CODEWITHSUNDEEP
c++vector
sushi7777
sushi7777

Reputation: 189

Whats the most efficient way to remove the first element from a large vector?

Say I have a vector with 100000 elements, and I wish to iterate through the vector, one by one, whilst copying the element into a map of some sort, but during each iteration, deleting the element from the vector - what is the most efficient way to do this?

Whilst iterating through the vector, I had done something like "it = vec.erase(it)" but it takes absolutely ages to complete. Is there not a quicker way? And as a side note, ordering is very important...

Upvotes: 5

Views: 2010

Answers (3)

SergeyA
SergeyA

Reputation: 62563

Since the order of the elements in the vector can not be important - you are putting them in the map anyways - the easy solution is to swap the first element with the last, and than remove the last element. That is going to be much faster.

Upvotes: 0

Jim Buck
Jim Buck

Reputation: 20726

If you need to use a vector, then given your requirements, you should use a vector of pointers to dynamically-allocated instances of the objects instead of a vector of objects themselves.

Upvotes: -3

paddy
paddy

Reputation: 63471

There isn't one. You are using the vector as a queue. This goes against its design.

You have a few choices. The following come immediately to mind:

  • Don't erase one item at a time. Do them in batches.

  • Use the vector as a ring buffer and just advance an index, but never remove elements.

  • Use a more appropriate container such as std::deque.

Upvotes: 5

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