The matrix class

This represents a matrix of arbitrary size (hence the X in MatrixXd), in which every entry is a double (hence the d in MatrixXd).
Matrix < typename Scalar, int RowsAtCompileTime, int ColsAtCompileTime >
typedef Matrix < float, 3, 1 > Vector3f;
typedef Matrix < double, Dynamic, Dynamic > MatrixXd;
All Eigen matrices default to column-major storage order.
rows(), cols() and size().
use fixed sizes for very small sizes where you can, and use dynamic sizes for larger sizes or where you have to.

Matrix and vector arithmetric

Therefore, the instruction a = a.transpose() does not replace a with its transpose(except multiplication)
the dot() and cross() methods.

The Array class and coefficient-wise operations

The Array class provides general-purpose arrays.
Furthermore, the Array class provides an easy way to perform coefficient-wise operations.
Array < typename Scalar, int RowsAtCompileTime, int ColsAtCompileTime >
For element-wise product?

Converting between array and matrix expressions

Matrix expressions have an .array() method that ‘converts’ them into array expressions.
Array expressions have a .matrix() method
Array: coefficient wise

Block operations

Individual columns and rows are special cases of blocks. Eigen provides methods to easily address them: .col() and .row().

Advanced initialization

The finished() method is necessary here to get the actual matrix object once the comma initialization of our temporary submatrix is done.

Reductions, visitors and broadcasting

Norm computations?
Partial reductions are applied with colwise() or rowwise() .
The concept behind broadcasting is similar to partial reduction.

Interfacing with raw buffers: the Map class

You can use a Map object just like any other Eigen type:

Reshape and Slicing


Aliasing occurs more naturally when trying to shrink a matrix
a = a.transpose(); // !!! do NOT do this !!!
mat.bottomRightCorner(2,2) = mat.topLeftCorner(2,2).eval();
Eigen provides the special-purpose function transposeInPlace() which replaces a matrix by its transpose.
If an xxxInPlace() function is available, then it is best to use it, because it indicates more clearly what you are doing.
Thus, if matA is a squared matrix, then the statement matA = matA * matA;
Aliasing occurs when the same matrix or array coefficients appear both on the left- and the right-hand side of an assignment operator.

Storage orders

If the storage order is not specified, then Eigen defaults to storing the entry in column-major.

Dense linear problems and decompositions

Linear algebra and decompositions


Sparse matrix manipulations