unify reduce/reducedim empty case behaviors

NEWS.md

@@ -41,6 +41,10 @@ New library features
 Standard library changes
 ------------------------
 
+* Empty dimensional reductions (e.g., `reduce` and `mapreduce` with the `dims` keyword
+  selecting one or more dimensions) now behave like their whole-array (`dims=:`) counterparts,
+  only returning values in unambiguous cases and erroring otherwise.
+
 #### JuliaSyntaxHighlighting
 
 #### LinearAlgebra

base/reducedim.jl

@@ -341,8 +341,10 @@ _mapreduce_dim(f, op, ::_InitialValue, A::AbstractArrayOrBroadcasted, ::Colon) =
 _mapreduce_dim(f, op, nt, A::AbstractArrayOrBroadcasted, dims) =
     mapreducedim!(f, op, reducedim_initarray(A, dims, nt), A)
 
-_mapreduce_dim(f, op, ::_InitialValue, A::AbstractArrayOrBroadcasted, dims) =
+function _mapreduce_dim(f, op, ::_InitialValue, A::AbstractArrayOrBroadcasted, dims)
+    isempty(A) && return fill(mapreduce_empty(f, op, eltype(A)), reduced_indices(A, dims))
     mapreducedim!(f, op, reducedim_init(f, op, A, dims), A)
+end
 
 """
     reduce(f, A::AbstractArray; dims=:, [init])
@@ -1128,10 +1130,7 @@ findmin(f, A::AbstractArray; dims=:) = _findmin(f, A, dims)
 function _findmin(f, A, region)
     ri = reduced_indices0(A, region)
     if isempty(A)
-        if prod(map(length, reduced_indices(A, region))) != 0
-            throw(ArgumentError("collection slices must be non-empty"))
-        end
-        similar(A, promote_op(f, eltype(A)), ri), zeros(eltype(keys(A)), ri)
+        _empty_reduce_error()
     else
         fA = f(first(A))
         findminmax!(f, isgreater, fill!(similar(A, _findminmax_inittype(f, A), ri), fA),
@@ -1201,10 +1200,7 @@ findmax(f, A::AbstractArray; dims=:) = _findmax(f, A, dims)
 function _findmax(f, A, region)
     ri = reduced_indices0(A, region)
     if isempty(A)
-        if prod(map(length, reduced_indices(A, region))) != 0
-            throw(ArgumentError("collection slices must be non-empty"))
-        end
-        similar(A, promote_op(f, eltype(A)), ri), zeros(eltype(keys(A)), ri)
+        _empty_reduce_error()
     else
         fA = f(first(A))
         findminmax!(f, isless, fill!(similar(A, _findminmax_inittype(f, A), ri), fA),

deps/checksums/SparseArrays-ac0cc6e8278bb614dc2a16e9353fa092dae6606a.tar.gz/md5

@@ -0,0 +1 @@
+edafbcbfc29cb7a86aef4a92bf7e7267

deps/checksums/SparseArrays-ac0cc6e8278bb614dc2a16e9353fa092dae6606a.tar.gz/sha512

@@ -0,0 +1 @@
+230151395d43b005e1285f5938a5800650314dc562e4cc6363775b54b45b33076a551276e45a977a0ea3b07ca0088a2ea27def63196f435ff1d6153b93999485

stdlib/SparseArrays.version

@@ -1,4 +1,4 @@
-SPARSEARRAYS_BRANCH = main
-SPARSEARRAYS_SHA1 = f3610c07fe0403792743d9c9802a25642a5f2d18
-SPARSEARRAYS_GIT_URL := https://github.com/JuliaSparse/SparseArrays.jl.git
-SPARSEARRAYS_TAR_URL = https://api.github.com/repos/JuliaSparse/SparseArrays.jl/tarball/$1
+SPARSEARRAYS_BRANCH = mb/mapreducedim_empty
+SPARSEARRAYS_SHA1 = ac0cc6e8278bb614dc2a16e9353fa092dae6606a
+SPARSEARRAYS_GIT_URL := https://github.com/mbauman/SparseArrays.jl.git
+SPARSEARRAYS_TAR_URL = https://api.github.com/repos/mbauman/SparseArrays.jl/tarball/$1

test/reducedim.jl

@@ -207,23 +207,34 @@ end
     @test isequal(prod(A, dims=(1, 2)), fill(1, 1, 1))
     @test isequal(prod(A, dims=3), fill(1, 0, 1))
 
-    for f in (minimum, maximum)
+    for f in (minimum, maximum, findmin, findmax)
         @test_throws "reducing over an empty collection is not allowed" f(A, dims=1)
-        @test isequal(f(A, dims=2), zeros(Int, 0, 1))
+        @test_throws "reducing over an empty collection is not allowed" f(A, dims=2)
         @test_throws "reducing over an empty collection is not allowed" f(A, dims=(1, 2))
-        @test isequal(f(A, dims=3), zeros(Int, 0, 1))
-    end
-    for f in (findmin, findmax)
-        @test_throws ArgumentError f(A, dims=1)
-        @test isequal(f(A, dims=2), (zeros(Int, 0, 1), zeros(Int, 0, 1)))
-        @test_throws ArgumentError f(A, dims=(1, 2))
-        @test isequal(f(A, dims=3), (zeros(Int, 0, 1), zeros(Int, 0, 1)))
-        @test_throws ArgumentError f(abs2, A, dims=1)
-        @test isequal(f(abs2, A, dims=2), (zeros(Int, 0, 1), zeros(Int, 0, 1)))
-        @test_throws ArgumentError f(abs2, A, dims=(1, 2))
-        @test isequal(f(abs2, A, dims=3), (zeros(Int, 0, 1), zeros(Int, 0, 1)))
+        @test_throws "reducing over an empty collection is not allowed" f(A, dims=3)
+        if f === maximum
+            # maximum allows some empty reductions with abs/abs2
+            z = f(abs, A)
+            @test isequal(f(abs, A, dims=1), fill(z, (1,1)))
+            @test isequal(f(abs, A, dims=2), fill(z, (0,1)))
+            @test isequal(f(abs, A, dims=(1,2)), fill(z, (1,1)))
+            @test isequal(f(abs, A, dims=3), fill(z, (0,1)))
+            z = f(abs2, A)
+            @test isequal(f(abs2, A, dims=1), fill(z, (1,1)))
+            @test isequal(f(abs2, A, dims=2), fill(z, (0,1)))
+            @test isequal(f(abs2, A, dims=(1,2)), fill(z, (1,1)))
+            @test isequal(f(abs2, A, dims=3), fill(z, (0,1)))
+        else
+            @test_throws "reducing over an empty collection is not allowed" f(abs, A, dims=1)
+            @test_throws "reducing over an empty collection is not allowed" f(abs, A, dims=2)
+            @test_throws "reducing over an empty collection is not allowed" f(abs, A, dims=(1, 2))
+            @test_throws "reducing over an empty collection is not allowed" f(abs, A, dims=3)
+            @test_throws "reducing over an empty collection is not allowed" f(abs2, A, dims=1)
+            @test_throws "reducing over an empty collection is not allowed" f(abs2, A, dims=2)
+            @test_throws "reducing over an empty collection is not allowed" f(abs2, A, dims=(1, 2))
+            @test_throws "reducing over an empty collection is not allowed" f(abs2, A, dims=3)
+        end
     end
-
 end
 
 ## findmin/findmax/minimum/maximum
@@ -720,3 +731,38 @@ end
     @test_broken @inferred(maximum(exp, A; dims = 1))[1] === missing
     @test_broken @inferred(extrema(exp, A; dims = 1))[1] === (missing, missing)
 end
+
+some_exception(op) = try return (Some(op()), nothing); catch ex; return (nothing, ex); end
+reduced_shape(sz, dims) = ntuple(d -> d in dims ? 1 : sz[d], length(sz))
+
+@testset "Ensure that calling, e.g., sum(empty; dims) has the same behavior as sum(empty)" begin
+    @testset "$r(Array{$T}(undef, $sz); dims=$dims)" for
+        r in (minimum, maximum, findmin, findmax, extrema, sum, prod, all, any, count),
+        T in (Int, Union{Missing, Int}, Number, Union{Missing, Number}, Bool, Union{Missing, Bool}, Any),
+        sz in ((0,), (0,1), (1,0), (0,0), (0,0,1), (1,0,1)),
+        dims in (1, 2, 3, 4, (1,2), (1,3), (2,3,4), (1,2,3))
+
+        A = Array{T}(undef, sz)
+        rsz = reduced_shape(sz, dims)
+
+        v, ex = some_exception() do; r(A); end
+        if isnothing(v)
+            @test_throws typeof(ex) r(A; dims)
+        else
+            actual = fill(something(v), rsz)
+            @test isequal(r(A; dims), actual)
+            @test eltype(r(A; dims)) === eltype(actual)
+        end
+
+        for f in (identity, abs, abs2)
+            v, ex = some_exception() do; r(f, A); end
+            if isnothing(v)
+                @test_throws typeof(ex) r(f, A; dims)
+            else
+                actual = fill(something(v), rsz)
+                @test isequal(r(f, A; dims), actual)
+                @test eltype(r(f, A; dims)) === eltype(actual)
+            end
+        end
+    end
+end