Find minmax of custom data type using algorithm header

Question

I have a vector of QPointF and I need to find the minimum and maximum y values because I need to know what is the amplitude of the data in the vector.

I use QPointF but for adding each new element I sum up the x value of other elements in the vector:

std::vector<QPointF> points;

int getTotalTime() {
    int time = 0;
    for(QPointF& p : points) {
        time += p.x();
    }

    return time;
}

void addPointToGraph(const QPointF& p) {
    if(points.size() == 0) {
        points.push_back(p);
        return;
    }

    points.push_back(QPointF(getTotalTime() + p.x(), p.y()));
}

So therefore I will have a continues waveform...this works fine! But now I need to find the amplitude of the waveform so I need to find minimum and maximum y values of the points vector.

E.g. I need a function to return min y and max y like a std::pair<float,float> I have seen in the algorithm header we have something like:

  std::array<int,7> foo {3,7,2,9,5,8,6};

  auto result = std::minmax_element (foo.begin(),foo.end());

  // print result:
  std::cout << "min is " << *result.first;
  std::cout << ", at position " << (result.first-foo.begin()) << '\n';
  std::cout << "max is " << *result.second;
  std::cout << ", at position " << (result.second-foo.begin()) << '\n';

The question is how can I use the same idea and go over my own vector and only check for y of the points?

Answer 1

std::minmax_elements, like many other algorithms, provides an overload that accepts a custom predicate:

template< class ForwardIt, class Compare >
std::pair<ForwardIt,ForwardIt> 
    minmax_element( ForwardIt first, ForwardIt last, Compare comp );

You can use that with a lambda expression to achieve what you want:

const auto result = std::minmax_element(foo.begin(), foo.end(), 
    [](const QPointF& a, const QPointF& b){ return a.y() < b.y(); });