Amortized analysis of std::vector insertion

Question

How do we do the analysis of insertion at the back (push_back) in a std::vector? It's amortized time is O(1) per insertion. In particular in a video in channel9 by Stephan T Lavavej and in this ( 17:42 onwards ) he says that for optimal performance Microsoft's implementation of this method increases capacity of the vector by around 1.5.

How is this constant determined?

Answer 1

Assuming you mean push_back and not insertion, I believe that the important part is the multiply by some constant (as opposed to grabbing N more elements each time) and as long as you do this you'll get amortized constant time. Changing the factor changes the average case and worst case performance.

Concretely: If your constant factor is too large, you'll have good average case performance, but bad worst case performance especially as the arrays get big. For instance, imagine doubling (2x) a 10000 size vector just because you have the 10001th element pushed. EDIT: As Michael Burr indirectly pointed out, the real cost here is probably that you'll grow your memory much larger than you need it to be. I would add to this that there are cache issues that affect speed if your factor is too large. Suffice it to say that there are real costs (memory and computation) if you grow much larger than you need.

However if your constant factor is too small, say (1.1x) then you're going to have good worst case performance, but bad average performance, because you're going to have to incur the cost of reallocating too many times.

Also, see Jon Skeet's answer to a similar question previously. (Thanks @Bo Persson)

A little more about the analysis: Say you have n items you are pushing back and a multiplication factor of M. Then the number of reallocations will be roughly log base M of n (log_M(n)). And the ith reallocation will cost proportional to M^i (M to the ith power). Then the total time of all the pushbacks will be M^1 + M^2 + ... M^(log_M(n)). The number of pushbacks is n, and thus you get this series (which is a geometric series, and reduces to roughly (nM)/(M-1) in the limit) divided by n. This is roughly a constant, M/(M-1).

For large values of M you will overshoot a lot and allocate much more than you need reasonably often (which I mentioned above). For small values of M (close to 1) this constant M/(M-1) becomes large. This factor directly affects the average time.

Answer 2

You can do the math to try to figure how this kind of thing works.

A popular method to work with asymptotic analysis is the Bankers method. What you do is markup all your operations with an extra cost, "saving" it for later to pay for an expensive operation latter on.

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Let's make some dump assumptions to simplify the math:

• Writing into an array costs 1. (Same for inserting and moving between arrays)
• Allocating a larger array is free.

And our algorithm looks like:

function insert(x){
    if n_elements >= maximum array size:
         move all elements to a new array that
         is K times larger than the current size
    add x to array
    n_elements += 1

Obviously, the "worst case" happens when we have to move the elements to the new array. Let's try to amortize this by adding a constant markup of d to the insertion cost, bringing it to a total of (1 + d) per operation.

Just after an array has been resized, we have (1/K) of it filled up and no money saved. By the time we fill the array up, we can be sure to have at least d * (1 - 1/K) * N saved up. Since this money must be able to pay for all N elements being moved, we can figure out a relation between K and d:

d*(1 - 1/K)*N = N
d*(K-1)/K = 1
d = K/(K-1)

A helpful table:

k    d     1+d(total insertion cost)
1.0  inf   inf
1.1  11.0  12.0
1.5  3.0   4.0
2.0  2.0   3.0
3.0  1.5   2.5
4.0  1.3   2.3
inf  1.0   2.0

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So from this you can get a rough mathematician's idea of how the time/memory tradeoff works for this problem. There are some caveats, of course: I didn't go over shrinking the array when it gets less elements, this only covers the worst case where no elements are ever removed and the time costs of allocating extra memory weren't accounted for.

They most likely ran a bunch of experimental tests to figure this out in the end making most of what I wrote irrelevant though.