# C++ Algorithm Library - is_heap_until() Function

## Description

The C++ function std::algorithm::is_heap_until() finds the first element from the sequence which violates the max heap condition. It uses binary function for comparison.

## Declaration

Following is the declaration for std::algorithm::is_heap_until() function form std::algorithm header.

### C++11

```template <class RandomAccessIterator>
template <class RandomAccessIterator, class Compare>
RandomAccessIterator is_heap_until(RandomAccessIterator first,
RandomAccessIterator last Compare comp);
```

## Parameters

• first − Random access iterator to the initial position.

• last − Random access iterator to the final position.

• comp − A binary function which accepts two arguments and returns bool.

## Return value

Returns an iterator to the first element which violates the max heap condition. If entire sequence is valid max heap then it returns last.

## Exceptions

Throws exception if either the element comparison or an operation on an iterator throws exception.

Please note that invalid parameters cause undefined behavior.

Linear.

## Example

The following example shows the usage of std::algorithm::is_heap_until() function.

```#include <iostream>
#include <vector>
#include <algorithm>

using namespace std;

bool ignore_case(char a, char b) {
return (tolower(a) == tolower(b));
}

int main(void) {
vector<char> v = {'E', 'd', 'C', 'b', 'A'};
auto result = is_heap_until(v.begin(), v.end());

cout << char(*result) << " is the first element which "
<< "violates the max heap." << endl;

v = {5, 4, 3, 2, 1};

result = is_heap_until(v.begin(), v.end(), ignore_case);

if (result == end(v))
cout << "Entire range is valid heap." << endl;

return 0;
}
```

Let us compile and run the above program, this will produce the following result −

```d is the first element which violates the max heap.
Entire range is valid heap.
```
algorithm.htm