The lt keyword for issorted and sort! is defying my intuition. From reading the source code, I understand why I get the following results, but I am finding it counterintuitive. From the documentation for sort!: the lt keyword allows providing a custom “less than” function Let’s try it: julia> …

[image] sostock: I don’t see where the current behavior contradicts the documentation. The documentation is just vague. I agree. It’s just that the behaviour is extremely counter-intuitive. To some, at least. Do you find the current behaviour intuitive? As in, if you just read the docstring f…

Personally, I would have never provided <= as lt. After all it stands for “less than” and not “less than or equal”. So, for me, the strict order constraint, although not explicitly mentioned, coincides with my intuition. (I would be fine with expanding the documentation in this regard or changing t…

[image] carstenbauer: Personally, I would have never provided <= as lt . After all it stands for “less than” and not “less than or equal”. I’m struggling to understand this argument. It seems to argue about what sort of comparisons one ought to input, not how the comparison should be used. Y…

[image] jzr: Aha, for that case, I would use sort ∘ unique to change the sequence, and allunique(seq) && issorted(seq) to check. Are sort ∘ unique and allunique(seq) && issorted(seq) really the most efficient way to accomplish those tasks? For the former, perhaps sorted_unique(x) = (unique! …

[image] DNF: You could use lt=(>) if you like, or even lt=(x,y)->rand(Bool) . That is a very good example. Yes, you can input any binary function that returns a Bool. But if you would actually use lt=(x,y)->rand(Bool), would you consider the result useful? If yes, do you think the current im…

[image] mkitti: Are sort ∘ unique and allunique(seq) && issorted(seq) really the most efficient way to accomplish those tasks? No, the most efficient would probably be a loop.

[image] sostock: would you consider the result useful? If you want a random shuffle, why not? Useful is in the eye of the beholder. [image] sostock: If no, then why do you think the result for lt=<= would be useful? Not sure I understand the question. It would be useful because every …

Okay. So let’s assume you did sort(A, lt=(x,y)->rand(Bool)). You now expect A to be sorted according to lt, so issorted(A, lt=(x,y)->rand(Bool)) should return true. You try it and it returns false! Do you consider that a bug in sort/issorted?

No, this is a reasonable exception, occurring because I input a ‘perverse’ comparison function. I can do shuffle!(A) but testing for ‘shuffleness’ wouldn’t make sense.

But <= is a ‘perverse’ comparison function as well, because it does not implement a strict total order.

Understanding issorted's lt keyword

New to Julia

sostock May 12, 2021, 10:47am 33

No, the most efficient would probably be a loop.

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Understanding issorted's lt keyword

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