[Eigen] reviewing linear algebra day.4 , slicing and redicing

//blocking a vector(partitioning) 
inline vxd::t v_block(vxd::t _v, int endIdx, int startIdx = 0 ){  return _v.segment(startIdx,endIdx); }
 
//slicing and redicing axpy algorithm
inline vxd::t v_axpy(double alpha, vxd::t _x, vxd::t _y){
 
    vxd::t v_(_x.rows()); v_.setZero();
 
    vxd::t xTop = _x.segment(0,0);
    vxd::t xBot = _x.segment(0,_x.rows());
 
    vxd::t yTop = _y.segment(0,0);
    vxd::t yBot = _y.segment(0,_y.rows());
 
    int    idx    = 0;
 
    while(xTop.rows() < _x.rows()){
        v_[idx++] = alpha * _x[idx] + _y[idx];
 
 
        xTop = _x.segment(  0,             idx );
        xBot = _x.segment(idx, _x.rows() - idx );
        
        yTop = _y.segment(  0,             idx );
        yBot = _y.segment(idx, _y.rows() - idx );
 
        co(xTop, "xTop");
        co(xBot, "xBot");
 
        co(yTop, "yTop");
        co(yBot, "yBot");
    }
    return v_;
}
 
    vxd::t v(3);
    vxd::t v2(3);
 
    v  << 1, 2, 3;
    v2 << 2, 2, 2;
 
 
    co(v_axpy(2, v, v2));

result :
xTop = 
1
xBot = 
2
3
yTop = 
2
yBot = 
2
2
xTop = 
1
2
xBot = 
3
yTop = 
2
2
yBot = 
2
xTop = 
1
2
3
xBot = 
 
yTop = 
2
2
2
yBot = 
 
output = 
4
6
8