填写结构，同时发现min_element，C ++

问题描述

我要填写一些结构，同时发现的最小元素。 PL在下面找到

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

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

我有个的载体 - 的std ::矢量＆lt;点和GT; V 中，我有几个千分点。

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

有另一种结构我有

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

现在我有两个任务：

1. 我需要找到结构V最小元素。



2. 填充结构的车，这是直接与V。

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

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

我可以做到这一点使用以下code。

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);
}




现在有一个明显的问题，这个实现。

Now there is an evident problem with this implementation.

有两个环。其中填充的结构和其他寻找最小元素（的std :: min_element（）必须有一个循环遍历所有的值）。我争取几毫秒'改善。因此，这不是一个好的code。此外，这似乎让 C_style

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

于是我想出了以下code。

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;
    }

现在的问题是 - 我得到分段错误。我不知道我应该如何使用迭代器插入值。我想提到过vCart，但vCart是调用min_element（..）之后空。我甚至尝试insert_iterator，但没有成功。

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.

所以PL建议。

推荐答案

看来你想要的东西，如：

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);
}

然后

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;
}

请注意，如果 val_1 / val_2 总是依赖 PT ，您可以添加 车该采取点

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

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