Instantiating a std::vector of boost::multi_array.. melting brain for cpp guru?

Question

I think I'm confused with instantiating objects.. well.. not properly object because new statements make a compile error. My background is in Python and Java and I'm stuck in front of C++ way of creating objects that aren't classes.

I'm translating an algorithm from C# and for machine learning and it uses an array of multidimensional arrays.

C# code:

public double Learn(int[][] observations, int symbols, int states, ...

    // ...

    double[][, ,] epsilon = new double[N][, ,]; // also referred as ksi or psi
    double[][,] gamma = new double[N][,];

    for (int i = 0; i < N; i++)
    {
        int T = observations[i].Length;
        epsilon[i] = new double[T, States, States];
        gamma[i] = new double[T, States];
    }

I've decided to use the Boost library for the multidimensional array, and I have:

typedef boost::multi_array<double, 2> matrix2D;
typedef boost::multi_array<double, 3> matrix3D;

vector<matrix3D> epsilon;
vector<matrix2D> gamma;

cout << "HMM::learn >> before" << endl;
for (int i = 0; i < N; i++)
{
    int T = observations[i].size();
    epsilon[i] = matrix3D(boost::extents[T][states][symbols]);
    gamma[i] = matrix2D(boost::extents[T][states]);
}

and I get this runtime error:

HMM::learn >> before
std::bad_alloc' what(): std::bad_alloc

Answer 1

The vectors have no space allocated (well it may be reserved already but you can't reference it with the array indexers). Change the lines:

epsilon[i] = matrix3D(boost::extents[T][states][symbols]);
gamma[i] = matrix2D(boost::extents[T][states]);

To:

epsilon.push_back(matrix3D(boost::extents[T][states][symbols]);
gamma.push_back(matrix2D(boost::extents[T][states]);

that should solve it. In your case since you know the array size you should reserve that much space in the vectors so that it reduces the reallocations needed:

epsilon.reserve(N);
gamma.reserve(N);