检查点是否在向量内

Question

I want to check if a given point with x and y value is inside a vector of points:

bool inside_vector(int x, int y, vector<Point2i>& points)
{
  for(vector<Point2i>::const_iterator it = points.begin();
    it != points.end();
    ++it)
  {
    if(it->x == y && it->y == y)
    {
      return true;
    }
  }
  return false;
}

Are there other approaches without the for loop?

Answer 1

You can use std::find or std::find_if with suitable functor to avoid writing your own loop. But you don't gain in terms of complexity: it is still O(n). For example,

bool operator==(const Point2i& lhs, const Point2i& rhs)
{
  return lhs.x == rhs.x && lhs.y == rhs.y;
}

Point2Di somePoint = Point2Di(x,y); // point to find
auto it = std::find(points.begin(), points.end(), somePoint);

or, without the equality operator,

auto it = std::find_if(points.begin(), 
                       points.end(), [&somePoint](const Point2Di& p)
                                     {return somePoint.x == p.x && somePoint.y == p.y;});

Answer 2

Assumming you have an operator== defined for your Point2i structure, you can use std::find() , like this:

std::vector<Point2i>::const_iterator findIt = std::find(
    points.begin(),
    points.end(),
    Point2i(x, y));

I also assume you have a Point2i constructor that takes the two coordinates.