let’s learn how to use heap in C++
| func | description |
|---|
| make_heap | Make heap from range |
| push_heap | Push element into heap range |
| pop_heap | Pop element from heap range |
| sort_heap | Sort elements of heap |
| is_heap | Test if range is heap |
| is_heap_until | Find first element not in heap order |
make_heap
make heap from range
We need to have a heap before we operate on a heap, make_heap let’s us rearrange elements in a range.
Usually we use a vector to hold these range of elements:
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| #include <iostream> // std::cout
#include <algorithm> // std::make_heap, std::pop_heap, std::push_heap, std::sort_heap
#include <vector> // std::vector
std::vector<int> heap{1,2,3,4,5,6,7};
std::make_heap(heap.begin(), heap.end()); // 7 5 6 4 2 1 3
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For now, you only need to know that after make_heap, the biggest element is *begin(heap) or heap[0]. Other elements are not necessarily sorted, but arranged in a certain way, to know the detail, see heap in C++ 2.
push_heap
Push element into heap range
Given a heap in the range [first,last-1), this function extends the range considered a heap to [first,last) by placing the value in (last-1) into its corresponding location within it.
Meaning to add an element to a heap:
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| heap.push_back(8); // 7 5 6 4 2 1 3 8
std::push_heap(heap.begin(), heap.end()); // 8 7 6 5 2 1 3 4
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push_heap will adjust the elements in the new range (old range + 1), and place the added element to where it fits.
pop_heap
Pop element from heap range
Given a heap in the range [first,last), this function rearranges the elements in the heap range [first,last) in such a way that the part considered a heap is shortened by one: The element with the highest value is moved to (last-1).
Meaning to pop out the largest element from the heap:
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| std::pop_heap(heap.begin(), heap.end()); // 7 5 6 4 2 1 3 8
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new heap range: [first, last-1)
is_heap
Test if range is heap
simple as description, code:
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| // heap: 7 5 6 4 2 1 3 8
std::is_heap(heap.begin(), heap.end()); // false
std::is_heap(heap.begin(), heap.end()-1); // true
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is_heap_until
Find first element not in heap order
We’ve already popped out 8 from the heap, so:
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| // 7 5 6 4 2 1 3 8
auto it = std::is_heap_until(heap.begin(), heap.end());
std::cout << *it << std::endl; // 8
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sort_heap
Sort elements of heap
Sorts the elements in the heap range [first,last) into ascending order.
This is actually quite simple, you may have as well implement this function yourself, just keep doing pop_heap until the heap is shortened to 1.
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| std::sort_heap(heap.begin(), heap.end() - 1, cmp); // 1 2 3 4 5 6 7 8
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Code Examples
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| #include <algorithm>
#include <iostream>
#include <random>
#include <vector>
template <typename T>
void printArr(const std::vector<T> &arr) {
for (const T &t : arr) std::cout << t << " ";
std::cout << std::endl;
}
// std::random_device rd;
// std::mt19937_64 urng(rd());
int main(int argc, char const *argv[]) {
std::cout << "original array" << std::endl;
std::vector<int> heap(7);
std::iota(heap.begin(), heap.end(), 1);
printArr(heap); // 1 2 3 4 5 6 7
// std::cout << "shuffle" << std::endl;
// std::shuffle(heap.begin(), heap.end(), urng);
// printArr(heap);
// auto cmp = std::greater<int>();
// auto cmp = std::less<int>();
std::cout << "make heap" << std::endl;
std::make_heap(heap.begin(), heap.end());
printArr(heap); // 7 5 6 4 2 1 3
std::cout << "push back 8" << std::endl;
heap.push_back(8);
printArr(heap); // 7 5 6 4 2 1 3 8
std::cout << "push heap" << std::endl;
std::push_heap(heap.begin(), heap.end());
printArr(heap); // 8 7 6 5 2 1 3 4
std::cout << "pop heap" << std::endl;
std::pop_heap(heap.begin(), heap.end());
printArr(heap); // 7 5 6 4 2 1 3 8
std::cout << "is heap" << std::endl;
std::cout << std::is_heap(heap.begin(), heap.end()) << std::endl; // 0
std::cout << "is heap until" << std::endl;
auto it = std::is_heap_until(heap.begin(), heap.end());
std::cout << *it << std::endl; // 8
std::cout << "sort heap (begin, end-1)" << std::endl;
std::sort_heap(heap.begin(), heap.end() - 1);
printArr(heap); // 1 2 3 4 5 6 7 8
return 0;
}
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