Pick out the even (i,j where both even) elements from a matrix

    m[0::2, 0::2]                        // Numpy
    m(0::2, 0::2)                        // Fortran [1]
    m(seq(0, last, 2), seq(0, last, 2))  // C++ Eigen

[1] If you have declared the array to start indexing from 0.