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#include"../src/All.h"
using namespace std;
using namespace Eigen;
int main(int, char **)
{
Matrix<double,2,2> m; // 2x2 fixed-size matrix with uninitialized entries
m(0,0) = 1;
m(0,1) = 2;
m(1,0) = 3;
m(1,1) = 4;
cout << "Here is a 2x2 matrix m:" << endl << m << endl;
cout << "Let us now build a 4x4 matrix m2 by assembling together four 2x2 blocks." << endl;
MatrixX<double> m2(4, 4); // dynamic matrix with initial size 4x4 and uninitialized entries
// notice how we are mixing fixed-size and dynamic-size types.
cout << "In the top-left block, we put the matrix m shown above." << endl;
m2.block(0,1,0,1) = m;
cout << "In the bottom-left block, we put the matrix m*m, which is:" << endl << m*m << endl;
m2.block(2,3,0,1) = m * m;
cout << "In the top-right block, we put the matrix m+m, which is:" << endl << m+m << endl;
m2.block(0,1,2,3) = m + m;
cout << "In the bottom-right block, we put the matrix m-m, which is:" << endl << m-m << endl;
m2.block(2,3,2,3) = m - m;
cout << "Now the 4x4 matrix m2 is:" << endl << m2 << endl;
cout << "The central 2x2 block of m2 is:" << endl << m2.block(1,2,1,2) << endl;
cout << "Row 0 of m2, written as a column vector, is:" << endl << m2.row(0) << endl;
cout << "Column 1 of m2 is:" << endl << m2.col(1) << endl;
cout << "The matrix m2 with row 0 and column 1 removed is:" << endl << m2.minor(0,1) << endl;
cout << endl << "Now let us study a tricky issue." << endl;
cout << "Recall that the matrix product m*m is:" << endl << m*m << endl;
cout << "We want to store that into m, i.e. do \"m = m * m;\"" << endl;
cout << "Here we must be very careful. For if we do \"m = m * m;\"," << endl
<< "the matrix m becomes" << endl;
Matrix<double,2,2> m_save = m;
m = m * m; // the bogus operation
cout << m << "," << endl;
cout << "which is not what was wanted!" << endl
<< "Explanation: because of the way expression templates work, the matrix m gets" << endl
<< "overwritten _while_ the matrix product m * m is being computed." << endl
<< "This is the counterpart of eliminating temporary objects!" << endl
<< "Anyway, if you want to store m * m into m, you can do this:" << endl
<< " m.alias() = m * m;" << endl;
m = m_save;
m.alias() = m * m;
cout << "And m is now:" << endl << m << endl << "as was expected." << endl;
return 0;
}
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