Does it make much of a difference here? It's passed by value for a one line function.

Yeah, from now on I will check the test data and the results expected and make up my own instructions from that.
That last fo99obar99 that comes out from nowhere 😅

Using a switch statement requires the data type to be an int, but hard converting a double to an int causes the decimal to be lost. OP multiplied by 10 and multiplied the ranges by 10 to circumnavigate that. Since input values only go to one decimal place, this works.

well because the form of the serie 1/1 + 1/4 ... etc
it will never grow bigger than 9

What part of the kata exactly makes you think the description is incomplete?

user - defined parentheses operator, used as a default constructor for struct object, that's how it works, I believe.

damn you really went this far lol

You'll have to look them for yourself.

It is called a fold expression

Added a red comment about reading the notes next to that line, I hope that's enough.

You wrote:

Please state clearly

And it's clearly stated as I quoted there. Sorry if that sounded rude, but you should read the specs fully before programming anything. It's a good thing to learn to do that, even for an 8 kyu kata. About the complains of other people, they didn't read the specs fully either.

You cannot swap the operands. For instance, in the given example you cannot get expression (1 + 3) * 2 = 8.

It's already there, and even in bold. Read better?

This solution is invalidated.

You're not the only one here lol. This had me thinking I'm doing something wrong haha