warning: advanced initialization in Eigen might confuse

[uukhas]

There is the following way to initialize an array:

Eigen::Array<double,2,1> a;
a << 1, 2;
std::cout << a << "\n";

And the output is as one would assume:

1
2

So, I have immediately concluded that << acts as =, i.e. the right side is calculated at the same time and only then the substitution is done. This is WRONG:

Eigen::Array<double,2,1> a;
a << 1, 2;
a << a[0] + a[1], a[0];
std::cout << a << "\n";

returns

3
3

and not 3 1. So that every comma-separated element is calculated right after the update in corresponding a element.

[Expander]

You are right, this is a potential pitfall. Did this cause problems in some parts of the code?

[uukhas]

@Expander, this caused a problem for l_i->3l_j decays in the code which is not fully merged in FlexibleSUSY.
About other places I do not know yet.

[pathron]

This is just eigen though, not sure what we can do other than not use the << incorrectly . Is it documented somewhere in eigen that it works this way?

[uukhas]

Dear @pathron, here they do not use things similar to a << a[0] + a[1], a[0];.

What we can do is to make sure that we know about this "tricky" behavior (maybe contact eigen developers as well and ask them to add this case in the linked page).

We can add this warning to our developer wiki.

[Expander]

@uukhas I like your idea to tell the Eigen developers to put a brief note into their documentation to make the users aware of this issue. Formally, however, this is not a FS bug or similar. It's just something we need to be aware of.

[pathron]

Yes I also agree with that. I also agree with @uukhas that putting a note in our wiki here to warn other FS people about it is also helpful.

[Expander]

I have created the wiki page https://github.com/FlexibleSUSY/FlexibleSUSY/wiki/Eigen-library to warn us about this issue.