# Heap in C++ STL | make_heap(), push_heap(), pop_heap(), sort_heap(), is_heap, is_heap_until()

• Difficulty Level : Medium
• Last Updated : 17 Feb, 2021

Heap data structure can be implemented in a range using STL which allows faster input into heap and retrieval of a number always results in the largest number i.e. largest number of the remaining numbers is popped out each time. Other numbers of the heap are arranged depending upon the implementation.

Operations on heap :

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1. make_heap() :- This function is used to convert a range in a container to a heap.

2. front() :- This function displays the first element of heap which is the maximum number.

 `// C++ code to demonstrate the working of ``// make_heap(), front()``#include ``using` `namespace` `std;``int` `main()``{``     ` `    ``// Initializing a vector``    ``vector<``int``> v1 = {20, 30, 40, 25, 15};``     ` `    ``// Converting vector into a heap``    ``// using make_heap()``    ``make_heap(v1.begin(), v1.end());``     ` `    ``// Displaying the maximum element of heap``    ``// using front()``    ``cout << ``"The maximum element of heap is : "``;``    ``cout << v1.front() << endl;``     ` `    ``return` `0;``}`

Output:

```The maximum element of heap is : 40
```

3. push_heap() :- This function is used to insert elements into heap. The size of the heap is increased by 1. New element is placed appropriately in the heap.

4. pop_heap() :- This function is used to delete the maximum element of the heap. The size of heap is decreased by 1. The heap elements are reorganised accordingly after this operation.

 `// C++ code to demonstrate the working of ``// push_heap() and pop_heap()``#include ``using` `namespace` `std;``int` `main()``{``     ` `    ``// Initializing a vector``    ``vector<``int``> v1 = {20, 30, 40, 25, 15};``     ` `    ``// Converting vector into a heap``    ``// using make_heap()``    ``make_heap(v1.begin(), v1.end());``     ` `    ``// Displaying the maximum element of heap``    ``// using front()``    ``cout << ``"The maximum element of heap is : "``;``    ``cout << v1.front() << endl;``     ` `    ``// using push_back() to enter element``    ``// in vector``    ``v1.push_back(50);``     ` `    ``// using push_heap() to reorder elements``    ``push_heap(v1.begin(), v1.end());``     ` `    ``// Displaying the maximum element of heap``    ``// using front()``    ``cout << ``"The maximum element of heap after push is : "``;``    ``cout << v1.front() << endl;``     ` `     ``// using pop_heap() to delete maximum element``    ``pop_heap(v1.begin(), v1.end());``    ``v1.pop_back();``     ` `    ``// Displaying the maximum element of heap``    ``// using front()``    ``cout << ``"The maximum element of heap after pop is : "``;``    ``cout << v1.front() << endl;``     ` `    ``return` `0;``}`

Output:

```The maximum element of heap is : 40
The maximum element of heap after push is : 50
The maximum element of heap after pop is : 40
```

5. sort_heap() :- This function is used to sort the heap. After this operation, the container is no longer a heap.

 `// C++ code to demonstrate the working of ``// sort_heap()``#include ``using` `namespace` `std;``int` `main()``{``     ` `    ``// Initializing a vector``    ``vector<``int``> v1 = {20, 30, 40, 25, 15};``     ` `    ``// Converting vector into a heap``    ``// using make_heap()``    ``make_heap(v1.begin(), v1.end());``     ` `    ``// Displaying heap elements ``    ``cout << ``"The heap elements are : "``;``    ``for` `(``int` `&x : v1) ``       ``cout << x << ``" "``;``    ``cout << endl;``     ` `    ``// sorting heap using sort_heap()``    ``sort_heap(v1.begin(), v1.end());``     ` `     ``// Displaying heap elements ``    ``cout << ``"The heap elements after sorting are : "``;``    ``for` `(``int` `&x : v1) ``       ``cout << x << ``" "``;``     ` `    ``return` `0;``}`

Output:

```The heap elements are : 40 30 20 25 15
The heap elements after sorting are : 15 20 25 30 40
```

6. is_heap() :- This function is used to check whether the container is heap or not. Generally, in most implementations, the reverse sorted container is considered as heap. Returns true if container is heap else returns false.

6. is_heap_until() :- This function returns the iterator to the position till the container is the heap. Generally, in most implementations, the reverse sorted container is considered as heap.

 `// C++ code to demonstrate the working of ``// is_heap() and is_heap_until()``#include ``using` `namespace` `std;``int` `main()``{``     ` `    ``// Initializing a vector``    ``vector<``int``> v1 = {40, 30, 25, 35, 15};``     ` `    ``// Declaring heap iterator``    ``vector<``int``>::iterator it1;``     ` `    ``// Checking if container is heap``    ``// using is_heap()``    ``is_heap(v1.begin(), v1.end())?``    ``cout << ``"The container is heap "``:``    ``cout << ``"The container is not heap"``;``    ``cout << endl;``     ` `    ``// using is_heap_until() to check position ``    ``// till which container is heap``    ``auto` `it = is_heap_until(v1.begin(), v1.end());``   ` `    ``// Displaying heap range elements``    ``cout << ``"The heap elements in container are : "``;``    ``for` `(it1=v1.begin(); it1!=it; it1++)``       ``cout << *it1 << ``" "``;``    ` `    ``return` `0;``}`

Output:

```The container is not heap
The heap elements in container are : 40 30 25
```

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