C ++ std :: map vs dynamic array

Question

I'm trying to make a 3 dimensional array of booleans that tells me if I previously visited a location in 3d space for a simple navigation algorithm. The array could be quite large (something along the lines of 1,000,000 x 1,000,000 x 1,000,000 or maybe larger), so I'm wondering if it would be faster to declare an array of that size and set each boolean value to false, or to make a map with a key of coordinate (x, y, z) and a value of type bool.

From what I figure, the array would take O(1) to find or modify a coordinate, and the map would take O(log n) to find or insert a value. Obviously, for accessing values, the array is faster. However, does this offset the time it takes to declare such an array?

Thanks

Answer 1

Even at 1 bit per bool, your array will take over 2**39 bytes. I'd suggest a set if there aren't too many elements that will be true .

You can use a class to hide the implementation details, and use a 1D set.

Answer 2

Have you tried calculating how much memory would be needed for an array like this? A lot! Use std::map if ordering of the points is important, or std::unordeded_map if not. Also the unordered map gives you a constant time insertion and lookup. I guess that some kind of search tree is probably what you're looking for (kd tree for example).

Answer 3

You're going to make an array that is one exabyte, assuming that you use 8 bits per point? Wow, you have a lot of RAM!

I think you should re-think your approach.