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> …

The definition Julia uses is that a list is sorted iff for every pair of elements, either: The elements are equal The first element is less than the second The reason for this design is it only relies on users to define ==(a::T,b::T) and <(a::T,b::T). The reason to define sort in terms of the mo…

The design perfectly fits the expectation of users who know the mathematical definition of a total order, and that is much more than 1% of users.

[image] Oscar_Smith: The elements are equal The first element is less than the second Then it would help a lot if the first clause was made very explicit. And that the lt input is subject to very strict requirements.

Yeah. The docs probably should say something along the lines of “lt should be a function that forms a total order when combined with isequal”

[image] Oscar_Smith: The reason for this design is it only relies on users to define ==(a::T,b::T) and <(a::T,b::T) . That is not correct. All you need is < / isless. Take for example julia> mutable struct Wrap _::Int end julia> Base.isless(a::Wrap, b::Wrap) = isless(a._, b._…

[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…

Understanding issorted's lt keyword

New to Julia

DNF May 12, 2021, 6:44pm 69

The number was for the sake of argument. But how big do you think that number is?

And as I said, the docs don’t even mention “total order” (or do they), so why would I expect that to be relevant?

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Meaning of `isless`, `<`, `cmp`, `isequal`, `==`, total order, partial order, unordered Internals & Design	24	2466	January 7, 2018
Sortperm with keyword lt = !isless throws BoundsError General Usage	9	586	January 8, 2023
Behavior of == when isless or < is defined General Usage	22	2272	April 4, 2018
What should `issorted` return when an order isn't defined for the elements, but a comparison works? General Usage sort	4	272	July 10, 2023
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Understanding issorted's lt keyword

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