Add a by keyword to maximum and minimum

[davidanthoff]

Essentially the same thing that already exists for sort: an option to specify a selector function, so that the maximum is determined by whatever the selector function returns, but then the maximum function returns an element from the original input sequence.

For example, maximum([(a=1,b=2),(a=2,b=1)], by=i->i.b) would return (a=1,b=2).

[StefanKarpinski]

Makes sense—any ordering-dependent function should support this kind of API.

[davidanthoff]

Just saw #27639, where argmax(f, v) was suggested as an alternative.

[stevengj]

See also #19190 and #19359.

[sam0410]

Is it fine if we write a function like this ?

function ($fname)(a::AbstractArray; dims=:, by=identity)
    val = ($_fname)(by, a, dims)
    for x in a
        if by(x) == val
            return x
        end
    end
end

[giordano]

A solution could be adding a by keyword argument to isless and then forwarding this keyword argument from maximum/minimum all the way down to isless, right?

E.g.

julia> isless((1,2),(2,1))
true

julia> function Base.isless(t1::Tuple, t2::Tuple; by = first)
           a, b = by(t1), by(t2)
           isless(a, b) || (isequal(a, b) && isless(tail(t1), tail(t2)))
       end

julia> isless((1,2),(2,1))
true

julia> isless((1,2),(2,1), by = i->i[2])
false

[chethega]

Any implementation that is compatible with Base.Order has minimum([NaN,0.1, 0.2, missing]; by = identity) == 0.1, which is notably different from minimum([NaN,0.1, 0.2, missing]) == missing, and also minimum(Any[0, 1.0]) isa Float64 while minimum(Any[0, 1.0]; by=identity) isa Int64.

I am in favor of using the Base.Order framework. This means that we need to duplicate some code: The usual min/max comparison is in an entirely different category than isless.

[tkluck]

Reviving this one as its related to #34719 .

Any implementation that is compatible with Base.Order has minimum([NaN,0.1, 0.2, missing]; by = identity) == 0.1

Here's another way to think about it. Let's just restrict attention to min(a, b) as minimum(...) should just be equal to reduce(min, ...), with keywords passed through.

An important reason why min(::Float64, ::Int) isa Float64 is type stability. But when we extend with keywords, we can't really expect type stability in every case; for example,

min((a=2, b=4), (a=5.0, b=2), by=x -> x.b)

or even

min((a=2,b=4), (a=4.0,b=2,c=6), by=x -> x.b)

is never going to have a sane type-stable interpretation. I see two options:

• Make min(::Number, ::Number; kwds...) type-stable through promotion even when by= is passed.
• Make it type-stable whenever promote allows it: something like

function min(a, b; lt=isless, by=identity, order=Forward)
    if isconcretetype(promote_type(typeof(a), typeof(b)))
        a, b = promote(a, b)
    end
    isordered = Base.Order.lt(_ord(lt, by, order), a, b)
    return isordered ? a : b
end

Julia will evaluate isconcretetype(...) at compile-time and collapse the branch. It also doesn't depend on inferred types so it's API stable. It's also backwards compatible with current min(a, b) behaviour with the default keyword arguments.

I have a preference for this solution because I'm a heavy user of non-Number types that do support promote. Any thoughts/objections before I make a pull request?

[jariji]

findmax needs by= too. For example:

> passnothing(f) = x->isnothing(x) ? nothing : f(x)
> findmax(passnothing(abs), [-2., -4., nothing]; by=x->isnothing(x) ? -Inf : x) == (4., 2)