using copy with multi-dimensional arrays

Question

I'm just trying to get more into stl semantics, and converting old loops over to algorithms where appropriate. I'm having trouble figuring out the best way to transform this loop into a call to copy. Any ideas?

    vector< vector<float> > rvec;
    const float * r[Max] = ...;

    // ...

    for (int ri=0; ri<N; ri++)
      for (int rj=0; rj<M; rj++)
        rvec[ri][rj] = r[ri][rj];

Answer 1

Not sure you can do this with only the standard algorithms and no functors (and with those, the code is bound to grow significantly than above).

You know, sometimes a plain loop is just best. STL's algorithms are very nice, but since C++ doesn't have anonymous functions or built in lambdas (yet), taking perfectly readable code such as you show above and converting it to STL algorithms is more of an intellectual exercise than actual improvement,

Answer 2

rvec.resize(Max);
for (int i = 0; i < Max; ++i) {
  rvec[i].resize(M);
  std::copy(r[i], r[i] + M, rvec[i].begin());
}

If rvec itself and each vector in rvec already has the correct size, then resizing isn't needed.

Answer 3

In this case just leaving the code as is isn't so bad. Though if you wrote it multiple times abstracting it into a separate function would be a good idea. Another point to consider, any solution that doesn't reserve or resize will waste time copying where you don't need to.

Answer 4

Here is an example I made a bit rapidly with iterators...

#include <algorithm>
using std::copy;
using std::for_each;
using std::random_shuffle;

#include <iostream>
using std::cout;
using std::endl;

#include <vector>
using std::iterator;
using std::vector;

void Write(int i)
{
    cout << i << endl;
}

int main()
{
    const short MAX_LIST = 1000,
       MAX_NUMBER = 100;
    float number = 0;
    vector<vector<float> > v (MAX_LIST),
            v2 (MAX_LIST);
    vector<float> list(MAX_NUMBER);

    //Fills list and fills v with list
    for(vector<vector<float> >::iterator v_iter = v.begin(); v_iter != v.end(); ++v_iter)
    {
     for(vector<float>::iterator list_iter = list.begin(); list_iter != list.end(); ++list_iter)
     {
      *list_iter = number;
      ++number;
     }
     *v_iter = list;
    }

    //write v to console
    for(vector<vector<float> >::iterator v_iter = v.begin(); v_iter != v.end(); ++v_iter)
    {
     for_each(v_iter->begin(), v_iter->end(), Write);
    }

    //copy v to v2
    cout << "Copying..." << endl;
    copy(v.begin(), v.end(), v2.begin());
    cout << "Finished copying!" << endl;

    cout<< "Shuffling..." << endl;
    random_shuffle(v2.begin(), v2.end());
    cout << "Finished shuffling!" << endl;

    //write v2 to console
    for(vector<vector<float> >::iterator v_iter = v2.begin(); v_iter != v2.end(); ++v_iter)
    {
     for_each(v_iter->begin(), v_iter->end(), Write);
    }
}