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] 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?

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

Why would I care about that. It implements a reasonable comparison. “Each element is less than or equal to the next” is a perfectly fine way of sorting, and is in fact what sort(A) returns.

[image] DNF: Why would I care about that. It implements a reasonable comparison. How do you define “reasonable comparison”? For me, in the context of sort/issorted a reasonable comparison is defined by a strict total order. [image] DNF: “Each element is less than or equal to the next” is…

[image] sostock: How do you define “reasonable comparison”? I don’t. Whatever works is fine. Including random. It’s just that you cannot expect random sorts to be repeatable. [image] sostock: For me, in the context of sort / issorted a reasonable comparison is defined by a strict total o…

Understanding issorted's lt keyword

New to Julia

DNF May 12, 2021, 10:56am 36

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.

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Understanding issorted's lt keyword

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