Matrix from std vector to vector from Eigen/Dense

Question

I try to read out elements from a 3D matrix created using the std c++ dynamic container for vectors. Below is how I initialize my matrix:

typedef vector<vector<vector<ClassA> > > matrix3D;

In my class named "ClassA", I have the following public members:

double a, b, c;

Then in my main file, I fill in the matrix with:

double varA=M_PI; double varB=varA; double varC=varA;

matrix3D[i][j][k].a = varA;

matrix3D[i][j][k].b = varB;

matrix3D[i][j][k].c = varC;

Now when I read the doubles into a vector created using Eigen/Dense library, the type of the vector becomes a matrix:

    Vector3d vectorEigen;
    vectorEigen << matrix3D[i][j][k].a, matrix3D[i][j][k].b, matrix3D[i][j][k].c;

and vectorEigen becomes a variable of the type Eigen::Matrix<double, 3,1,0,3,1>

Does anybody have a clue what I have missed here?

C++ is a statically typed language. A variable won't "become" a different type. — Weak to Enuma Elish
I presume the question about the type of vectorEigen is not the real issue anymore. Maybe you're wrestling with some other issue? Actually, don't you need vectorEigen << matrix3D[i][j][k].a << matrix3D[i][j][k].b << matrix3D[i][j][k].c; instead of vectorEigen << matrix3D[i][j][k].a, matrix3D[i][j][k].b, matrix3D[i][j][k].c; — WhiteViking
vectorEigen << matrix3D[i][j][k].a << matrix3D[i][j][k].b << matrix3D[i][j][k].c; is giving me an error, unfortunately :( The problem occurs when I call the function which takes a Vector3d as input parameter. Substituting vectorEigen as input parameter of the function causes the compiler to complain. And the reason is the function with an input of the type Eigen::Matrix<double, 3,1,0,3,1> is undefined. — bull

WhiteViking WhiteViking · Accepted Answer · 2015-11-06T08:53:49

Internally Eigen represents vectors as matrices with only one column. So vectors (just like "ordinary" matrices) are really instances of an Eigen::Matrix template class.

For simplicity towards the programmer however, Eigen uses C++'s typedef to define vector classes that are synonyms for an Eigen::Matrix<> with specific options. For example, the Vector3d type in Eigen is a typedef for a matrix whose elements are doubles and that has 3 rows and 1 column:

typedef Matrix<double, 3, 1> Vector3d

The Matrix template class actually has 6 template arguments, the last 3 ones being default arguments. Here is the full signature:

template<typename _Scalar, int _Rows, int _Cols, int _Options, int _MaxRows, int _MaxCols>
class Eigen::Matrix< _Scalar, _Rows, _Cols, _Options, _MaxRows, _MaxCols >

If the compiler refers to Eigen::Matrix<double, 3,1,0,3,1> in error messages it's talking about an Eigen::Matrix with the following template parameters:

_Scalar= double
_Rows = 3
_Cols = 1
_Options = 0 (by default)
_MaxRows = _Rows (by default) = 3
_MaxCols = _Cols (by default) = 1

So Eigen::Matrix<double, 3,1,0,3,1> is just the full type of Vector3d that the compiler sees after resolving typedef and template arguments.

The type hasn't changed at all, you just use the Vector3d shorthand notation in your code, whereas the compiler refers to it by its explicit type.

Matrix from std vector to vector from Eigen/Dense

2 Answers