How do I define unary functioon used in the artificial neural network C++ class

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C++

artificial-intelligence

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What is the definition of the unary function in the following procedure?

void train(NeuralNetwork& net)
{
    cout << "Training:" << endl; RowVector input(3), output(3);
    int stop = 0;
    for (int i = 0; stop < 8 && i < 50000; i++)
    {
        cout << i + 1 << endl;
        for (int num = 0; stop < 8 && num < 8; num++)
        {
            input.coeffRef(0) = (num >> 2) & 1;
            input.coeffRef(1) = (num >> 1) & 1;
            input.coeffRef(2) = num & 1;
            output.coeffRef(0) = ((num + 1) >> 2) & 1;
            output.coeffRef(1) = ((num + 1) >> 1) & 1;
            output.coeffRef(2) = (num + 1) & 1;
            net.train(input, output);
            double mse = net.mse();
            cout << "In [" << input << "] " << " Desired [" << output << "] " << " Out [" << net.mNeurons.back()->unaryExpr(ptr_fun(unary)) << "] " << " MSE [" << mse << "]" << endl;
            stop = mse < 0.1 ? stop + 1 : 0;
        }
    }
}

What I have tried:

I searched your source code for unary definition.
I searched the Eigen library for unary function definition.
I searched for unary function on the Web.
I defined my own arbitrary unary function to get the example to compile.