如何以块为单位初始化“本征”矩阵？

Question

Sorry for the subject - I wasn't able to create a better one...

What I mean is: I have a 2d-Vector a and a 3d-Vector b. When I initialize b, b(0,1) should be set to a and b(2) should be set to 1. This code works:

    const Vector2d a(1,2);
    Vector3d b( 0,0,1 );
    b.head<2>() = a;

But what I would rather have (among others because of the const ):

    const Vector2d a(1,2);
    const Vector3d b( a, 1 );

This doesn't work. Is there a way to achieve this?

Answer 1

Without the const you would do:

Vector3d b;
b << a, 1;

If you really want it to be const, then you can do:

const Vector3d b = (Vector3d() << a, 1).finished();

Answer 2

Here's what you could do using a lambda (assuming there's not an easier way by using a constructor call):

auto generateB = [&]() -> Vector3d {
    Vector3d b(0, 0, 1);
    b.head<2>() = a;
    return b;
  };

And then you'd use it like this:

auto const actualB = generateB();
Vector3d const anotherB = generateB();