std :: set 2D点的自定义比较器 [英] std::set custom comparator for 2D points

问题描述

我需要一个非重复2D点的列表，因此我使用具有自定义比较功能的std::set.我使用的函数在插入点后出现问题，因为有时std::find找不到已插入的点.

I need a list of non-duplicated 2D points, so I use a std::set with a custom comparison function. The function I use has problems after inserting points because sometimes the std::find does not find the already inserted points.

const double tolerance = 0.1;
struct MyPoint2D
{
  MyPoint2D(double x, double y) : _x(x), _y(y) {}
  double _x, _y;
};
auto compMyPoint2D = [&](const MyPoint2D& pointA, const MyPoint2D& pointB) -> bool
{
  if (pointA._x < pointB._x - tolerance) return true;
  if (pointA._x > pointB._x + tolerance) return false;
  if (pointA._y < pointB._y - tolerance) return true;
  return false;
};
std::set<MyPoint2D, decltype(compMyPoint2D)> orderedMyPoints(compMyPoint2D);
MyPoint2D pointA(0.66,1.14);
MyPoint2D pointB(0.75, 0.0);
MyPoint2D pointC(0.57,1.19);
orderedMyPoints.insert(pointA);
orderedMyPoints.insert(pointB);
orderedMyPoints.insert(pointC);
if (orderedMyPoints.find(pointC)==orderedMyPoints.end())
{
  std::cout << "Not found" << std::endl;
  orderedMyPoints.insert(pointC);
  if (orderedMyPoints.find(pointC)==orderedMyPoints.end())
    std::cout << "Still not found" << std::endl;
}

在插入std::set之前我是否需要对2d点进行预排序，或者2d点有更好的比较功能?

Would I need to preorder the 2d points before inserting into the std::set or there is a better comparison function for 2d points?

在插入所有点以获得最终点索引之后，我需要使用std::find.

I need to use std::find after inserting all points to obtain the final point indexes.

我在Microsoft Visual Studio 2010上使用本机C ++.

I'm using native C++ on Microsoft Visual Studio 2010.

推荐答案

您的比较功能错误.取出+公差.在尝试确定浮点值之间的绝对顺序时，这没有用.例如，它不强制等效性的可传递性.也就是说，如果A == B(即f(A, B)和f(B, A)均为假)和B == C，则当您在其中进行公差调整时，A == C并不一定是这种情况.

Your comparison function is wrong. Take out the +-tolerance. That's not useful when trying to determine an absolute order among floating point values. It doesn't enforce transitivity of equivalence, for example. That is, if A == B (i.e. f(A, B) and f(B, A) are both false) and B == C, then it is not necessarily the case that A == C when you have that tolerance adjustment in there.

只需执行以下操作:

if (pointA._x < pointB._x) return true;
if (pointA._x > pointB._x) return false;
if (pointA._y < pointB._y) return true;
return false;

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