Most efficient way to grow array C++

Question

Apologies if this has been asked before, I can't find a question that fully answers what I want to know. They mention ways to do this, but don't compare approaches.

I am writing a program in C++ to solve a PDE to steady state. I don't know how many time steps this will take. Therefore I don't know how long my time arrays will be. This will have a maximum time of 100,000s, but the time step could be as small as .001, so it could be as many as 1e8 doubles in length in the worst case (not necessarily a rare case either).

What is the most efficient way to implement this in terms of memory allocated and running time?

Options I've looked at:

• Dynamically allocating an array with 1e8 elements, most of which won't ever be used.
• Allocating a smaller array initially, creating a larger array when needed and copying elements over
• Using std::vector and it's size increasing functionality

Are there any other options?

I'm primarily concerned with speed, but I want to know what memory considerations come into it as well

Answer 1

If you are concerned about speed just allocate 1e8 doubles and be done with it.

In most cases vector should work just fine. Remember that amortized it's O(1) for the append.

Unless you are running on something very weird the OS memory allocation should take care of most fragmentation issues and the fact that it's hard to find a 800MB free memory block.