STL Priority Queue (1)

Priority Queue Usage

In the priority queue, elements with high priority are first out of the queue.

The standard library uses the < operator of the element type by default to determine the precedence relationship between them.

First use of the priority queue:

is also the most common usage

Priority_queue<int> Qi;

The < operator shows the high precedence of the elements in integers.

Therefore, the output in example 1 is:9 6 5 3 2

Second usage:

In example 1 , what if we want to export the element from small to large?

At this point we can pass in a comparison function, using the functional.h function Object as the comparison function.

Priority_queue<int, Vector<int>, greater<int> >qi2; priority queue from small to large

You can change the greater to less, which is from big to small

Where the first parameter is a container type. The second argument is a comparison function.

Therefore, the output in example 2 is:2 3 5 6 9

Third usage: Custom precedence.

struct node

{

friend bool operator< (node n1, node N2)

{

return n1.priority < n2.priority; "<" arranges ">" for Small to large

}

int priority;

int value;

};

In this structure, value is the values and priority is the precedence.

The priority in the element is compared by customizing the operator< operator.

In Example 3 , the output is:

Priority Value

9 5

8 2

6 1

2 3

1 4

But if the structure is defined as follows:

struct node

{

friend bool Operator> (node n1, node N2)

{

return n1.priority > n2.priority;

}

int priority;

int value;

};

It will compile but (g++ compiler)

Because the standard library uses the < operator of the element type by default to determine the precedence relationship between them.

and the < operator of the custom type has no direct contact with the > operator, so it compiles.

Code Listing

1#include <iostream>2#include <functional>3#include <queue>4 using namespacestd;5 structnode6 {7FriendBOOLoperator<(node n1, node N2)8     {9 returnN1.priority <n2.priority;Ten     } OneintPriority ; Aintvalue; - }; - intMain () the { -ConstintLen =5; -inti; -intA[len] = {3,5,9,6,2}; +//Example 1 -priority_queue<int>Qi; + for(i =0; i < Len; i++) A Qi.push (A[i]); at for(i =0; i < Len; i++) -     { -Cout<<qi.top () <<" "; - Qi.pop (); -     } -cout<<Endl; in//Example 2 -priority_queue<int, vector<int, greater<int> >Qi2; to for(i =0; i < Len; i++) + Qi2.push (A[i]); - for(i =0; i < Len; i++) the     { *Cout<<qi2.top () <<" "; $ Qi2.pop ();Panax Notoginseng     } -cout<<Endl; the //Example 3 +Priority_queue<node>qn; A node B[len]; theb[0].priority =6; b[0].value =1;  +b[1].priority =9; b[1].value =5;  -b[2].priority =2; b[2].value =3;  $b[3].priority =8; b[3].value =2;  $b[4].priority =1; b[4].value =4;  -  - for(i =0; i < Len; i++) the Qn.push (B[i]); -cout<<"Priority Level"<<'\ t'<<"value"<<Endl;Wuyi for(i =0; i < Len; i++) the     { -Cout<<qn.top () .priority<<'\ t'<<qn.top () .value<<Endl; Wu Qn.pop (); -     } Aboutreturn0; $}

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STL Priority Queue (1)