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MathNetInterop.ToSparseMatrix

Converts a CsrMatrix into a Math.NET SparseMatrix.

public static SparseMatrix ToSparseMatrix(CsrMatrix matrix)

Parameters — matrix is the matrix to convert.

Returns — a SparseMatrix of the same shape holding the same values, sharing no array with the source.

Exceptions — ArgumentNullException when matrix is null. ArgumentException when it is not a valid CSR matrix — built through CsrMatrix.CreateUnchecked, or its arrays changed after construction — which Math.NET would answer with an error of its own base type, or accept with a column outside the matrix (#1549).

Example — a row whose columns arrive out of order still reads correctly.

using Lodestar.Abstractions;
using Lodestar.Extensions.MathNet;
using MathNet.Numerics.LinearAlgebra.Double;

var unsorted = new CsrMatrix(
    rowCount: 1,
    columnCount: 5,
    values: [9.0, 7.0, 8.0],
    columnIndices: [4, 0, 2],
    rowPointers: [0, 3]);

SparseMatrix sparse = MathNetInterop.ToSparseMatrix(unsorted);

double first = sparse[0, 0];    // => 7
double middle = sparse[0, 2];   // => 8
double last = sparse[0, 4];     // => 9
double gap = sparse[0, 3];      // => 0

Remarks — the rows are sorted, and duplicate columns are added together, because CsrMatrix never promised an order and Math.NET reaches a cell by searching the row. The example above is exactly that case, and the tests pin it (#1402). A matrix whose rows are already strictly increasing — what every vectorizer here produces — is copied into Math.NET's storage as it is; any other is sorted here first, a repeated column summed in stored order as CsrMatrix.ToDense sums it. Math.NET's compressed-row factory is not used: its sort is unstable past 16 entries in a row, so a row holding 1e16, 1, -1e16 in one column could read 1 where ToDense reads 0, and it would sort again what is already sorted (#1548).

Explicit zeros are carried across rather than dropped — Math.NET's own compressed-row storage counts "stored values including explicit zeros", so a matrix with sorted rows and no repeated column keeps its structure unchanged; an unsorted row comes back sorted, and a repeated column as one stored sum, zero if the two cancel.

Applies to — net10.0, netstandard2.0.

See also — MathNetInterop, MathNetInterop.ToCsrMatrix.