Adrian Ratnapala Adrian Ratnapala - 1 year ago 107
C++ Question

O(1) deletion in an unordered vector

In a C++ program I have a collection of plain-old-data for which I need be able to efficiently:

  • Add elements.

  • Iterate over the whole collection.

  • Remove some of those elements (see update below).iteration, see below).

I do not have to use a set or map type because there is no external requirement for:

  • Indexing into it

  • Maintaining ordering.

  • Efficiently finding elements or test for them being in the collection.

If I had to write it from scratch in C, I would use an exponentially growing dynamic array and the deletion operation would move the last element into the freed-up slot. That way deletion is O(1).

If I want to use a C++ standard container it seems I could (a) use a
but write the deletion operation by hand or (b) use an
. I have a mild, but definite, dislike of linked lists.

Both solutions are acceptable to me, but I would really rather have it both ways: to use a vector but using only standard operations. Is there a way to do this.


The comments below made clear that I am not looking for a general deletion operation. My problem requires me to iterate over the collection frequently and on each pass I expect to discover that some elements have to be removed.

I can do that whole operation in O(n) time regardless of the number of deletions. But this is obviously a bespoke operation and I should have known I need to write my own loop. Surprisingly though
almost solves my problem. If the only thing I needed to do in the iteration pass was identify the out-of-date elements, then
would do the job, but I need to do other processing too.

Answer Source

Actually, this is all already in the standard library:

The std::remove_if also lets you remove a whole bunch of elements in a single iteration.

And, probably most importantly, iteration will be maximally fast due to your data being stores contiguously -> No cache misses.

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