我可以在没有地图执行任何旋转的情况下预加载STL地图吗？

Question

I have sorted data coming from a database to initialize an STL map. Only 5% of data will be changed later on inside the map.

As I understand, there will be an overhead of rotations incurred for each insertions. Is it possible to bypass the overhead for sorted data? eg is there an option to skip rotation, and another STL algoritm to do create a balanced tree with sorted data?

PS : I am aware there will be only 2 max rotations, but was wondering if I can improve performance further.

Answer 1

I assume you're interesting only in efficient loading of the initial sorted data?

The standard map :: map (InputIterator first, InputIterator last) constructor seems to do the right thing.

"For the iterator constructor, linear in the distance between the iterators (copy constructions) if the elements are already sorted according to comp. For unsorted sequences, linearithmic (N*logN) in that distance (sorting,copy constructions)."

Answer 2

http://www.sgi.com/tech/stl/Map.html

I think map(InputIterator f, InputIterator l) might help you, but I don't know if it takes into account the data being sorted :/

Answer 3

I don't believe you have any guarantees as to the actual data structure used under the hood for an STL map. Additionally, consider that the order you insert the data (you state it is sorted) could have a negative impact on performance if the map did not perform rotations! Of course rotations implies that a self-balancing tree is used rather than a skip-list, splay tree, or any other data structure decided upon by the library author.

Likely the time spent retrieving the data from the database will dwarf the time spent adding the sorted data into the map. A possible optimization would be to NOT retrieve the data in sorted order. The map will not care about the sorting.

Answer 4

The std::map::insert(iterator, pair) function has amortized constant cost if the input is sorted. Reading in the entire data set means you get O(N). (Note that this program has correct semantics regardless of whether the input is sorted.)

#include <map>
#include <iostream>
int main() {
 std::map<int, int> m;
 int a, b;
 for(std::map<int,int>::iterator it = m.begin();
     std::cin >> a >> b;
     it = m.insert(it, std::pair<int,int>(a,b))) {
  /* nothing */
 }
}

Answer 5

If you must use a map, then consider using the gcc policy based containers if you can. They are much faster than the STL.

http://gcc.gnu.org/onlinedocs/libstdc++/ext/pb_ds/interface.html