How to work out the angle between two 2D vectors using cross product?

Question

So here's a link for the same question but the best answer doesn't explain it fully:

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It's the cross product that I'm stuck with, since the formula in that link can only be applied in mathematics outside of computing.

What is the actual formula to calculate the cross product in computing form? If you can post it as C++ code that would be great.

Keep in mind I'm looking for the cross product between 2 2D vectors, not 3D.

Answer 1

The title says you are interested in computing the angle between two 2D vectors, so thats what I'm going with.

If you look at for instance http://mathworld.wolfram.com/DotProduct.html, It is fairly straightforward to implement in code.

There is however the atan2 function which makes this a cinch:

double angle = atan2(p2y, p2x) - atan2(p1y, p1x);