Question

I’ve got a bidimensional array of objects (a 2D vector containing GridCell instances) as in here:

typedef vector<GridCell> CellArray;
typedef vector<CellArray> TwoDCellArray;
TwoDCellArray CellArr2D;

I am currently drawing all the cells like this:

for (int i = 0; i < rows; i++){
    for (int j = 0; j < cols; j++){
        CellArr2D[i][j].draw()
    }
}

However, I’ve got a depth problem and I should draw the instances depending on its size (size property, CellArr2D[i][j].size).

What should I do to sort this array without changing it’s i,j values? Copying all the objects to a secondary array and sort that one?
And more imporntant, hence this post… how can I sort an Array (Vector) by it’s contained object property?

Thanks in advance.

Answer 1

Create a vector of pairs (i,j) and sort it by size property.

typedef std::pair<int, int> int_pair_t;
typedef std::vector< int_pair_t > size_index_t;

namespace {
struct sort_helper {
    sort_helper( const TwoDCellArray& arr ) : arr_(arr) {}      
    bool operator()( const int_pair_t& ind1, const int_pair_t& ind2 ) { 
      return arr_[ind1.first][ind1.second].size > arr_[ind2.first][ind2.second].size; 
    }
private:
    const TwoDCellArray& arr_;
};

struct draw_helper {
    draw_helper( TwoDCellArray& arr ) : arr_(arr) {}        
    void operator()( const int_pair_t& ind ) { 
        arr_[ind.first][ind.second].draw();
    }
private:
    TwoDCellArray& arr_;
};
}

void some_func()
{
    // initialize helper array of indices
    size_index_t index;
    index.reserve( rows*cols );
    for ( int i = 0; i < rows*cols; ++i )
        index.push_back( make_pair( i/cols%rows, i%cols ) );

    // sort according to the `size` field
    std::sort( index.begin(), index.end(), sort_helper( CellArr2D ) );

    // draw
    std::for_each( index.begin(), index.end(), draw_helper( CellArr2D ) );
}