Development build. This page describes main, not a released package. The latest published Lodestar.Abstractions is 0.2.0 — read its documentation.

Home › Abstractions › The shared sparse primitive

CsrMatrix.ToDense

The same matrix with its zeros written out, for inspection.

public double[,] ToDense()

Returns — double[,] of RowCount × ColumnCount, every cell present, the stored values in their places and zeros everywhere else.

Exceptions — InvalidOperationException when the matrix has more cells than a two-dimensional array holds — uint.MaxValue in all, the runtime's largest array along one side — rather than failing inside the allocation. A 50,000-square matrix is within that; it still needs 20 GB (#1412).

Example — the cell that says the appears twice in the third document.

using Lodestar.Abstractions;
using Lodestar.Text.Vectorization;

string[] docs = ["the cat eats", "the dog eats", "the cat and the dog"];
var cv = new CountVectorizer();
CsrMatrix counts = cv.FitTransform(docs);

// Features are sorted: and, cat, dog, eats, the — so "the" is column 4.
double[,] dense = counts.ToDense();
double theInThird = dense[2, 4];  // => 2

Remarks — allocates RowCount × ColumnCount doubles, which is exactly the array the sparse layout exists to avoid. On the three-document corpus above that is fifteen cells; on a real corpus it is the number that made sparsity necessary in the first place. Reach for it to look at a small matrix, to hand a small one to something that wants a rectangular array, or in a test — not as a step in a pipeline.

To read one cell without materialising the rest, walk RowPointers and ColumnIndices directly; to reduce the whole matrix against a vector, Multiply does it without densifying.

A column stored twice in one row is written out as the sum of its entries, which is what Multiply computes from the same row and what scipy's toarray() returns.

A stored NaN or infinity is written out as it is, not refused, as toarray() writes it.

Before .NET 6 the cap can be lower. .NET Framework without gcAllowVeryLargeObjects refuses any array past 2 GB, so a result within this bound but past that fails there with the runtime's own out-of-memory error, raised before any memory is taken. Otherwise a double[,] before .NET 6 is capped at UInt32.MaxValue elements in all and 0x7FEFFFFF along one side, below .NET 6's 0x7FFFFFC7: the check reads the running runtime's bound, so a side between the two is refused there too (#1614).

Applies to — net10.0, netstandard2.0.

See also — CsrMatrix, CsrMatrix.Multiply.