问题
In the Eigen documentation I see a lot of these:
Array (const Scalar &val0, const Scalar &val1)
Array (const Scalar &val0, const Scalar &val1, const Scalar &val2)
Array (const Scalar &val0, const Scalar &val1, const Scalar &val2, const Scalar &val3)
According to the documentation (http://eigen.tuxfamily.org/dox/classEigen_1_1Array.html) these constructors "constructs an initialized ND vector with given coefficients".
What does that mean? If I do Array(1,2,3), what is the result?
Specifically, what coefficients does it place in each dimension, and how large is the array in each of those respective dimensions? The constructor Array(1,2,3), according to the docs, should construct an 3D array, and initialise its contents with "the given coefficients". How should the result look?
回答1:
The constructors are for fixed sized Arrays. Assuming the declaration is Eigen::Array3i then the constructor you mentioned initializes a 1D int array with three elements initialized to the stated values.
回答2:
Those constructos allow you to create arrays of sizes up to 4:
Eigen::Array<int, 1, 4> a(1, 2, 3, 4)
Eigen will throw a compile error if you attempt to initialize like this with anything but an N(1-4) x 1 array. For example:
Eigen::Array<int, 1, 3> a(1, 2, 3) //Fine
Eigen::Array<int, 3, 1> a(1, 2, 3) //Fine
Eigen::Array<int, 3, 3> a(1, 2, 3) //Compile error
error: ‘THIS_METHOD_IS_ONLY_FOR_VECTORS_OF_A_SPECIFIC_SIZE
I would not be initializing them like this though. There is a tutorial on Eigen initialization here which provides good suggestions for how to initialize arrays for eg:
Eigen::Array33 a;
a << 1, 2, 3, 4 ...
Which has documentation here.
Or you can use special initializations like:
Eigen::ArrayXXf a = Eigen::ArrayXXf::Zero(1, 4) //0, 0, 0, 0
来源：https://stackoverflow.com/questions/24502837/what-do-the-initialized-array-constructors-do