CODEWITHSUNDEEP
c++11priority-queue
waas1919
waas1919

Reputation: 2595

How to force reorganization of a priority_queue?

I have a priority_queue that contains a vector with some objects.

std::priority_queue<std::shared_ptr<Foo>, std::vector<std::shared_ptr<Foo>>, foo_less> foo_queue;

It has a foo_queue function that will order the priority_queue.

Now, from outside of the priority_queue, I want to change some object value that must influenciate the ordering of the priority_queue.

My question is:

How can I set some kind of "refresh" that will trigger the priority_queue to run the foo_queue() in order to keep it orderes all the time?

Upvotes: 2

Views: 365

Answers (1)

ooga
ooga

Reputation: 15501

Make your own priority queue using the standard heap algorithms and a vector. When you want to change a key, find and remove that value from the underlying vector and call make_heap on the vector. Alter the key and then push it back onto the heap. So the cost is a linear search of the vector to find the value and a call to make_heap (which I think is also linear).

#include <iostream>
#include <vector>
#include <algorithm>

template <class T, class Container = std::vector<T>,
          class Compare = std::less<T> >
class my_priority_queue {
protected:
    Container c;
    Compare comp;
public:
    explicit my_priority_queue(const Container& c_  = Container(),
                            const Compare& comp_ = Compare())
        : c(c_), comp(comp_)
    {
        std::make_heap(c.begin(), c.end(), comp);
    }
    bool empty()       const { return c.empty(); }
    std::size_t size() const { return c.size(); }
    const T& top()     const { return c.front(); }
    void push(const T& x)
    {
        c.push_back(x);
        std::push_heap(c.begin(), c.end(), comp);
    }
    void pop()
    {
        std::pop_heap(c.begin(), c.end(), comp);
        c.pop_back();
    }
    void remove(const T& x)
    {
        auto it = std::find(c.begin(), c.end(), x);
        if (it != c.end()) {
            c.erase(it);
            std::make_heap(c.begin(), c.end(), comp);
        }
    }
};

class Foo {
    int x_;
public:
    Foo(int x) : x_(x) {}
    bool operator<(const Foo& f) const { return x_ < f.x_; }
    bool operator==(const Foo& f) const { return x_ == f.x_; }
    int get() const { return x_; }
    void set(int x) { x_ = x; }
};

int main() {
    my_priority_queue<Foo> q;

    for (auto x: {7, 1, 9, 5}) q.push(Foo(x));
    while (!q.empty()) {
        std::cout << q.top().get() << '\n';
        q.pop();
    }

    std::cout << '\n';

    for (auto x: {7, 1, 9, 5}) q.push(Foo(x));
    Foo x = Foo(5);
    q.remove(x);
    x.set(8);
    q.push(x);
    while (!q.empty()) {
        std::cout << q.top().get() << '\n';
        q.pop();
    }
}

Upvotes: 1

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