This violates certain invariances that users may expect, such as

findmax(x)[1] == maximum(x)

so I would say it is a bug. If we take the "NaN should propagate" view, it both should be NaN, but I have no good suggestion for the position returned by findmax (maybe 0? yet another corner case). If we take the view that "NaN encodes missing data", both should return 2.0.

By the way, the original motivation of this question was to find a way to perform NaN-ignoring maximum. I thought I could use findmax for this if the above reported behavior of findmax was intended.

However, now that the above NaN-ignoring behavior of findmax seems like a bug, I need a better way to find the maximum ignoring NaN. Some discussion thread suggested something like maximum(v[!isnan.(v)]), which I don’t like because of too many unnecessary allocations.

The best way I’ve figured out is to define nanmax that returns the greater of the two numbers ignoring NaN, and to use it in reduce:

julia> nanmax(x,y) = isnan(x) ? y : (isnan(y) ? x : max(x,y))
nanmax (generic function with 1 method)
julia> reduce(nanmax, v)
2.0

A couple of notes:

You can generalize this to multidimensional arrays A using reducedim, as reducedim(nanmax, A, 2) that finds the maxima along the second dimension ignoring NaN.

When A is StaticArray, you can use the type-stable version of this: reducedim(nanmax, A, Val{2}). By using Val{2} instead of 2 as the search dimension indicator, you can make reducedim to produce a StaticArray. See https://github.com/JuliaArrays/StaticArrays.jl/pull/86.

Edit: @DNF pointed out that y > x returns false for all x if y = NaN, so the above nanmax can be simplified to this:

For the array maximum you can save quite a bit of time by only checking for isnan up to the point where the first non-NaN element is encountered, and then switching to ordinary > comparison, since NaN > maxval is false.

The above nanmax does not perform isnan at all, so it’s a bit weird to call it nanmax, but it works by the following reason.

If x is guaranteed to be non-NaN, the above nanmax always returns the correct non-NaN maximum between x and y, regardless of y being NaN or non-NaN. Now, note that reduce(nanmax, v) performs reduction by putting the intermediate reduction result up to the current entry of v into x and the next entry of v into y. Therefore, if the first entry of v is non-NaN, x of nanmax(x,y) stays always non-NaN throughout the entire reduction procedure of reduce(nanmax, v).

The only case reduce(nanmax, v) does not work is when the first entry of v is NaN. We can avoid this case by using reduce(nanmax, -Inf, v), which is effectively the same as reduce(nanmax, w) for w = [-Inf; v] whose first entry is non-NaN.

The problem of this is that it produces -Inf even if v is completely filled with NaN, for which case we may want to get NaN. However, if we know v has at least one non-NaN entry, this technique works.

Inspired by this idea, we can define nanmaximum more generally as follows:

nanmaximum(v::AbstractVector{T}) where {T<:Number} = reduce((x,y) -> x<y ? y : x, typemin(T), v)

Yes, this is pretty much what I was thinking as well. Here’s my (ugly) code which also handles all NaNs. Speed is the same as your code:

function _maximum(y, istart)
val = typemin(eltype(y))
for i in eachindex(y)[istart:end]
if y[i] > val
val = y[i]
end
end
return val
end
function nanmaximum(y)
for i in eachindex(y)
if !isnan(y[i])
return _maximum(y, i)
end
end
return convert(eltype(y), NaN)
end

@bjarthur above linked to NaNMath.jl, which I believe is the intended way of dealing with this, i.e. you could use NaNMath.maximum rather than implementing your own replacemen… The code for maximum in NaNMath looks a lot like @DNF’s

function maximum{T<:AbstractFloat}(x::AbstractArray{T})
result = convert(eltype(x), NaN)
for i in x
if !isnan(i)
if (isnan(result) || i > result)
result = i
end
end
end
return result
end

Note that this (and DNF’s code) may not work for all iterable collection of all eltypes, which is why NaNMath restricts it to {T<:AbstractFloat}(x::AbstractArray{T}). This can be annoying in practice if your code sometimes passes something else, like a UnitRange{Int} to the function.
In Plots this was resolved by defining an internal function that dispatched to the NaNMath versoin for {T<:AbstractFloat}(x::AbstractArray{T}), and to the Base version as fallback.