[Eigen] extracting a vector from the vector space

 col extrating
    mxd::t m(3,3);
 
    m << 1,2,3,
         4,5,6,
         7,8,9;
 
    co(m , "m");
    co(m.col(0), "col[0]");
 
    m.col(2) = m.col(0) * 3;
 
    co(m,"m");
 

result:

row[0] = 
1
4
7
m = 
 1  2  3
 4  5 12
 7  8 21

vector combination 

    v2d::t v1;
    v2d::t v2;
 
    v1 << 1, 2;
    v2 << 3, 4;
 
    mxd::t m(2,2);
 
    m << v1, v2;
 
    co(m,"m");
 
 
result:
m = 
1 3
2 4