daydayup - 8 months ago 46

C++ Question

I am always confused with defining the comparator for a priority queue container and don't know how to understand it.

For example, I have a

`vector`

`pair<int,int>`

So the codes looks like this:

`struct Compare`

{

bool operator()(pair<int,int> const &p1, pair<int,int> const &p2) const

{

return p1.second < p2.second;

}

};

priority_queue<pair<int,int>,vector<pair<int,int> >, Compare> pqueue;

How to understand the operator

`"<"`

`">"`

`"<"`

`">"`

`priority_queue`

Answer

Priority queue returns the *top* element based on the comparison operator, meaning that when you retrieve items one by one, you get them in *descending* order.

The meaning of the comparison operator always stays *"less than"*, meaning that when `compare(A, B)`

is true, `B`

has higher priority than `A`

, and would be returned earlier from the priority queue.

Inverting the comparison function inverts the order in which you get the items from your priority queue. Specifically, using `>`

in place of `<`

reverses the order to *ascending*.