[dataflow]

That's everyone's instinctive reaction, but it's completely wrong, and everyone will know this too, if you just give the right examples. Like say you compare people by their age. That's a perfectly fine operation sufficient for sorting, min, max, etc. Now you find two people have the same age. Are they the same person?

Or let's say you're sorting strings case-insensitively. "a" might be neither less than nor greater than "A", meaning they'd be unordered with respect to each other. But that doesn't mean you suddenly wouldn't care if one got duplicated and the other got deleted.

Fundamentally "unordered" just means "neither comes before nor comes after". It doesn't imply "equal to". You can wrap this in a lot of fancy math terminology about partial and total orders, but it's not that complicated conceptually; the idea is just that the lack of an ordering doesn't imply equality, just like how you and your twin aren't the same person. This is one aspect of why C++ is introducing more sophisticated comparisons in C++20 (I would suggest looking it up).

[lmm]

> Like say you compare people by their age. That's a perfectly fine operation for sorting, min, max, etc. Now you find two people have the same age. Are they the same person?

They have the same age, which might be the minimum age of the group. I don't think it makes sense to talk about the minimum person of the group; minimum/maximum imply that it's the thing itself which is ordered.

[JadeNB]

The reaction to this point seems to have been dismissive (ironic in a thread that started with a complaint about dismissing small distinctions!), but I think the distinction you're making is an important one. In Haskell, it's the difference between `minimum` and `minimumBy`.

https://hackage.haskell.org/package/base-4.12.0.0/docs/GHC-L...

https://hackage.haskell.org/package/base-4.12.0.0/docs/Data-...

The point about strings is different. It doesn't make sense in general to speak of the minimum string in a list, or, more generally, the minimum of a partially ordered set; but it does make perfectly good sense to sort a partially ordered set, with the idea that there are multiple correct sorts when elements are incomparable. That, of course, is where the notion of a stable sort becomes important.

[Tyr42]

How about a priority heap? It's much more useful to sort things by their min priority, but just telling me the priority at the end isn't helpful. You'll want the thing as well.

(Even more concretely consider a min heap used in pathfinding, where you store the (distance_so_far, (x,y)) pairs, and they sort by distance_so_far.)

[lmm]

> How about a priority heap? It's much more useful to sort things by their min priority, but just telling me the priority at the end isn't helpful. You'll want the thing as well.

Sure, but again, at that point you're not taking the minimum thing, you're taking the thing with the minimum priority. And it wouldn't be strange to say that two things have the same priority (even if they're different things).

[mehrdadn]

> I don't think it makes sense to talk about the minimum person of the group; minimum/maximum imply that it's the thing itself which is ordered.

Uh, it totally makes sense, I just gave you a string example too. The comparator doesn't have to distinguish unequal elements, and a min (and argmin and sort et al.) is perfectly fine and sensible with such a comparator.

Do you code in Python at all? Do you find the fact that 1.0 and 1 are both simultaneously equal in some sense and unequal in another sense to be just complete nonsense? Do you expect min(1.0, 1) to error...? Or for sorted([1.0, 1]) to return [1, 1] just because the elements are "equal" so obviously what right do you have to care which one is returned?

[lmm]

> Uh, it totally makes sense, I just gave you a string example too. The comparator doesn't have to distinguish unequal elements, and a min (and argmin and sort et al.) is perfectly fine and sensible with such a comparator.

Sorting is not the same thing as taking the minimum.

> Do you code in Python at all? Do you find the fact that 1.0 and 1 are both simultaneously equal in some sense and unequal in another sense to be just complete nonsense? Do you expect min(1.0, 1) to error...?

Yes, and this is a large reason why I prefer to work in languages that let me manage such distinctions more carefully.