Eigen defines several typedef shortcuts for most common 1D and 2D array types.
The general patterns are the following:
Cols can be
4 for fixed size square matrices or
X for dynamic size, and where
Type can be
i for integer,
f for float,
d for double,
cf for complex float,
cd for complex double.
Array33d is a fixed-size 3x3 array type of doubles, and
ArrayXXf is a dynamic-size matrix of floats.
There are also
ArraySizeType which are self-explanatory. For example,
Array4cf is a fixed-size 1D array of 4 complex floats.