11 12 12
21 21 22 22
21 21 22 22

I am looking for a generic solution that could scale to any given row and column sizes of A.

Answer 1

Using the head of Eigen you write such a repelem in a few lines of code:

template<typename XprType, typename RowFactorType, typename ColFactorType>
auto repelem(const XprType &xpr, RowFactorType row_factor, ColFactorType col_factor) {
    using namespace Eigen;

    const int RowFactor = internal::get_fixed_value<RowFactorType>::value;
    const int ColFactor = internal::get_fixed_value<ColFactorType>::value;
    const int NRows = XprType::RowsAtCompileTime == Dynamic || RowFactor == Dynamic ? Dynamic : XprType::RowsAtCompileTime*RowFactor;
    const int NCols = XprType::ColsAtCompileTime == Dynamic || ColFactor == Dynamic ? Dynamic : XprType::ColsAtCompileTime*ColFactor;
    const int nrows = internal::get_runtime_value(row_factor) * xpr.rows();
    const int ncols = internal::get_runtime_value(col_factor) * xpr.cols();

    return xpr(
        Array<int,NRows,1>::LinSpaced(nrows,0,xpr.rows()-1),
        Array<int,NCols,1>::LinSpaced(ncols,0,xpr.cols()-1)
    );
}

Usage:

Array22i A;
A << 1, 2, 3, 4;
std::cout << repelem(A,2,2) << "\n";

Full demo: https://godbolt.org/z/rYgDxF

This code might look a bit complicated but it is so to preserve compile-time sizes as proved by the following test:

Array22i A;
static_assert(decltype(repelem(A,fix<2>,fix<2>))::RowsAtCompileTime==4,"Bad compile-time size");

People can track its inclusion within Eigen there.