C ++中的稀疏数组

Question

I need a vector-like container, with integer indexation, but where some indices are omitted. So what is the common way to represent such sparse array in C++? I have an intuition that std::map is mostly used for such purposes. But it is rather slow for container where new items aren't usually added. What can you propose?

UPD: Not very "sparse". Maybe about 5%. Items mostly added during initialization step (and not very often after). But access is frequent (obviously I would not start this topic, if it was not crucial).

Answer 1

Yes, a map is usually the correct approach.

I would suggest a C++11 unordered_map (based on hash tables) to get lightning-fast lookup: it's pretty much the best you can do without a contiguous incrementing key.

Answer 2

Maybe something like

std::vector<boost::optional<your_type>> 

would be enough for you.

Answer 3

Items mostly added during initialization step (and not very often after). But access is frequent

In this case boost::container::flat_map can be a good option for you. It is basically just a sorted vector. Advantages (stolen from the website):

• Faster lookup than standard associative containers
• Much faster iteration than standard associative containers
• Less memory consumption for small objects (and for big objects if shrink_to_fit is used)
• Improved cache performance (data is stored in contiguous memory)

A potential drawback:

• Worst case linear-time insertions and linear-time deletions

Even if the worst case happens during insertion or deletion (moving elements of the underlying vector), it is still not that bad thanks to (1) the good use of the cache, (2) relocation of the underlying elements may be vectorized (vector instructions). You would have to try it in your application to see if insertion / deletion is a problem, given your usage pattern.

If flat_map is not suitable for you, I would try std::unordered_map .