本征：累积可变大小的数组

Question

I have a class holding an Eigen::Array of data and a method that adds new data (number of rows may vary) by appending to the array along the first axis. I solved the accumulation by creating a new Array of the right size and initializing it with the old and the new data.

typedef Eigen::Array<double, Eigen::Dynamic, 3> DataArray

class Accumulator {
    void add(DataArray &new_data) {
        DataArray accu(accumulated_data_.rows() + new_data.rows(), 3)
        accu << accumulated_data_, new_data;
        accumulated_data_ = accu;
    }

    DataArray accumulated_data_;
}

Is there anything wrong with doing it like this? Or is it preferred to resize the accumulated data array:

• .resize() and copy in both old and new
• or .conservative_resize() and copy in the new data (requires block operations if new data is longer than 1 row)

Answer 1

First of all, two easy-to-fix flaws with your current implementation:

• Eigen stores Arrays (and Matrices) in column-major order by default, so if you are appending rows, you should prefer the RowMajor storage order:

  Eigen::Array<double, Eigen::Dynamic, 3, Eigen::RowMajor>

• Since accu will not be used anymore, you should move it to the accumulator: accumulated_data_ = std::move(accu);

  If you are pre-C++11, you can also swap the data:

  accumulated_data_.swap(accu);

Then your approach is nearly equivalent to

accumulated_data_.conservativeResize(accumulated_data_.rows() + new_data.rows(), 3);
accumulated_data_.bottomRows(new_data.rows()) = new_data;

You will still have memory (re-)allocations and memory-copies at every call.

A more efficient approach would be to resize the accumulated_data_ only occasionally (ideally just once at the beginning), and keep track of how much of it is currently actually valid:

typedef Eigen::Array<double, Eigen::Dynamic, 3, Eigen::RowMajor> DataArray;

class Accumulator {
public:
    Accumulator(Eigen::Index initialCapacity=10000) : accumulated_data_(initialCapacity, 3), actual_rows_(0) {}
    void add(DataArray &new_data) {
        if(actual_rows_+new_data.rows() > accumulated_data_.rows())
        { // TODO adapt memory-growing to your use case
             accumulated_data_.conservativeResize(2*actual_rows_+new_data.rows(), 3);
        }
        accumulated_data_.midRows(actual_rows, new_data.rows()) = new_data;
        actual_rows_+=new_data.rows();
    }

    DataArray accumulated_data_;
    Eigen::Index actual_rows_;
};