broadcast should not drop zero-dimensional results to scalars

[ZeppLu]

julia> map(typeof, (zeros(), ones(), fill(3.14)))
(Array{Float64,0}, Array{Float64,0}, Array{Float64,0})

julia> typeof(ones() .* fill(3.14))
Float64

julia> typeof(zeros() + ones())  # in fact not only after broadcast
Float64

Is it a bug, or a feature?

[mbauman]

This is an intentional feature. Zero-dimensional arrays act like scalars in broadcast.

[ZeppLu]

OK seems like it's a feature.

But I still wonder why typeof(zeros() + ones()) == Float64, note there's no broadcast. Intuitively speaking, the sum of two arrays should always be an array as well, shouldn't it?

[mbauman]

Oh my, you're right — I missed your third example there. That indeed is a bug.

[mbauman]

In short: we have long depended upon broadcasting to implement a number of array functions that implicitly work elementwise. We just add additional size checks. Unfortunately we now also need to add a zero-dimensional check, too.

Just around the definition of +, I can see that this also impacts -, conj, real, and imag.

[ZeppLu]

IMHO, we should make things consistent here, if ones() .+ ones() returns a scalar while ones() + ones() returns an array, people will get confused.

My suggestion is, broadcast SHOULD treat 0d arrays as scalars, but SHOULD NOT implicitly cast them to scalars in the final result.

[mbauman]

Yes, I'm in complete agreement. That's the bug, and it needs to be fixed.

Edit (a year later): I'm confused by my comment. I'm pretty sure I didn't mean to say what this says. I just wanted to fix the + behavior but keep broadcast the same.

[andyferris]

The casting of 0D arrays to scalars at the end of broadcast has been bothering me too.

[yurivish]

I just noticed this too while working to update the tests for https://github.com/mbauman/InvertedIndices.jl to 1.0.

Specifically,

    A = fill(1)
    @test A[Not(A.==1)] == []

fails with

Test threw exception
  Expression: A[Not(A .== 1)] == []
  ArgumentError: invalid index: true of type Bool

since fill(1) .== 1 returns true on 1.0.