Question

Why do I get different output? How can I fix this? I want the trainingVector[0] to reference A.

vector<double> A(4,0);
vector<vector<double > > trainingVector;
A[0]=1;
trainingVector.push_back(A); 
A[0]=2;
cout << A[0] << endl ;
cout << trainingVector[0][0] << endl ;

Answer 1

You cannot store references in STD containers, so what you ask for is impossible. If you want trainingVector to store a pointer to A, that’s entirely doable:

vector<double> A(4,0);
vector<vector<double>*> trainingVector;

A[0] = 1;
trainingVector.push_back(&A);
A[0] = 2;

// notice that you have to dereference trainingVector[0] to get to A
cout << (*trainingVector[0])[0] << endl; // prints 2