10

According to the Wikipedia page, search is "not an operation" on binary heaps (see complexity box at top-right).

Why not? Binary heaps may not be sorted, but they are ordered, and a full graph traversal can find any object in $O(n)$ time, no?

Is the page wrong or am I?

Raphael
  • 73,212
  • 30
  • 182
  • 400
Barry Fruitman
  • 755
  • 3
  • 10
  • 17

2 Answers2

12

Actually it depends on your point of view, or level of detail.

The heap, or better priority queue, as abstract data structure usually supports operations like is-empty, add-element, delete-min. And usually not find-element. This is the data structure seen as a specification, fixing the set of operations and their behaviour. The implementation is unknown, it can be a linked tree or array.

The wikipedia page however is concerned with the implementation. That is even clear from the title binary heap. (You understand the binary heap by looking as if it is a tree, you implement it using an array). In this implementation it is easily seen how to program your operation find-element. Technically, that is another abstract data structure (as it has an additional operation aprt from standard heap operations).

In practice heaps (priority queues) are often used for graph algorithms like Dijkstra. There it is important that the priorities stored in the heap can be changed (if we find a shorter route). That is only possible if we are able to find the element in the heap so we can adjust its priority (and reposition the element in the heap). Usually that is done by building another structure on top of the heap so we can efficiently find the positions where elements are stored.

Hendrik Jan
  • 31,459
  • 1
  • 54
  • 109
0

In a binary heap, values are indeed ordered, and a search operation degenerates to a scan of the array if the value/key is >= last value in the array. If however the value you are searching is close to the first (i.e index close to 0), then you will be able to stop early and not scan the array looking for a value that is not there.

For example(C++), an implementation such as this for a Vector that holds the binary heap:

template <class Compare>
uint32_t SearchHeapImpl(const uint32_t idx, const T V, const Compare &cmp) const
{       
    if (slots[idx] == V)
        return idx;
     else if (cmp(slots[idx], V))
         return UINT32_MAX;

     const auto leftChild = 2 * idx + 1;

     if (leftChild >= nElements)
         return UINT32_MAX;

     const auto i = SearchHeapImpl(leftChild, V, cmp);

     if (i != UINT32_MAX)
         return i;

     const auto rightChild = leftChild + 1;

     return rightChild >= nElements ? UINT32_MAX : SearchHeapImpl(rightChild, V, cmp);
}

template<class Compare = std::greater<T>>
uint32_t SearchHeap(const T V, const Compare cmp = Compare()) const
{
    return nElements ? SearchHeapImpl(0, V, cmp) : UINT32_MAX;
}

will abort search early if cmp(slots[idx], V) == true

As others have mentioned, such data structures are suited for O(1) peek, and O(log n) insert and delete operations. A skip list or a self balancing binary tree(e.g red black trees) are usually a better alternative, if you need O(log n) search (you trade O(1) for peek, for O(log n) for search). Of course, they usually more complex, require more memory, and random access patterns may impact performance due to cache hit misses, etc. Win some, lose some.

markpapadakis
  • 1
  • 1