Mapping array back to an existing Eigen matrix

问题

I want to map an array of double to an existing MatrixXd structure. So far I've managed to map the Eigen matrix to a simple array, but I can't find the way to do it back.

void foo(MatrixXd matrix, int n){

 double arrayd = new double[n*n];
 // map the input matrix to an array
 Map<MatrixXd>(arrayd, n, n) = matrix;  

  //do something with the array 
             .......
// map array back to the existing matrix

}

回答1:

I'm not sure what you want, but I'll try to explain.

You're mixing double and float in your code (a MatrixXf is a matrix where every entry is a float). I'll assume for the moment that this was unintentional amd that you want to use double everywhere; see below for if this was really your intention.

The instruction Map<MatrixXd>(arrayd, n, n) = matrix copies the entries of matrix into arrayd. It is equivalent to the loop

for (int i = 0; i < n; ++i)
   for (int j = 0; j < n; ++j)
      arrayd[i + j*n] = matrix(i, j);

To copy the entries of arrayd into matrix, you would use the inverse assignment: matrix = Map<MatrixXd>(arrayd, n, n).

However, usually the following technique is more useful:

void foo(MatrixXd matrix, int n) {
   double* arrayd = matrix.data();
   // do something with the array 
}

Now arrayd points to the entries in the matrix and you can process it as any C++ array. The data is shared between matrix and arrayd, so you do not have to copy anything back at the end. Incidentally, you do not need to pass n to the function foo(), because it is stored in the matrix; use matrix.rows() and matrix.cols() to query its value.

If you do want to copy a MatrixXf to an array of doubles, then you need to include the cast explicitly. The syntax in Eigen for this is: Map<MatrixXd>(arrayd, n, n) = matrix.cast<double>() .

回答2:

You do not need to do any reverse operation.

When using Eigen::Map you are mapping a raw array to an Eigen class. This means that you can now read or write it using Eighen functions.

In case that you modify the mapped array the changes are already there. You can simply access the original array.

float buffer[16]; //a raw array of float

//let's map the array using an Eigen matrix
Eigen::Map<Eigen::Matrix4f> eigenMatrix(buffer); 

//do something on the matrix
eigenMatrix = Eigen::Matrix4f::Identity();


//now buffer will contain the following values
//buffer = [1 0 0 0  0 1 0 0  0 0 1 0  0 0 0 1]

来源：https://stackoverflow.com/questions/12003692/mapping-array-back-to-an-existing-eigen-matrix