Fix float grouping

NEWS.md

@@ -14,7 +14,9 @@
 
 ## Bug fixes
 
-* fix bug in how `groupby` handles grouping of float columns
+* fix bug in how `groupby` handles grouping of float columns;
+  now `-0.0` is treated as *not integer* when deciding on which
+  grouping algorithm should be used
   ([#2791](https://github.com/JuliaData/DataFrames.jl/pull/2791))
 * fix bug in how `issorted` handles custom orderings and improve performance
   of sorting when complex custom orderings are passed

src/groupeddataframe/groupeddataframe.jl

@@ -79,6 +79,11 @@ an `AbstractDict` can be used to index into a grouped data frame where
 the keys are column names of the data frame. The order of the keys does
 not matter in this case.
 
+A column is considered to be an integer column when deciding on the grouping
+algorithm choice if its `eltype` is a subtype of `Union{Missing, Real}`, all its
+elements are either `missing` or pass `isinteger` test, and none of them is
+equal to `-0.0`.
+
 # See also
 
 [`combine`](@ref), [`select`](@ref), [`select!`](@ref), [`transform`](@ref), [`transform!`](@ref)

src/groupeddataframe/utils.jl

@@ -145,22 +145,16 @@ function refpool_and_array(x::AbstractArray)
         end
     elseif x isa AbstractArray{<:Union{Real, Missing}} && length(x) > 0
         if !(x isa AbstractArray{<:Union{Integer, Missing}})
-            if !all(v -> (ismissing(v) | isinteger(v)) & (!isequal(v, -0.0)), x)
+            if !all(v -> (ismissing(v) | isinteger(v)) & !isequal(v, -0.0), x)
                 return nothing, nothing
             end
         end
         if Missing <: eltype(x)
             smx = skipmissing(x)
-            y = iterate(smx)
-            y === nothing && return nothing, nothing
-            (v, s) = y
-            minval = maxval = v
-            while true
-                y = iterate(smx, s)
-                y === nothing && break
-                (v, s) = y
-                maxval = max(v, maxval)
-                minval = min(v, minval)
+            if isempty(smx)
+                return nothing, nothing
+            else
+                minval, maxval = extrema(smx)
             end
         else
             minval, maxval = extrema(x)