Respect IOContext while displaying a SparseMatrixCSC (#423)

* Respect IOContext in printing column indices

* Add docstring to ColumnIndices

Author: jishnub

src/sparsematrix.jl

@@ -344,6 +344,26 @@ function Base.print_array(io::IO, S::AbstractSparseMatrixCSCInclAdjointAndTransp
     end
 end
 
+"""
+    ColumnIndices(S::AbstractSparseMatrixCSC)
+
+Return the column indices of the stored values in `S`.
+This is an internal type that is used in displaying sparse matrices,
+and is not a part of the public interface.
+"""
+struct ColumnIndices{Ti,S<:AbstractSparseMatrixCSC{<:Any,Ti}} <: AbstractVector{Ti}
+    arr :: S
+end
+
+size(C::ColumnIndices) = (nnz(C.arr),)
+# returns the column index of the n-th non-zero value from the column pointer
+@inline function getindex(C::ColumnIndices, i::Int)
+    @boundscheck checkbounds(C, i)
+    colptr = getcolptr(C.arr)
+    ind = searchsortedlast(colptr, i)
+    eltype(C)(ind)
+end
+
 # always show matrices as `sparse(I, J, K)`
 function Base.show(io::IO, _S::AbstractSparseMatrixCSCInclAdjointAndTranspose)
     _checkbuffers(_S)
@@ -358,21 +378,7 @@ function Base.show(io::IO, _S::AbstractSparseMatrixCSCInclAdjointAndTranspose)
         print(io, "transpose(")
     end
     print(io, "sparse(", I, ", ")
-    if length(I) == 0
-        print(io, eltype(getcolptr(S)), "[]")
-    else
-        print(io, "[")
-        il = nnz(S) - 1
-        for col in 1:size(S, 2),
-            k in getcolptr(S)[col] : (getcolptr(S)[col+1]-1)
-            print(io, col)
-            if il > 0
-                print(io, ", ")
-                il -= 1
-            end
-        end
-        print(io, "]")
-    end
+    show(io, ColumnIndices(S))
     print(io, ", ", K, ", ", m, ", ", n, ")")
     if _S isa Adjoint || _S isa Transpose
         print(io, ")")

test/sparsematrix_constructors_indexing.jl

@@ -1574,6 +1574,15 @@ end
     _show_with_braille_patterns(ioc, _filled_sparse(8, 16))
     @test String(take!(io)) == "⎡⣿⣿⎤\n" *
                                "⎣⣿⣿⎦"
+
+    # respect IOContext while displaying J
+    I, J, V = shuffle(1:50), shuffle(1:50), [1:50;]
+    S = sparse(I, J, V)
+    I, J, V = I[sortperm(J)], sort(J), V[sortperm(J)]
+    @test repr(S) == "sparse($I, $J, $V, $(size(S,1)), $(size(S,2)))"
+    limctxt(x) = repr(x, context=:limit=>true)
+    expstr = "sparse($(limctxt(I)), $(limctxt(J)), $(limctxt(V)), $(size(S,1)), $(size(S,2)))"
+    @test limctxt(S) == expstr
 end
 
 @testset "issparse for specialized matrix types" begin