fill structure while finding min_element, C++

问题:

I want to fill some structure while finding minimum element. Pl find the code below

tyoedef struct Point
{
    double x, y;
}Point;

I have a vector of points - std::vector<Point> V in which i have few thousand points.

There is another struct I have

typedef struct cart
{
    Point pt;
    double val_1; // computed using only Pt
    double val_2; // computer using only Pt
}cart;

Now I have two tasks:

1. I need to find minimum element from structure V.

2. Fill the structure cart, which is directly dependent on V.

   I can do this using following code.

   std::vector<cart> vCart;
   for(unsigned i = 0; i < V.size(); ++i)
   {
       cart thsElement;
       thsElement.pt = V[i];
       thsElement.val_1 = compute_val_1(V[i]);
       thsElement.val_2 = compute_val_2(V[i]);
       vCart.push_back(thsElement)
   }
   
   auto it = std::min_element(vCart.begin(), vCart.end(), lex_sort);
   
   bool lex_sort(cart const &a, cart const &b)
   {
       if(a.pt.x < b.pt.x) return true;
       if(a.pt.x == b.pt.x) return (a.pt.y < b.pt.y);
   }
   

Now there is an evident problem with this implementation.

There are two loops. One for filling the structure and other for finding the min element (std::min_element() has to have a loop to iterate over all the values). I am fighting for few miliseconds' improvement. So this is not a good code. Moreover, this seems so C_style

So I came up with following code.

    std::vector<cart> vCart;
    std::iterator <vCart> st_ite;

    auto it = std::min_element(V.begin(), V.end(), boost::bind(FillStruct_LexSort, st_ite, _1, _2)); // V is a vector of Point

    bool FillStruct_LexSort(std::insert_iterator< std::vector<Cart>> vcpInput, const Point &a, const Point &b)
    {
        Cart thsPt;
        if(a.x() < b.x())
        {
            thsPt.pt = b;
            thsPt.val_1 = compute_val_1(b);
            thsPt.val_2 = compute_val_2(b);
            (*vcpInput++) = (thsPt);
            return true;
        }
        if (a.x() == b.x())
        {
            if(a.y() < b.y())
            {
                thsPt.pt = b;
                thsPt.val_1 = compute_val_1(b);
                thsPt.val_2 = compute_val_2(b);

                (*vcpInput++) = (thsPt);
                return true;
            }
        }

        thsPt.pt = a;
        thsPt.val_1 = compute_val_1(b);
        thsPt.val_2 = compute_val_2(b);

        (*vcpInput++) = (thsPt);
        return false;
    }

Now, the problem is - I get segmentation fault. I do not know how should I use iterator to insert a value. I tried passing reference to vCart, but vCart is empty after calling min_element(..). I even tried insert_iterator, but with no success.

So pl suggest.

回答1:

It seems you want something like:

bool lex_sort(const Point& lhs, const Point& rhs)
{
    return std::tie(lhs.x, lhs.y) < std::tie(rhs.x, rhs.y);
}

and then

auto it = std::min_element(V.begin(), V.end(), &lex_sort);
if (it == V.end()) {
    // V is empty.
} else {
    Cart thsPt;
    thsPt.pt = it;
    thsPt.val_1 = compute_val_1(*it);
    thsPt.val_2 = compute_val_2(*it);
    return thsPt;
}

Note that if val_1/val_2 always depend of pt, you can add a constructor for Cart which take a Point