Handle empty matrix

Author: YingboMa

src/coloring/matrix2graph.jl

@@ -5,7 +5,7 @@ Returns a vector of rows where each row contains
 a vector of its column indices.
 """
 function _cols_by_rows(rows_index,cols_index)
-    nrows = maximum(rows_index)
+    nrows = isempty(rows_index) ? 0 : maximum(rows_index)
     cols_by_rows = [eltype(rows_index)[] for _ in 1:nrows]
     for (i,j) in zip(rows_index,cols_index)
         push!(cols_by_rows[i],j)

test/test_specialmatrices.jl

@@ -1,7 +1,9 @@
 using SparseDiffTools
+using SparseArrays
 using LinearAlgebra
 using BandedMatrices
 using BlockBandedMatrices
+using Test
 
 n=10
 dense=fill(1.,(n,n))
@@ -13,6 +15,7 @@ bidiagonalU=Bidiagonal(dense,:U)
 bidiagonalL=Bidiagonal(dense,:L)
 tridiagonal=Tridiagonal(dense)
 symtridiagonal=SymTridiagonal(dense)
+emptymat=sparse(Int[], Int[], true, 1, 1)
 
 banded=BandedMatrix(dense,(1,2))
 blockbanded1=BlockBandedMatrix(dense,[1,2,3,4],[4,3,2,1],(1,0))
@@ -60,3 +63,4 @@ _testvalidity(blockbanded1)
 _testvalidity(blockbanded2)
 _testvalidity(bandedblockbanded1)
 _testvalidity(bandedblockbanded2)
+_testvalidity(emptymat)