STL priority_queue is the implementation of Heap Data-structure. By default, it’s a max heap and we can easily use it for primitive datatypes. There are some important applications of it which can be found here
Prerequisite: Prioirty_queue Basics
In this article, we will see how can we use priority_queue for custom datatypes like class or structure.
suppose we have a structure name Person which consist of two variables Age and height
and we want to store that in priority_queue then a simple method won’t work here.
Given below is an example of the declaration of struct Person:
struct Person {
int Age;
float Height;
} |
On defining the Priority Queue as shown below, it’ll give us error since priority_queue doesn’t know on what order(min or max) we need to arrange the objects.
priority_queue<Person> pq; |
To rectify the error above, we will use operator overloading to define the priority. So that priority_queue can decide how to store the structure object.
Given below is the priority_queue implementation with the structure below:
// program in c++ to use priority_queue with structure #include <iostream> #include <queue> using namespace std;
#define ROW 5 #define COL 2 struct Person {
int age;
float height;
// this will used to initialize the variables
// of the structure
Person( int age, float height)
: age(age)
, height(height)
{
}
}; // this is an structure which implements the // operator overloading struct CompareHeight {
bool operator()(Person const & p1, Person const & p2)
{
// return "true" if "p1" is ordered
// before "p2", for example:
return p1.height < p2.height;
}
}; int main()
{ priority_queue<Person, vector<Person>, CompareHeight> Q;
// When we use priority_queue with structure
// then we need this kind of syntax where
// CompareHeight is the function or comparison function
float arr[ROW][COL] = { { 30, 5.5 },
{ 25, 5 },
{ 20, 6 },
{ 33, 6.1 },
{ 23, 5.6 } };
for ( int i = 0; i < ROW; ++i) {
Q.push(Person(arr[i][0], arr[i][1]));
// insert an object in priority_queue by using
// the Person structure constructor
}
while (!Q.empty()) {
Person p = Q.top();
Q.pop();
cout << p.age << " " << p.height << "\n" ;
}
return 0;
} |
33 6.1 20 6 23 5.6 30 5.5 25 5
Given below is the implementation of priority_queue using Class
// program in c++ to use priority_queue with class #include <iostream> #include <queue> using namespace std;
#define ROW 5 #define COL 2 class Person {
public :
int age;
float height;
// this is used to initialize the variables of the class
Person( int age, float height)
: age(age)
, height(height)
{
}
bool operator<( const Person& p) const
{
// this will return true when second person
// has greater height. Suppose we have p1.height=5
// and p2.height=5.5 then the object which
// have max height will be at the top(or
// max priority)
return this ->height < p.height;
}
}; int main()
{ priority_queue<Person> Q;
float arr[ROW][COL] = { { 30, 5.5 },
{ 25, 5 },
{ 20, 6 },
{ 33, 6.1 },
{ 23, 5.6 } };
for ( int i = 0; i < ROW; ++i) {
Q.push(Person(arr[i][0], arr[i][1]));
// insert an object in priority_queue by using
// the Person class constructor
}
while (!Q.empty()) {
Person p = Q.top();
Q.pop();
cout << p.age << " " << p.height << "\n" ;
}
return 0;
} |
33 6.1 20 6 23 5.6 30 5.5 25 5