unordered_multimap::equal_range slow

Question

I would expect unordered_multimap::equal_range to be in average constant complexity, however the following does not scale linearly with n as expected:

#include <iostream>
#include <tr1/unordered_map>
#include <cstdlib>

using namespace std::tr1;
using namespace std;

int main(){
        int n;
        cin >> n;
        unordered_map<int, int> um;
        for(int i=0; i<n; ++i){
                um.insert(make_pair(i%100000, i));
                pair<unordered_map<int, int>::iterator,unordered_map<int,int>::iterator > t = um.equal_range(i);
        }
}

$ g++ testbr.cpp
$ time echo 10000 | ./a.out 

real    0m0.065s
user    0m0.060s
sys     0m0.003s
$ time echo 100000 | ./a.out 

real    0m4.492s
user    0m4.490s
sys     0m0.003s

Is there a way to fix this?

Edit: Without equal_range it scales perfectly as expected.
Also if I insert all elements with the same key 0 (and alway call equal_range(0)) it scales as expected, even though boost doc states that equal range is in average O(count(k))...?

Answer 1

This seems to be a bug in libstdc++, I cannot find a bug report anywhere, but with #include <unordered_map> and compilation with -std=c++11 I get the expected behaviour.

Answer 2

This doesn't scale O(n), but O(n * n) (you're calling std::equal_range n times)!

Move the std::equal_range out of the inner loop.