[MCrossover97]

Hey everybody, I came across a strange case.

I generated a mesh with blockMesh in which all cells are the same size:

convertToMeters 0.000508;

vertices
(
    (0 0 0)
    (100 0 0)
    (100 40 0)
    (0 40 0)
    (0 0 40)
    (100 0 40)
    (100 40 40)
    (0 40 40)
);

blocks
(
    hex (0 1 2 3 4 5 6 7) (100 1 1) simpleGrading (1 1 1)
);

Then I compared the volumes of two random cells:

	if (mesh.V()[0] == mesh.V()[2]){
		std :: cout <<"Yes"<< std::endl;
	}else{
		std :: cout <<"No"<< std::endl;
	}
	
	std :: cout <<mesh.V()[0] - mesh.V()[2]<< std::endl;

And this is the result:

No
2.64698e-23

But when I compare the volumes of mesh(0) and mesh(1), I get a different answer:

Yes
0

How is this possible?

[adhiraj]

The difference is , which is very small.

I think you should not be comparing floating point umbers directly, as you have here:

if (mesh.V()[0] == mesh.V()[2])
{
    std :: cout <<"Yes"<< std::endl;
}

You are better off trying to see if the absolute difference between them is smaller than some , such as .

[MCrossover97]

Thank you for your answer! Do you know why some are exactly equal, but some are slightly different?

[Santiago]

Originally Posted by MCrossover97

Thank you for your answer! Do you know why some are exactly equal, but some are slightly different?

Because you are working with discrete mathematics here: 1 + 1 = 2 is a mathematical abstraction, for a computer 1 + 1 ~ 2, which implies moving bytes around