C++ Programming Code Examples
C++ > Visual C++ 5.0 Standard C++ Library Code Examples
Priority queue pop - Removes an element with the highest priority
Priority queue pop - Removes an element with the highest priority
pop
Header
<queue>
void pop()
Removes an element with the highest priority from the priority_queue. If you call pop on an empty priority_queue the results are undefined.
Sample
//Compiler options: /GX
#include <queue>
#include <functional>
#include <iostream>
int main()
{
std::priority_queue<int> pqi ; //Constructs an empty priority_queue, uses vector as default container.
int i ;
std::priority_queue<int>::allocator_type a1 = pqi.get_allocator() ;
std::cout << "call pqi.empty()" << std::endl ;
if (pqi.empty())
{
std::cout << "priority_queue is empty" << std::endl ;
}
else
{
std::cout << "priority_queue contains some elements" << std::endl ;
}
std::cout << "pqi.size() = " << pqi.size() << std::endl ;
std::cout << "Push Values on pqi = " ;
for(i = 0; i < 10; i++)
{
std::cout << i << ", " ;
pqi.push(i) ;
}
std::cout << std::endl ;
std::cout << "pqi.size() = " << pqi.size() << std::endl ;
std::cout << "Pop Values from pqi = " ;
while (!pqi.empty())
{
std::cout << pqi.top() << ", " ;
pqi.pop() ;
}
std::cout << std::endl ;
return 0 ;
}
Program Output
call pqi.empty()
priority_queue is empty
pqi.size() = 0
Push Values on pqi = 0, 1, 2, 3, 4, 5, 6, 7, 8, 9,
pqi.size() = 10
Pop Values from pqi = 9, 8, 7, 6, 5, 4, 3, 2, 1, 0,
Access top element. Returns a constant reference to the top element in the priority_queue. The top element is the element that compares higher in the priority_queue, and the next that is removed from the container when priority_queue::pop is called. This member function effectively calls member front of the underlying container object. It does not take any parameter. Function returns a reference to the top element in the priority_queue.
Return size. Returns the number of elements in the priority_queue. size() function is used to return the size of the priority queue container or the number of elements in the container. This member function effectively calls member size of the underlying container object. This function does not accept any parameter.
#include is a way of including a standard or user-defined file in the program and is mostly written at the beginning of any C/C++ program. This directive is read by the preprocessor and orders it to insert the content of a user-defined or system header file into the following program. These files are mainly imported from an outside source into the current program. The process of importing such files that might be system-defined or user-defined is known as File Inclusion. This type of preprocessor directive tells the compiler to include a file in the source code program.
Priority queue. Priority queues are a type of container adaptors, specifically designed such that its first element is always the greatest of the elements it contains, according to some strict weak ordering criterion. The priority queue in C++ is a derived container in STL that considers only the highest priority element. The queue follows the FIFO policy while priority queue pops the elements based on the priority, i.e., the highest priority element is popped first. The priority_queue uses this function to maintain the elements sorted in a way that preserves heap properties (i.e., that the element popped is the last according to this strict weak ordering). This can be a function pointer or a function object, and defaults to less<T>, which returns the same as applying the less-than operator (a<b).
In computer programming, loops are used to repeat a block of code. For example, when you are displaying number from 1 to 100 you may want set the value of a variable to 1 and display it 100 times, increasing its value by 1 on each loop iteration. When you know exactly how many times you want to loop through a block of code, use the for loop instead of a while loop. A for loop is a repetition control structure that allows you to efficiently write a loop that needs to execute a specific number of times.
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Test whether container is empty. Returns whether the priority_queue is empty: i.e. whether its size is zero. empty() function is used to check if the priority queue container is empty or not. This member function effectively calls member empty of the underlying container object. It does not take any parameter. Function returns true if the underlying container's size is 0, false otherwise.
In while loop, condition is evaluated first and if it returns true then the statements inside while loop execute, this happens repeatedly until the condition returns false. When condition returns false, the control comes out of loop and jumps to the next statement in the program after while loop. The important point to note when using while loop is that we need to use increment or decrement statement inside while loop so that the loop variable gets changed on each iteration, and at some point condition returns false. This way we can end the execution of while loop otherwise the loop would execute indefinitely. A while loop that never stops is said to be the infinite while loop, when we give the condition in such a way so that it never returns false, then the loops becomes infinite and repeats itself indefinitely.
A program shall contain a global function named main, which is the designated start of the program in hosted environment. main() function is the entry point of any C++ program. It is the point at which execution of program is started. When a C++ program is executed, the execution control goes directly to the main() function. Every C++ program have a main() function.
Insert element. Inserts a new element in the priority_queue. The content of this new element is initialized to val. This member function effectively calls the member function push_back of the underlying container object, and then reorders it to its location in the heap by calling the push_heap algorithm on the range that includes all the elements of the container. Function returns none.
Remove top element. Removes the element on top of the priority_queue, effectively reducing its size by one. The element removed is the one with the highest value. The value of this element can be retrieved before being popped by calling member priority_queue::top. This member function effectively calls the pop_heap algorithm to keep the heap property of priority_queues and then calls the member function pop_back of the underlying container object to remove the element. This calls the removed element's destructor. This function does not accept any parameter. This function does not return any value.
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