Function that takes VectorXf and can modify its values

Question

I am trying to understand how to manipulate Eigen Vector/Matrix. I would like to implement a least-square gauss-newton algorithm (hence why I am learning to use the Eigen library). I have a 1x6 vectors of parameters that I need to update each iteration. Right now, I just want to figure out how a function can take a vector as argument and change its values...

Eigen::VectorXf betas = Eigen::VectorXf::Zero(6);

void SomeFunc(const Eigen::VectorXf& v){  // as per the Eigen guide, one must pass as const
    v(0) = 5;  // error: expression must be a modifiable lvalue
    return;
}

int main()
{
    betas(5) = 5.f;  // works
    SomeFunc(&betas);
    std::cout << "Hello World" << std::endl;
    std::cout << betas(0) << "\t" << betas(5) << std::endl;
}

My question is: How would you make a function take a vector and modify its values?

Edit: Eigen guide

Answer 1

As the documentation says:

Functions taking writable (non-const) parameters must take const references and cast away constness within the function body.

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How would you make a function take a vector and modify its values?

You can use const_cast as shown below:

Eigen::VectorXf betas = Eigen::VectorXf::Zero(6);

void SomeFunc(const Eigen::VectorXf& v){  // as per the Eigen guide, one must pass as const
    const_cast<Eigen::VectorXf&>(v)(0) = 5;//cons_cast used here
    return;
}

int main()
{
    betas(5) = 5.f;  // works
    SomeFunc(betas);
    std::cout << "Hello World" << std::endl;
    std::cout << betas(0) << "\t" << betas(5) << std::endl;
}