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
}
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>()
.