`findmin` with arbitrary comparison operator?

[Erik Schnetter]

Is there a version of `findmin` that allows passing in an arbitrary comparison operator?

In throw-away code, I often have an array containing tuples of both the result and some additional operator. I want to find the minimum and its location, but want to ignore the additional information. I would like to use a user-defined comparison operator that looks only at one of the tuple elements, yet `findmin` doesn't seem to offer this.

-erik

--

Erik Schnetter <schn...@gmail.com> http://www.perimeterinstitute.ca/personal/eschnetter/

[Júlio Hoffimann]

Hi Erik,

All you need to do is specialize the < operator for your tuple type.

Please check the definition of findmin by running:

@edit findmin([1,2,3])

You'll see the implementation is quite trivial. Let us know if you weren't able to make it work.

Best,

-Júlio

[Erik Schnetter]

Júlo

My "tuple type" is an actual tuple. While I could create custom comparison operator for e.g. `Tuple{Int, Int, AbstractString}`, this would be unorthodox, as this comparison operator would be globally visible. This could break many things, such as e.g. dictionaries that hold tuples of arbitrary type.

-erik

[Júlio Hoffimann]

Hi Erik,

What I meant was a derived type from Julia's Tuple type. Though I agree with you that this may be too much of an overhead for such a simple task.

I'll open an issue on GitHub to ask if others are ok with an additional predicate option a la C++:

findmin(array, pred=<)

If they are all positive, I'll add it for you.

Best,
-Júlio

[Dan]

suggestion: call the named argument `lt` to match the argument of `sort`.

[Júlio Hoffimann]

The issue on GitHub: https://github.com/JuliaLang/julia/issues/14216

Thanks,

-Júlio

[Josh Langsfeld]

Doesn't 'min' imply/require the usage of '<' ? A whole lot of methods would need the extra argument added for consistency, including min, minimum, maximum, etc...

Could a workable solution be to define a function that maps a tuple to a real number instead of a comparison function and do 'findmin(map(score_func, my_tuples))' ?

[Erik Schnetter]

One of the tuple entries is the number by which I'd like to sort, so the score_func is trivial. However, in the end I want not only the minimum value, but also the additional tuple elements, which would be stripped off by the score_func.

-erik

[Josh Langsfeld]

That's true; my_tuples[findmin(map(score_func, my_tuples))[2]] is a fair bit uglier than just interesting the map.

[Júlio Hoffimann]

Actually, I am starting to like the simple approach of filtering the tuple suggested by Josh. It's cleaner and probably faster in general.

scores = map(t -> t[1], mytuplelist) # first tuple element as score

idx = indmin(scores)

mintuple = mytuplelist[idx]

I will close the issue.

Best,

-Júlio