Sort a vector using its elements

Question

i need to know how can we sort a vector of user define class using its elements. Say i have a class called "coordinates" with getX and getY method that return an int value. I have created and array of vector "vector PointTwoD vcP2D(5);"

 class coordinates {
 int getX();
 int getY();

  )

Now the issue, 1) I need to sort the vector "vcP2D" using getX() and sort in asc order 2) Say an user enters the "2" as the x coordinate. And using that info i need to find which vector contains 2

Please advice

Answer 1

This will do:

std::sort(v.begin(), v.end(), [](const coordinates& c, const coordinates& d){ return c.getX() < d.getX(); });

It uses a C++11 Lambda expression as a binary predicate for std::sort.

A short demonstration:

#include <algorithm>
#include <vector>

#include <iostream>

struct coordinates
{
  int x;
  int y;
};

int main()
{
  std::vector<coordinates> v{ {2,3}, {0,0}, {1,5} };

  std::sort(v.begin(), v.end(), [](const coordinates& c, const coordinates& d) { return c.x < d.x; });

  std::cout << "sorted by x values, values of \"x\": " << v[0].x << " " << v[1].x << " " << v[2].x << "\n";

  std::sort(v.begin(), v.end(), [](const coordinates& c, const coordinates& d) { return c.y < d.y; });

  std::cout << "sorted by y values, values of \"x\": "  << v[0].x << " " << v[1].x << " " << v[2].x << "\n";
}

---

A demo of how to find an element in the same way:

#include <algorithm>
#include <vector>

#include <iostream>

struct coordinates
{
  int x;
  int y;
};

int main()
{
  std::vector<coordinates> v{ {2,3}, {0,0}, {1,5} };

  auto result = std::find_if(v.begin(), v.end(), [](const coordinates& c){ return c.x == 1 && c.y == 5; });
  if(result != v.end())
    std::cout << "point (1,5) is number " << std::distance(v.begin(), result)+1 << " in the vector.\n";
  else
    std::cout << "point (1,5) not found.\n";
 }

If you are looking to search in the sorted vector, you can use std::binary_search which takes a comparison function (the same as std::sort above). It also does not give an iterator to that element, only a true or false.

Answer 2

You need to define a strict weak order on your elements, either using operator< () or a binary predicate, and then use std::sort().

The easiest approach is to create a less than operator<():

bool operator< (coordinates const& c0, coordinates const& c1) {
    // return a suitable result of comparing c0 and c1 such that operator<()
    // become a strict weak order
}

With this all you need to do to sort a std::vector<coordinates> is to use std::sort(). To locate a specific object you would use std::lower_bound().