Using a priority_queue of int with custom compare

Question

I have numbers from 1 to n, and an array of priorities P of size n. I want to declare a priority queue using P as their priorities.

I don't want to define a new type of object that contains the number and its priority, I want to use a priority queue of int objects and pass a custom comparison object that depends on P to std::priority_queue. I tried the following but it doesn't work:

std::priority_queue<int, vector<int>, [P](int i, int j){ return P[i]<P[j]; }> PQ;

I also tried defining a class with a bool operator(int i, int j) {P[i] < P[j]} member and a constructor where I can pass P to it but that also didn't work.

How should I declare my priority queue?

The following seems to work:

class MyLess{
 public:
  MyLess(const int* p) {p_ = p;}
  bool operator()(const int &lhs, const int &rhs) const { return p_[lhs] < p_[rhs]; }

 private:
  const int* p_;
};
...
std::priority_queue<int, vector<int>, MyLess> PQ(P);

How exactly did not it work?

Petr
– Petr

2015-07-31 10:03:14 +00:00
Commented Jul 31, 2015 at 10:03 — Petr
– Petr, Commented Jul 31, 2015 at 10:03

Piotr Skotnicki · Accepted Answer · 2015-07-31 10:08:08Z

2

Option #1

#include <functional>
// ...
std::priority_queue<int
                  , std::vector<int>
                  , std::function<bool(int,int)>
                  >
                  PQ([P] (int i, int j) { return P[i] < P[j]; });

Option #2

auto cmp = [P] (int i, int j) { return P[i] < P[j]; };

std::priority_queue<int, std::vector<int>, decltype(cmp)> PQ(cmp);

answered Jul 31, 2015 at 10:08

Piotr Skotnicki

48.8k7 gold badges122 silver badges167 bronze badges

Sign up to request clarification or add additional context in comments.

4 Comments

Kaveh Over a year ago

The first one gives the following error similar to what I was getting: "no matching function for call to 'std::priority_queue<int, std::vector<int>, std::function<bool(int, int)> >::priority_queue(methodname::__lambda0)' PQ([T] (int i, int j) { return T[i] < T[j]; });"

Piotr Skotnicki Over a year ago

@Kaveh and the compiler you use is... ?

Kaveh Over a year ago

gcc-4.8.1. The second one causes internal compiler error (gcc-4.8.1): 0x4ea72c check_initializer ../../../work/gcc-4.8.1/gcc/cp/decl.c:5590 0x4edab2 cp_finish_decl(tree_node*, tree_node*, bool, tree_node*, int) ../../../work/gcc-4.8.1/gcc/cp/decl.c:6347 ...

Piotr Skotnicki Over a year ago

@Kaveh demo1, demo2

Collectives™ on Stack Overflow

Using a priority_queue of int with custom compare

1 Answer 1

Option #1

Option #2

4 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Option #1

Option #2

4 Comments

Your Answer

Sign up or log in

Post as a guest

Related