Question

I’m extracting the min from a vector.

Say vector = [0, inf, inf, inf];

ExtractSmallest(vector) = 0;

and then vector = [0, 1, inf, inf];

but now, we’ve already seen 0. Thus,

ExtractSmallest(vector) = 1;

I represent this in my code by doing nodes.erase(nodes.begin() + smallestPosition);

But, I now realize that erasing is very bad. Is there a way to achieve this without erasing the vectors? Just skipping over the ones we’ve already seen?

Node* CGraph::ExtractSmallest(vector<Node*>& nodes)
{


    int size = nodes.size();
    if (size == 0) return NULL;
    int smallestPosition = 0;
    Node* smallest = nodes.at(0);


    for (int i=1; i<size; ++i)
    {

        Node* current = nodes.at(i);

        if (current->distanceFromStart <
            smallest->distanceFromStart)
        {
            smallest = current;
            smallestPosition = i;

        }
    }

    nodes.erase(nodes.begin() + smallestPosition);      


    return smallest;


}

Answer 1

Option 1 You can have an additional vector<bool> on which you iterate in parallel. When you find the smallest element, mark that position in the bool vector as true. Whenever you iterate, skip the positions in both vectors that are marked as true.

Option 2 If order is not important, keep the number of elements removed so far. When you find the minimum, swap positions with the first non-excluded element. On a new iteration, start from the first non-excluded element.

Option 3 If order is not important, sort the array. (this takes O(n*log(n))). Removal will now take O(1) – you just exclude the first non-excluded element.

Option 4 If there are no duplicates, you can keep a std::set on the side with all excluded elements to this point. When you iterate, check whether the current element was already excluded or not.