Custom object's vector attribute median c++ - c++

Question

I am trying to find the median value from the third attribute of my objects. My objects are in a vector.
std::vector<Object> obj =
{ Object {V , H , 5},
Object {C , B , 2},
Object {D , F , 6},
Object {B , H , 4},
Object {B , H , 4}
};
answer:
4
I tried to use the nth_element method but it seems that I need to access the attribute.
std::nth_element(obj.begin(), (obj.begin() + obj.end())/2, obj.end());

Answer 1

By default, std::nth_element() applies the operator< to the dereferenced iterators provided as arguments.
In C++, there is no operator< defined for custom classes but the author is free to define (overload) one.
Alternatively, std::nth_element() can be called with a custom less predicate to force a specific order.
Could you show me a generic example ?
Here we go:
an example to get median according to a specific attribute of an object with
partial sorting (aka. std::nth_element()) and
a custom predicate to sort for Object::attr.
#include <algorithm>
#include <iostream>
#include <vector>
#include <string>
struct Object {
std::string id1, id2;
int attr;
};
std::ostream& operator<<(std::ostream &out, const Object &obj)
{
return out << "Object { id1: " << obj.id1
<< ", id2: " << obj.id2
<< ", attr: " << obj.attr << " }";
}
// demonstrate
int main()
{
std::vector<Object> obj =
{ Object {"V", "H" , 5},
Object {"C", "B" , 2},
Object {"D", "F" , 6},
Object {"B", "H" , 4},
Object {"B", "H" , 4}
};
// partial sorting with custom predicate (provided as lambda):
std::nth_element(obj.begin(), obj.begin() + obj.size() / 2, obj.end(),
[](const Object &obj1, const Object &obj2) { return obj1.attr < obj2.attr; });
// get result:
const std::vector<Object>::iterator iter
= obj.begin() + obj.size() / 2;
std::cout << "Median: " << *iter << '\n';
}
Output:
Median: Object { id1: B, id2: H, attr: 4 }
Live demo on coliru
Note:
OP mentioned the following snippet:
std::nth_element(obj.begin(), obj.begin() + obj.end()/2, obj.end());
obj.begin() + obj.end()/2 is yet another error.
It had to be obj.begin() + (obj.end() - obj.begin()) / 2.
I used instead the more compact obj.begin() + obj.size() / 2.