# Finding the left-most and right-most points of a list. std::find_if the right way to go?

I have a list of Point objects, (each one with x,y properties) and would like to find the left-most and right-most points. I've been trying to do it with find_if, but i'm not sure its the way to go, because i can't seem to pass a comparator instance. Is find_if the way to go? Seems not. So, is there an algorithm in `<algorithm>` to achieve this?

`#include <iostream> #include <list> #include <algorithm> using namespace std; typedef struct Point{ float x; float y; } Point; bool left(Point& p1,Point& p2) { return p1.x < p2.x; } int main(){ Point p1 ={-1,0}; Point p2 ={1,0}; Point p3 ={5,0}; Point p4 ={7,0}; list <Point> points; points.push_back(p1); points.push_back(p2); points.push_back(p3); points.push_back(p4); //Should return an interator to p1. find_if(points.begin(),points.end(),left); return 0; } `

Use `std::min_element` and `std::max_element` instead.

```list<Point>::iterator left = std::min_element(points.begin(), points.end(), left); list<Point>::iterator right = std::max_element(points.begin(), points.end(), left); ```

I would also change the signature of `left` to:

```bool left(const Point& p1, const Point& p2) ```

even better would be to use the boost minmax element:

http://www.boost.org/doc/libs/1_42_0/libs/algorithm/minmax/index.html

```#include <boost/algorithm/minmax_element.hpp> ... auto res = boost::minmax_element(points.begin(), points.end(), left);```

``` ```

```std::cout << "min: " << res.first << std::endl; std::cout << "max: " << res.second << std::endl; ```

If you use `pair<float, float>` instead of your own `Point`, there's no need for a special comparator. There would also be an ordering on the y-axis of points with the same x-coordinate, which can be useful.

There are various ways to imbue `typedef pair<float, float> Point;` with custom behavior, if you're so inclined. For example,

`typedef pair<float, float> Point;`

``` enum AxisUnit { x, y }; float &operator*( Point &p, AxisUnit c ) // "special case" of inner product { return c == x? p.first : p.second; } ```

```Point my_point( 2.5, 6.3 ); float x_coord = my_point * x; ```

Category:c# Views:0 Time:2010-04-16

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