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50 lines
2.0 KiB
50 lines
2.0 KiB
Eigen
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=====
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`Eigen <http://eigen.tuxfamily.org>`_ is C++ header-based library for dense and
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sparse linear algebra. Due to its popularity and widespread adoption, pybind11
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provides transparent conversion support between Eigen and Scientific Python linear
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algebra data types.
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Specifically, when including the optional header file :file:`pybind11/eigen.h`,
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pybind11 will automatically and transparently convert
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1. Static and dynamic Eigen dense vectors and matrices to instances of
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``numpy.ndarray`` (and vice versa).
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2. Returned matrix expressions such as blocks (including columns or rows) and
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diagonals will be converted to ``numpy.ndarray`` of the expression
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values.
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3. Returned matrix-like objects such as Eigen::DiagonalMatrix or
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Eigen::SelfAdjointView will be converted to ``numpy.ndarray`` containing the
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expressed value.
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4. Eigen sparse vectors and matrices to instances of
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``scipy.sparse.csr_matrix``/``scipy.sparse.csc_matrix`` (and vice versa).
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This makes it possible to bind most kinds of functions that rely on these types.
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One major caveat are functions that take Eigen matrices *by reference* and modify
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them somehow, in which case the information won't be propagated to the caller.
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.. code-block:: cpp
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/* The Python bindings of these functions won't replicate
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the intended effect of modifying the function arguments */
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void scale_by_2(Eigen::Vector3f &v) {
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v *= 2;
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}
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void scale_by_2(Eigen::Ref<Eigen::MatrixXd> &v) {
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v *= 2;
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}
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To see why this is, refer to the section on :ref:`opaque` (although that
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section specifically covers STL data types, the underlying issue is the same).
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The :ref:`numpy` sections discuss an efficient alternative for exposing the
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underlying native Eigen types as opaque objects in a way that still integrates
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with NumPy and SciPy.
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.. seealso::
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The file :file:`tests/test_eigen.cpp` contains a complete example that
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shows how to pass Eigen sparse and dense data types in more detail.
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