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Clarify documentation on the restrictions of writable sparse block expressions.

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Gael Guennebaud
2016-02-03 16:08:43 +01:00
parent e6fd3fa177
commit cc26185d91
2 changed files with 58 additions and 28 deletions
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doc/TutorialSparse.dox
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@@ -241,11 +241,11 @@ In the following \em sm denotes a sparse matrix, \em sv a sparse vector, \em dm
sm1.real() sm1.imag() -sm1 0.5*sm1
sm1+sm2 sm1-sm2 sm1.cwiseProduct(sm2)
\endcode
However, a strong restriction is that the storage orders must match. For instance, in the following example:
However, <strong>a strong restriction is that the storage orders must match</strong>. For instance, in the following example:
\code
sm4 = sm1 + sm2 + sm3;
\endcode
sm1, sm2, and sm3 must all be row-major or all column major.
sm1, sm2, and sm3 must all be row-major or all column-major.
On the other hand, there is no restriction on the target matrix sm4.
For instance, this means that for computing \f$ A^T + A \f$, the matrix \f$ A^T \f$ must be evaluated into a temporary matrix of compatible storage order:
\code
@@ -307,6 +307,26 @@ sm2 = sm1.transpose() * P;
\endcode
\subsection TutorialSparse_SubMatrices Block operations
Regarding read-access, sparse matrices expose the same API than for dense matrices to access to sub-matrices such as blocks, columns, and rows. See \ref TutorialBlockOperations for a detailed introduction.
However, for performance reasons, writing to a sub-sparse-matrix is much more limited, and currently only contiguous sets of columns (resp. rows) of a column-major (resp. row-major) SparseMatrix are writable. Moreover, this information has to be known at compile-time, leaving out methods such as <tt>block(...)</tt> and <tt>corner*(...)</tt>. The available API for write-access to a SparseMatrix are summarized below:
\code
SparseMatrix<double,ColMajor> sm1;
sm1.col(j) = ...;
sm1.leftCols(ncols) = ...;
sm1.middleCols(j,ncols) = ...;
sm1.rightCols(ncols) = ...;
SparseMatrix<double,RowMajor> sm2;
sm2.row(i) = ...;
sm2.topRows(nrows) = ...;
sm2.middleRows(i,nrows) = ...;
sm2.bottomRows(nrows) = ...;
\endcode
In addition, sparse matrices expose the SparseMatrixBase::innerVector() and SparseMatrixBase::innerVectors() methods, which are aliases to the col/middleCols methods for a column-major storage, and to the row/middleRows methods for a row-major storage.
\subsection TutorialSparse_TriangularSelfadjoint Triangular and selfadjoint views
Just as with dense matrices, the triangularView() function can be used to address a triangular part of the matrix, and perform triangular solves with a dense right hand side: