# C++ Algorithm Library - is_heap_until() Function

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## Description

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

## Declaration

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

### C++11

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

## Parameters

• first − Random access iterator to the initial position.

• last − Random access iterator to the final position.

## 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;

int main(void) {
vector<int> v = {4, 3, 5, 1, 2};

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

cout << *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());

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 −

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