silhouette(): fix empty clusters case

fixes #241

Author: alyst

src/silhouette.jl

@@ -82,8 +82,9 @@ function silhouettes(assignments::AbstractVector{<:Integer},
         a[j] = r[l, j]
 
         v = typemax(eltype(b))
-        for i = 1:k
-            @inbounds rij = r[i,j]
+        @inbounds for i = 1:k
+            counts[i] == 0 && continue # skip empty clusters
+            rij = r[i,j]
             if (i != l) && (rij < v)
                 v = rij
             end

test/silhouette.jl

@@ -1,5 +1,6 @@
 using Test
 using Clustering
+using Distances
 
 @testset "silhouettes()" begin
 
@@ -54,4 +55,10 @@ end
     @test_throws ArgumentError silhouettes(a, d)
 end
 
+@testset "empty clusters handled correctly (#241)" begin
+    X = rand(MersenneTwister(123), 10, 5)
+    pd = pairwise(Euclidean(), X, dims=1)
+    @test all(>=(-0.5), silhouettes([5, 2, 2, 3, 2, 2, 3, 2, 3, 5], pd))
+end
+
 end