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] DNF: And as I said, the docs don’t even mention “total order” (or do they), so why would I expect that to be relevant? The documentation for Base.isless does mention “total order” ( Understanding issorted's lt keyword - #2 by goretkin ) . And isless is the default argument. The connecti…

The default argument creates(?) a total ordering, but it doesn’t follow that all arguments must do so. Oh, well, it’s getting late. Maybe sleeping on it will help.

[image] goretkin: It sounds like you are willing to accept the claim that isless and isequal is more fundamental than <= . I don’t see a ranking of fundamentalness between < and <=. It’s possible to define either in terms of the other. A total order is normally defined in terms of <=. Checki…

[image] sostock: [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. My contention is that issorted already contains that loop. What I’m not sure about is if I wo…

[image] jzr: I don’t see a ranking of fundamentalness between < and <=. It’s possible to define either in terms of the other. Yes, both choices are possible. And a choice has already been made. This allows Base to have fallback methods. Can you explain what you don’t see here in the previous…

[image] goretkin: That it is defined in terms of something like isless is reasonable. I don’t think running isstrictlysorted(::ArrayWithDuplicates) is usually the user’s intent when they run issorted(lst), and having footguns lying around is a bad property in a language. [image] goretkin: …

[image] jzr: I don’t think running isstrictlysorted(::ArrayWithDuplicates) is reasonable First of all, what does this mean? Secondly, just because the answer to something is false, doesn’t mean it’s unreasonable to ask the question. Please show an actual piece of code that you think is a foo…

Yeah I deleted that line :slight_smile:

[image] jzr: I don’t think running isstrictlysorted(::ArrayWithDuplicates) is usually the user’s intent when they run issorted(lst) This edit still doesn’t help me understand what you meant. Current behavior: julia> issorted([1, 1]) true it’s not a test of whether the list (which, by the w…

Sorry for the confusion, I wrote inaccurately above. The unexpected behavior is julia> issorted([1,1], lt=<=) false and more generally that issorted(sort(seq, lt=f), lt=f) isn’t necessarily true for collections with duplicates. Maybe a solution is to provide an le= parameter.

Understanding issorted's lt keyword

New to Julia

jzr May 12, 2021, 11:15pm 83

I guess that’s true.

Could you spell out why this follows?

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Meaning of `isless`, `<`, `cmp`, `isequal`, `==`, total order, partial order, unordered Internals & Design	24	2450	January 7, 2018
Sortperm with keyword lt = !isless throws BoundsError General Usage	9	578	January 8, 2023
Behavior of == when isless or < is defined General Usage	22	2265	April 4, 2018
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Understanding issorted's lt keyword

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